JavaScript quiz — 75 questions with answers and explanations

23 Beginner · 52 Intermediate

All 75 JavaScript quiz questions below come with a short explanation, visible right on the page — no sign-in needed. They're grouped by beginner and intermediate levels and 24 subtopics, so you can revise one part at a time. Choose Try JavaScript quiz above to take a scored run and review the questions you got wrong.

What the JavaScript quiz covers

Array Methods9
Coercion8
Objects7
Async5
Types5
Functions4
Operators4
Scope4
Events3
Numbers3
Strings3
Browser2
Closures2
Destructuring2
Error Handling2
Event Loop2
Modules2
this2
API1
Array1
Arrays1
Classes1
Hoisting1
Object1

Want long-form answers? Read 165 JavaScript interview questions with answers — same topic, in more depth.

Beginner level — 23 JavaScript questions

Question 1Coercion

What is the core difference between `==` and `===` in JavaScript?

A.== compares by reference, === compares by value
B.There is no difference, they are just two ways to write the same operation
C.== compares with type coercion, === compares both value and type
D.=== is faster, so it always yields a different result than ==

Answer: C

== coerces both operands to the same type before comparing (0 == "" → true), while === strictly compares both value and type with no coercion (0 === "" → false). Common practice is to prefer === to avoid surprising results.

References

MDN — Equality comparisons and sameness

Question 2Array Methods

What is the difference between `Array.prototype.map()` and `Array.prototype.forEach()`?

A.map() cannot access the index, but forEach() can
B.Both return a new array — there is no difference
C.forEach() mutates the original array in place, while map() does not
D.map() returns a new array of the transformed values

Answer: D

map() transforms each element and returns a new array of the same length — use it when you need the result. forEach() runs the callback on each element but returns undefined — use it for side effects (logging, writing to a DB...). If you are assigning the result of forEach, that is a sign you should use map.

References

MDN — Array.prototype.map()

Question 3Coercion

Which of the following values is NOT falsy in JavaScript?

A.0 (zero)
B.[] (an empty array)
C."" (the empty string)
D.NaN (not a number)

Answer: B

[] (an empty array) is truthy — every object in JS is truthy, including empty arrays and objects. There are exactly 8 falsy values: false, 0, -0, 0n, "", null, undefined, NaN. So if ([]) always takes the true branch, while [] == false is also true (due to type coercion) — two different things.

References

MDN — Falsy

Question 4Scope

What happens when you reassign a value to a variable declared with `const`?

A.The variable takes the new value normally; const is only a naming convention
B.It only errors if the new value has a different type from the old one
C.The program throws a TypeError at runtime — const does not allow reassignment
D.The assignment is silently ignored and the variable keeps its old value

Answer: C

Reassigning a const variable throws TypeError: Assignment to constant variable at runtime. const locks the binding (no reassignment), while let allows it. Note: const doesn't freeze the value — the object/array it points to can still be mutated. Rule of thumb: default to const, use let only when you need to reassign.

References

MDN — const

Question 5Strings

How do template literals (backtick strings) differ from regular quoted strings?

A.They allow embedded expressions with ${...} and multi-line strings written directly
B.${...} works in every kind of string, including single and double quotes
C.It is just another way of writing single quotes, with no extra features
D.Template literals run faster, so they should always replace normal quoted strings

Answer: A

Template literals use backticks ` and have two main features: expression interpolation ${expression} (e.g. `Hello ${name}, 1+1=${1+1}`) and multi-line strings written directly without \n. Single/double-quoted strings print ${name} literally instead of evaluating it. There are also tagged templates for advanced cases.

References

MDN — Template literals

Question 6Array Methods

What does `Array.prototype.filter()` do to the original array, and what does it return?

A.It returns true/false indicating whether any element matches
B.It returns a NEW array of the elements for which the callback returned truthy; the original array is unchanged
C.It returns the FIRST element matching the condition, or undefined if there is none
D.It deletes the failing elements from the original array in place

Answer: B

filter() runs the callback on each element and returns a new array containing the elements for which the callback returned truthy — the original array is unchanged. For example [1,2,3,4].filter(n => n % 2 === 0) gives [2, 4]. If nothing matches, it returns an empty array []. Use find for the first matching element, or some for a boolean.

References

MDN — Array.prototype.filter()

Question 7Types

What is the difference between `undefined` and `null` in JavaScript?

A.undefined only appears when the code throws an error
B.undefined is the default value when a variable or property has not been assigned
C.null means the variable has not been declared in the program
D.They are identical and interchangeable everywhere

Answer: B

undefined = "no value yet", produced automatically by the engine: a declared but unassigned variable, a missing property, a function with no return, an argument not passed. null = "intentionally no value", assigned deliberately by the developer. They have different types (typeof undefined → "undefined", typeof null → "object"), so null == undefined is true but null === undefined is false.

References

Question 8Destructuring

Destructuring with defaults and rest — what does the following code print?

javascript

const [a, b = 5, ...rest] = [1]
console.log(a, b, rest)
const { p: renamed, q = 9 } = { p: 7 }
console.log(renamed, q)

A.1 undefined [] 7 9
B.1 5 [] undefined 9
C.1 5 undefined 7 9
D.1 5 [] 7 9

Answer: D

It prints 1 5 [] then 7 9. Array destructuring: a=1, b has no element so it takes the default 5, and ...rest collects the leftovers into []. Object destructuring: { p: renamed } renames the key p to the variable renamed (7), while q doesn't exist so it takes the default 9. Defaults only kick in when the value is undefined.

References

MDN — Destructuring assignment

Question 9Operators

When does `a ?? b` (nullish coalescing) give a different result from `a || b`?

A.?? falls back on every falsy value, while || only falls back on null/undefined
B.When a is null or undefined
C.When a is falsy but NOT null/undefined
D.They always give the same result, only the spelling differs

Answer: C

They differ when a is falsy but not null/undefined — i.e. 0, "", false. ?? only falls back on null/undefined, so 0 ?? 5 → 0; || falls back on any falsy value, so 0 || 5 → 5. Use ?? when 0/empty string is a valid value (counts, flags, volume); use || when you want to treat empty as "missing" too.

References

MDN — Nullish coalescing operator (??)

Question 10Coercion

The `+` and `-` operators on numeric strings — what does the following code print?

console.log('5' + 2)
console.log('5' - 2)

A.52 3
B.52 52
C.7 7
D.7 3

Answer: A

52 then 3. The + operator is overloaded: if any operand is a string, it performs string concatenation, so "5" + 2 yields "52". Meanwhile - is purely arithmetic, forcing "5" to coerce to the number 5, so "5" - 2 yields 3.

References

MDN — Addition (+)

Question 11Coercion

`parseInt` on mixed strings — what does the following code print?

console.log(parseInt('08'))
console.log(parseInt('12px'))
console.log(parseInt('px12'))

A.8 NaN NaN
B.8 12 NaN
C.8 12 0
D.0 12 NaN

Answer: B

8, 12, NaN. parseInt reads left to right, taking the valid numeric prefix until it hits an invalid character: "08" → 8 (default radix is 10, so leading-zero octal is no longer recognized), "12px" → 12, while "px12" fails at the very first character → NaN.

References

MDN — parseInt()

Question 12Types

Nested `typeof` — what does the following code print?

console.log(typeof typeof 1)

A.string
B.number
C.undefined
D.object

Answer: A

string. typeof always returns a string naming the type. So typeof 1 is "number", and applying typeof again to the string "number" yields "string". Any typeof typeof x is always "string".

References

MDN — typeof

Question 13Arrays

Assigning to an array `length` — what does the following code print?

javascript

const arr = [1, 2, 3, 4, 5]
arr.length = 2
console.log(arr)
console.log(arr[3])

A.[1, 2] undefined
B.[1, 2, 3, 4, 5] 4
C.[1, 2] 4
D.TypeError

Answer: A

Prints [1, 2] then undefined. An array's length property is writable: assigning length = 2 truncates every element from index 2 onward, and the removal is real — arr[3] afterwards returns undefined. const cannot prevent this because it only locks the binding, not the object's contents.

References

MDN — Array.prototype.length

Question 14Types

Why does `typeof null` return `"object"`?

A.Because null inherits from Object.prototype like every other object
B.It is a bug from the first version of JavaScript, kept because fixing it would break existing code
C.Because null is a special empty object the runtime creates up front
D.Because typeof returns "object" for every value that is not a number or a string

Answer: B

This is a historical bug from the first version of JavaScript in 1995. Values were stored with a type tag, and null was represented by a null pointer, so it was misread as the object tag. A proposed fix was rejected because it would break a huge amount of code already running on the web.

References

MDN — typeof

Question 15Array Methods

What does `[10, 9, 100].sort()` return?

javascript

const nums = [10, 9, 100]
console.log(nums.sort())

A.[10, 100, 9], because by default the elements are compared as strings
B.[100, 10, 9], because the default order is descending by magnitude
C.[10, 9, 100], because an array of integers keeps its original order
D.[9, 10, 100], because the method recognises a numeric array and sorts by value

Answer: A

[10, 100, 9]. With no comparator, sort() converts each element to a string and sorts by UTF-16 code unit order. The string "10" comes before "9" because the character "1" precedes "9". To sort by numeric value you must write nums.sort((a, b) => a - b).

References

MDN — Array.prototype.sort

Question 16Array Methods

What is the difference between `find()` and `filter()`?

A.find returns the first matching element; filter returns an array of all matches
B.find mutates the original array while filter creates an entirely new one
C.find only works with arrays of numbers while filter works with any type
D.find returns the element's index while filter returns the element itself

Answer: A

find returns the first element that satisfies the condition, or undefined if none, and stops iterating as soon as it finds one. filter always iterates the whole array and returns an array of all matching elements, or an empty array.

References

MDN — Array.prototype.find()

Question 17Objects

What does optional chaining `?.` do when the left-hand value is `null` or `undefined`?

A.It returns null to preserve the type of the left-hand value
B.It still throws, but with a clearer message than the usual error
C.It stops and returns undefined instead of throwing
D.It automatically creates an empty object so the expression can continue

Answer: C

It short-circuits the expression and returns undefined instead of throwing a TypeError. user?.address?.city gives undefined if user or address is null/undefined, instead of a "Cannot read property of undefined" error. The operator also works for function calls fn?.() and index access arr?.[0].

References

MDN — Optional chaining (?.)

Question 18Functions

When does the default parameter in `function f(a = 1)` apply?

A.When the argument passed is null or undefined
B.When the argument passed is undefined, or nothing is passed at all
C.When fewer arguments are passed than parameters were declared
D.When the argument passed is any falsy value

Answer: B

Only when the argument is undefined — including when nothing is passed. Passing null, 0, "", or false gives the parameter that exact value. This is the key difference from the old a = a || 1 pattern, which also overrides valid falsy values.

References

MDN — Default parameters

Question 19Async

What states can a Promise be in?

A.pending, fulfilled, rejected — and it transitions exactly once
B.open, closed, cancelled — matching the cancellation of a task midway
C.waiting, resolved, failed — with the ability to return to pending
D.pending, running, done — following the lifecycle of the async task

Answer: A

Three states: pending (no result yet), fulfilled (has a value), rejected (has a failure reason). The transition happens only once and is irreversible — once fulfilled or rejected, any further resolve or reject calls are silently ignored.

References

MDN — Promise

Question 20Browser

How long does data in `sessionStorage` persist?

A.Until the user logs out of their account on the site
B.Forever, until code explicitly calls a delete
C.Until the expiry date set when the data was written
D.Until the open tab is closed

Answer: D

Until the tab is closed. sessionStorage is scoped per tab — opening the same page in another tab gives it a separate storage area. localStorage persists indefinitely until deleted and is shared across all tabs of the same origin.

References

MDN — Window.sessionStorage

Question 21Strings

After `let s = "abc"; s.toUpperCase()`, what is the value of `s`?

A.undefined, because the result was not assigned to any variable
B."ABC", because the method transforms the string held by the variable directly
C.A runtime error, because a method was called without using its return value
D."abc", because strings are immutable so the method returns a new string

Answer: D

"abc" — unchanged. Strings in JavaScript are immutable, so transforming methods like toUpperCase, trim, replace, slice all return a new string and leave the original alone. To change it you have to reassign: s = s.toUpperCase().

References

MDN — String (immutability)

Question 22Numbers

What type does `(1.005).toFixed(2)` return?

A.A string
B.An object representing the number formatted for the locale
C.An integer, because the decimals were dropped during formatting
D.A number rounded to two decimal places

Answer: A

A string. toFixed is meant for formatting numbers for display, so it always returns a string. Using the result in an addition turns it into string concatenation: (1.5).toFixed(1) + 1 gives "1.51", not 2.5.

References

MDN — Number.prototype.toFixed()

Question 23Scope

Before ES6, what was an IIFE mainly used for?

A.Creating a private scope so variables did not leak into the global one
B.Speeding up execution by hinting to the engine that it should optimise that code
C.Allowing await at the top level of a source file
D.Guaranteeing the enclosed code runs exactly once

Answer: A

To create a separate scope. Before ES6 there was only var, and var is function-scoped rather than block-scoped, so the only way to keep variables out of the global scope was to wrap the code in a function and call it immediately. Since let, const and ES Modules, this need has mostly disappeared.

References

MDN — IIFE

Intermediate level — 52 JavaScript questions

Question 1Closures

What is a closure in JavaScript?

A.A function that can still access variables from the scope where it was defined
B.An object frozen using Object.freeze()
C.A way to call a function immediately after declaration (IIFE)
D.A function that takes no parameters

Answer: A

A closure is a function that "remembers" the variables in the lexical scope where it was defined and can still access them after the parent function has returned. This mechanism allows keeping private state, creating factory functions, and is the foundation of patterns like modules and currying.

References

MDN — Closures

Question 2Event Loop

Microtask vs `setTimeout` — what is the output order of the following code?

console.log('A')
setTimeout(() => console.log('B'), 0)
Promise.resolve().then(() => console.log('C'))
console.log('D')

A.A B C D
B.A D B C
C.A D C B
D.A C D B

Answer: C

Prints A D C B. The two synchronous logs run first (A, D). Then the event loop drains all microtasks (Promise callback → C) before taking a macrotask (setTimeout → B). Microtasks always take priority over macrotasks.

References

MDN — The event loop

Question 3this

How is the value of `this` determined inside an arrow function?

A.this always points to the object that called the function, like a normal function
B.You can change an arrow function's this with .bind()
C.No this of its own — it inherits this from the enclosing lexical scope
D.this is always window/globalThis inside an arrow function

Answer: C

An arrow function does not create its own this; this is taken from the surrounding lexical scope where it is written. Therefore call, apply, bind cannot change an arrow function's this. This helps callbacks (e.g. in setTimeout, map) keep the correct this of the method containing them.

References

MDN — Arrow function expressions

Question 4Hoisting

Hoisting with `var` — what does the following code print?

console.log(x)
var x = 5
console.log(x)

A.null 5
B.5 5
C.undefined 5
D.ReferenceError

Answer: C

Prints undefined then 5. The var x declaration is hoisted to the top of the function/global scope and initialized to undefined, while the assignment x = 5 stays in place. So the first log (before the assignment) is undefined, and the later log is 5.

References

MDN — Hoisting

Question 5Scope· select all that apply

Which of the following are correct differences between `var`, `let`, and `const`? (select all that apply)

A.var cannot be redeclared with the same name in the same scope
B.var is function-scoped, while let/const are block-scoped
C.let and const sit in the Temporal Dead Zone until they are declared
D.const prevents reassigning the variable, but the object/array it points to can still be mutated

Answer: B, C, D

var is function-scoped, while let/const are block-scoped and have a TDZ; const locks reassignment but does not freeze object contents. Conversely, var allows redeclaring the same name in the same scope — it is let/const that forbid this.

References

MDN — let

Question 6Closures

Closures in a `var` loop — what does the following code print?

const fns = []
for (var i = 0; i < 3; i++) {
  fns.push(() => i)
}
console.log(fns[0](), fns[1](), fns[2]())

A.3 3 3
B.0 1 2
C.undefined undefined undefined
D.0 0 0

Answer: A

Prints 3 3 3. With var, all three closures share a single i variable (function-scoped). The loop ends when i === 3, so when the functions are called afterward they all read 3. Changing var to let gives 0 1 2 because each loop iteration has its own i binding.

References

MDN — Closures: Creating closures in loops

Question 7Async

What is the main difference between `async/await` and Promise `.then()` chaining?

A.async/await cannot catch errors, so you must always use .catch()
B.It is a syntax layer over Promises, letting you write asynchronous code in a sequential flow
C.async/await runs synchronously and blocks the main thread until it finishes
D.Only async/await can run several tasks in parallel

Answer: B

async/await is a syntactic layer over Promises: await pauses the async function body (without blocking the main thread) until the Promise settles, making asynchronous code read sequentially and allowing errors to be caught with try/catch. Under the hood it is still a Promise, so the two styles can be mixed.

References

MDN — async function

Question 8Coercion

`typeof null`, `NaN` and float equality — what does the following code print?

console.log(typeof null)
console.log(typeof NaN)
console.log(0.1 + 0.2 === 0.3)

A.null NaN true
B.object NaN false
C.object number true
D.object number false

Answer: D

Output: object, number, false. typeof null returns "object" (a historic bug kept for backward compatibility). NaN is a special value of type number. 0.1 + 0.2 yields 0.30000000000000004 due to IEEE-754 floating point representation, so === 0.3 is false.

References

MDN — typeof

Question 9Coercion

The `+` operator on two arrays — what does the following code print?

console.log([1, 2, 3] + [4, 5, 6])

A.1,2,3,4,5,6
B.[1,2,3,4,5,6]
C.1,2,34,5,6
D.NaN

Answer: C

It prints 1,2,34,5,6. The + operator applied to arrays calls ToPrimitive, turning each array into a string via Array.prototype.join(","): "1,2,3" and "4,5,6". Since one operand is a string, + does string concatenation → "1,2,34,5,6". To really merge arrays, use concat or [...a, ...b].

References

MDN — Addition (+)

Question 10Objects· select all that apply

Regarding shallow copy vs deep copy of an object, select all the correct statements. (select all that apply)

A.A shallow copy only copies the top level; nested properties (child objects/arrays) still share references with the original
B.JSON.parse(JSON.stringify(obj)) is a safe deep copy for every data type
C.structuredClone(obj) is the modern standard deep copy, handling Date/Map/Set and circular references
D.Spread {...obj} and Object.assign({}, obj) both make a shallow copy, not a deep copy

Answer: A, C, D

Shallow copy (spread {...obj}, Object.assign) copies only one level; nested objects/arrays are still shared by reference. Deep copy (prefer structuredClone) clones recursively so everything is fully independent. Note that JSON.parse(JSON.stringify()) loses functions/undefined/Date/Map and breaks on circular references.

References

MDN — structuredClone()

Question 11Array Methods

What is `Array.prototype.reduce()` used for, and what are its callback parameters?

A.Transforms each element and returns a new array of the same length
B.Sorts the array in place using a comparator
C.Drops the elements that fail a condition and returns a shorter array
D.Folds the array down to a single value

Answer: D

reduce reduces an entire array to one value by running the callback (accumulator, currentValue, index, array) over each element; the value returned at each step becomes the accumulator of the next step. So pass initialValue as the second argument to avoid errors on empty arrays and to make the accumulator type explicit.

References

MDN — Array.prototype.reduce()

Question 12Functions· select all that apply

Regarding the `...` operator as spread and rest, select all the correct statements. (select all that apply)

A.Object spread and array spread use the same mechanism: both iterate via Symbol.iterator
B.Object spread { ...obj } only makes a shallow copy of own enumerable properties (it does not copy the prototype); nested objects/arrays still share references
C.A rest parameter must be the LAST parameter — function f(a, ...rest, b) {} is a SyntaxError
D.Spread in call/array literal expands an iterable; object literal copies own enumerable props; rest gathers leftovers

Answer: B, C, D

Same ... symbol but opposite directions: spread expands an iterable into individual elements (function calls, [...a], { ...o }), while rest collects the remaining items into an Array ((...args) parameters or destructuring). Rest must come last. Spread only does a shallow copy. Note that array spread uses the iterator protocol, whereas object spread copies own enumerable properties, so the two mechanisms differ.

References

Question 13Modules

Which statement about ES Modules (`import`/`export`) is correct?

A.import can appear anywhere and runs dynamically at runtime like an ordinary function call
B.A file may have at most one export statement
C.An imported binding is a "live binding"
D.Default and named exports differ only in syntax; you can import a default with braces { }

Answer: C

Imported bindings are live bindings — read-only references to the exported value, so they always see the latest value when the source module updates, unlike CommonJS require which copies the value at require time. Also: a module can have many named exports plus at most one default export; import is static (hoisted, analyzed at parse time), and for dynamic loading you use import().

References

MDN — JavaScript modules

Question 14Events

What is event delegation and what mechanism does it rely on?

A.Attach ONE listener on a parent element and rely on event bubbling to catch events from its children
B.Attach a separate listener to every child so each one handles its own events independently
C.event.target and event.currentTarget always point to the same element
D.Delegation relies on event capturing (the downward phase), which is the browser default

Answer: A

Event delegation attaches one listener to a parent element instead of many listeners on each child, relying on event bubbling: the event bubbles up from the originating element (event.target) to the parent, where the handler processes it centrally. Benefits: fewer listeners, and it automatically applies to elements added dynamically later. Distinguish event.target (the child affected) from event.currentTarget (the parent the listener is attached to).

References

MDN — Event bubbling and capture

Question 15Objects· select all that apply

Regarding `Map`/`Set` versus objects/arrays, select all the correct statements. (select all that apply)

A.A Map accepts a key of any type, including an object
B.A Set can hold duplicate elements just like an array, differing only in access speed
C.A Map preserves insertion order, has .size, and is directly iterable
D.WeakMap/WeakSet accept only objects and hold weak references

Answer: A, C, D

Map accepts any key type (object/function), keeps insertion order, has .size, and is directly iterable; plain objects coerce keys to string/symbol and require counting via Object.keys. Set holds unique values (deduplicates), disallowing repeats unlike arrays. WeakMap/WeakSet accept only objects, hold weak references (entries vanish when garbage collected), and are not iterable — ideal for attaching metadata without memory leaks.

References

Question 16Coercion

`map(parseInt)` and the index argument — what does this code print?

console.log([1, 2, 3].map(parseInt))

A.[1, 2, 3]
B.[1, 2, NaN]
C.[NaN, NaN, NaN]
D.[1, NaN, NaN]

Answer: D

[1, NaN, NaN]. map passes the index as the second argument, and parseInt(string, radix) interprets that as the radix: parseInt(1, 0)=1, parseInt(2, 1)=NaN, parseInt(3, 2)=NaN. To map properly, wrap it: .map(Number) or .map((x) => parseInt(x, 10)).

References

MDN — parseInt()

Question 17Array Methods

`sort()` without a comparator — what does the following code print?

console.log([10, 1, 2, 21].sort())

A.[1, 10, 2, 21]
B.[10, 1, 2, 21]
C.[21, 10, 2, 1]
D.[1, 2, 10, 21]

Answer: A

[1, 10, 2, 21]. Array.prototype.sort() without a comparator converts each element to a string and compares them by UTF-16 code order (lexicographic): "1" < "10" < "2" < "21". To sort numbers correctly, pass a comparator: .sort((a, b) => a - b).

References

MDN — Array.prototype.sort()

Question 18Functions

`typeof arguments` in a normal function — what does the following code print?

console.log(
  (function () {
    return typeof arguments
  })(),
)

A.object
B.arguments
C.array
D.undefined

Answer: A

object. In a regular function, arguments is an array-like object: it has length and index access, but isn't a real Array (no map, forEach, etc.). typeof returns "object" for all objects — JS has no "array" result.

References

MDN — The arguments object

Question 19Operators

Chained `>` comparisons — what does the following code print?

console.log(3 > 2 > 1)

A.false
B.1
C.true
D.ReferenceError

Answer: A

Prints false. The > operator is left-associative, so 3 > 2 is evaluated first → true. Then true > 1: true is coerced to the number 1, making it 1 > 1 → false. JS does not read 3 > 2 > 1 as a chained inequality the way math does.

References

MDN — Greater than (>)

Question 20Numbers

`Math.max()` and `Math.min()` with no arguments — what does the following code print?

console.log(Math.max())
console.log(Math.min())

A.NaN NaN
B.Infinity -Infinity
C.-Infinity Infinity
D.undefined undefined

Answer: C

Prints -Infinity then Infinity. When called with no arguments, Math.max() returns -Infinity and Math.min() returns Infinity. Those are the "identity" elements of each operation: every real number is greater than -Infinity and less than Infinity, so results stay consistent as elements are added.

References

MDN — Math.max()

Question 21Coercion

Unary `+` coercing arrays and objects — what does the following code print?

console.log(+[])
console.log(+[1])
console.log(+[1, 2])
console.log(+{})

A.NaN NaN NaN NaN
B.0 1 3 0
C.0 1 NaN NaN
D.0 1 2 NaN

Answer: C

Prints 0, 1, NaN, NaN. Unary + coerces the operand to a number via ToPrimitive→ToNumber. Arrays are converted to strings with join(","): []→""→0, [1]→"1"→1, [1, 2]→"1,2"→NaN (invalid comma), {}→"[object Object]"→NaN.

References

MDN — Unary plus (+)

Question 22Destructuring

Default parameters with `undefined` vs `null` — what does the following code print?

javascript

function pick({ a = 1, b = 2 } = {}) {
  return [a, b]
}
console.log(pick({ a: undefined, b: null }))
console.log(pick())

A.[1, null] TypeError
B.[undefined, null] [1, 2]
C.[1, 2] [1, 2]
D.[1, null] [1, 2]

Answer: D

Prints [1, null] then [1, 2]. A destructuring default only kicks in when the value is undefined — including an explicit undefined. null is a "deliberate" value and is kept as is: b receives null. Calling pick() with no arguments lets the = {} default cover the whole parameter, then a and b each fall back to their own defaults.

References

MDN — Destructuring assignment (default values)

Question 23Operators

`?.` and `??` vs `||` — what does the following code print?

javascript

const user = { profile: null, age: 0 }
console.log(user.profile?.name)
console.log(user.profile?.name ?? 'guest')
console.log(user.age ?? 18)
console.log(user.age || 18)

A.TypeError guest 0 18
B.null guest 0 18
C.undefined guest 0 18
D.undefined guest 18 18

Answer: C

Prints undefined, guest, 0, 18. ?. short-circuits on null/undefined and returns undefined (always undefined, never null). ?? falls back only when the left side is null/undefined, so 0 ?? 18 keeps 0. || falls back for every falsy value (0, "", false...), so 0 || 18 yields 18 — the crucial difference between the two operators.

References

Question 24Objects

Getters and setters in an object literal — what does the following code print?

javascript

const counter = {
  count: 0,
  get double() {
    return this.count * 2
  },
  set double(value) {
    this.count = value / 2
  },
}
counter.double = 10
console.log(counter.count)
console.log(counter.double)
counter.count++
console.log(counter.double)

A.10 20 22
B.5 10 10
C.0 10 12
D.5 10 12

Answer: D

Prints 5, 10, 12. double is an accessor property: assigning counter.double = 10 calls the setter → count = 10/2 = 5. Reading counter.double calls the getter → 5 × 2 = 10. The getter does not cache — after count++ (now 6), the next read recomputes → 12. The syntax looks like an ordinary property, but every read/write is a function call.

References

MDN — getter

Question 25Types

Why is `NaN === NaN` false, and how do you correctly test for NaN?

A.Because the IEEE-754 standard says NaN is not equal to itself; use Number.isNaN()
B.Because JavaScript creates a separate NaN object each time one is produced
C.Because === cannot compare the special values of the number type
D.Because NaN carries an error trace, so two NaNs have different contents

Answer: A

Because the IEEE-754 standard says NaN is not equal to any value, including itself — it represents "not a determinate number", so asserting that two indeterminate values are equal is meaningless. Test correctly with Number.isNaN(x), or Object.is(x, NaN). Avoid the global isNaN() because it coerces first.

References

MDN — NaN

Question 26Types

What is a Symbol used for in JavaScript?

A.Creating a unique property key that cannot clash with keys chosen by others
B.Representing special characters that an ordinary string cannot express
C.Marking a property as private so nothing outside can reach it
D.Creating a constant whose value cannot be changed after initialisation

Answer: A

To create unique property keys. Each call to Symbol("description") yields a completely new value, even when the descriptions match, so you can add properties to a shared object with no chance of overwriting anyone else's key. Symbols are also used for the language's built-in extension points like Symbol.iterator.

References

MDN — Symbol

Question 27Array Methods

How do `slice()` and `splice()` differ?

A.slice returns the removed elements while splice returns what is left
B.slice is for arrays while splice also works on strings
C.slice returns a new array and leaves the original alone; splice modifies the original in place
D.slice can only cut from the start of the array while splice can cut anywhere

Answer: C

slice(start, end) returns a new array containing the extracted portion and leaves the original array untouched. splice(start, deleteCount, ...items) mutates the original array in place — removing and inserting — and returns an array of the removed elements.

References

MDN — Array.prototype.splice()

Question 28Array Methods

Why does `[NaN].indexOf(NaN)` give `-1` while `[NaN].includes(NaN)` gives `true`?

A.Because indexOf only works correctly with ordinary primitive values
B.Because includes converts the elements to strings before comparing
C.Because the two methods use different comparison algorithms
D.Because includes scans backwards from the end and meets NaN differently

Answer: C

Because they use different comparison algorithms. indexOf uses strict equality (===), and NaN === NaN is false, so it never finds it. includes uses SameValueZero — like strict equality but treating NaN as equal to itself — so it finds it.

References

MDN — Array.prototype.includes()

Question 29Array Methods

What combination is `flatMap()` equivalent to?

A.map() followed by flat() with infinite depth for every level of nesting
B.map() followed by flat() with a depth of 1
C.flat() followed by map() on the fully flattened array
D.filter() followed by map() to drop empty entries before transforming

Answer: B

arr.flatMap(fn) is equivalent to arr.map(fn).flat(1) — map each element, then flatten exactly one level. Useful when each input element produces zero or more output elements: returning [] from the callback drops that element from the result.

References

MDN — Array.prototype.flatMap()

Question 30Objects

After `Object.freeze(obj)`, which operation still works?

A.Assigning a new value to a top-level property
B.Changing a property of an object nested inside obj
C.Deleting an existing property with the delete operator
D.Adding an entirely new property to the frozen object

Answer: B

Modifying a nested object. Object.freeze only freezes one level: it blocks adding, changing, and deleting properties at the top level, but if a property points to another object, that object can still be modified normally. Deep freezing requires your own recursion.

References

MDN — Object.freeze()

Question 31Objects

How does a getter declared with `get` in an object literal behave?

A.It only has an effect when the object is accessed from within itself
B.It is read like an ordinary property but actually calls a function on every read
C.It is only valid when declared alongside a matching setter
D.It computes the value once at creation and stores it for subsequent reads

Answer: B

It's a property with a getter: reading obj.x looks like a normal property read but actually calls the getter function. The value is recomputed on every read, not stored. With only a getter the property is read-only; you need a setter to write to it.

References

MDN — getter (get)

Question 32this

Passing `obj.method` as a callback loses `this` inside it. Why?

javascript

const counter = {
  count: 0,
  increment() { this.count++ },
}

setTimeout(counter.increment, 0)

A.Because the function is copied onto another object when passed as an argument
B.Because a callback always runs in strict mode, which strips this
C.Because an object's methods are not inherited through the prototype chain
D.Because this is decided by how the function is called, not by where it is defined

Answer: D

Because in JavaScript this is decided at call time, not at definition time. counter.increment() has this set to counter thanks to the dot syntax. Passing counter.increment around passes a bare function reference; when it's called there's no object in front of it, so this is undefined or the global object.

References

MDN — Function.prototype.bind()

Question 33Classes

Why must `super()` be called before using `this` in a subclass constructor?

A.Because it is a style convention that makes the code easier to read, not a requirement
B.Because without it the parent's methods would not be inherited
C.Because the subclass object is only initialised after the parent constructor has run
D.Because the call copies the parent's properties down onto the subclass

Answer: C

Because in a derived class this doesn't exist until super() has run. The parent class is what creates the actual object; the subclass receives that object and only then adds its own parts. Using this before calling it throws a ReferenceError.

References

MDN — super

Question 34Functions

Why can an arrow function not use the `arguments` object?

A.Because it has no binding of its own and takes it from the enclosing function, just like this
B.Because an arrow function accepts at most a single parameter
C.Because an arrow function always runs in strict mode, which forbids that object
D.Because that object was removed entirely from newer versions of JavaScript

Answer: A

Because an arrow function doesn't create its own arguments binding, just as it doesn't create its own this binding. Referencing arguments inside an arrow takes the enclosing normal function's, or throws a ReferenceError if there's no enclosing function. The correct replacement is the rest parameter (...args).

References

MDN — Arrow function expressions

Question 35Async

Why does a `try/catch` around `setTimeout` not catch an error thrown inside its callback?

A.Because catching an async error requires a Promise catch
B.Because the timer function silently swallows every error raised inside the callback
C.Because the callback runs on a separate thread, so the error cannot travel back
D.Because the callback runs on a later turn, long after the try block has ended

Answer: D

Because try/catch only guards what runs on the same call stack. setTimeout registers the callback and returns immediately, so the try block ends. When the event loop pulls the callback out to run it, that starts a brand-new stack — the try block no longer exists by then.

References

MDN — setTimeout()

Question 36Async

How does `for await (const x of arr)` differ from `arr.forEach(async x => ...)`?

A.The loop runs in parallel while forEach goes through elements one at a time
B.The loop waits for each element to finish before moving on; forEach waits for nothing
C.Both wait sequentially, they only differ in how they are written
D.The loop only works on arrays while forEach works on any iterable

Answer: B

for await waits for each Promise to settle before moving to the next iteration — sequential, and errors are caught with try/catch. forEach ignores the callback's return value, so the async functions overlap, the outer function finishes before they do, and errors in the callback become unhandled rejections.

References

MDN — for await...of

Question 37Event Loop

When does `setTimeout(fn, 0)` actually run the function?

A.Immediately, because the delay is set to zero milliseconds
B.Exactly 0 milliseconds after the call, regardless of synchronous code still running
C.After the current synchronous code and every pending microtask have finished
D.Before any pending Promises, because the timer queue has higher priority

Answer: C

After all current synchronous code finishes and the microtask queue has been fully drained. The 0 only means "don't wait extra", not "run immediately". Also, browsers enforce a minimum of about 4 milliseconds for nested timers from level 5 onward.

References

MDN — setTimeout(): reasons for delays

Question 38Async

What is `AbortController` used for?

A.Automatically retrying a failed request according to a preset policy
B.Limiting how many requests may be sent concurrently to the same server
C.Cancelling an in-flight request or asynchronous task
D.Catching errors raised in a Promise so the program does not stop

Answer: C

To cancel a running asynchronous task. Create const c = new AbortController(), pass c.signal to fetch (or addEventListener), then call c.abort() when you need to stop. The request is cancelled and the Promise rejects with an error named AbortError.

References

MDN — AbortController

Question 39Error Handling

What does the function below return?

javascript

function f() {
  try {
    return 1
  } finally {
    return 2
  }
}

console.log(f())

A.A syntax error, because return is not allowed inside a finally block
B.2, because the return in finally overrides the value from try
C.undefined, because the two return statements conflict so nothing is returned
D.1, because the return in try executes first and therefore wins

Answer: B

2. The finally block always runs before the function actually returns, and if it contains a return, that value replaces the one prepared in try. This is why you should not put return inside finally — it silently swallows both the value and any exception.

References

MDN — try...catch

Question 40Error Handling

Why define a custom error class extending `Error` instead of throwing a plain object?

A.So try/catch can catch it, since a plain object is not caught by a catch block
B.To preserve the stack trace and classify errors with instanceof
C.So the error is automatically written to the monitoring system's log
D.So the error automatically becomes an HTTP response with a matching status code

Answer: B

To keep the stack trace and to classify errors. Plain objects usually lack a stack, so the origin information is lost. A dedicated class lets you write if (e instanceof ValidationError) to handle each kind differently, and carry extra fields like an error code or list of invalid fields.

References

MDN — Error

Question 41Modules

How does the function-form `import()` differ from a top-level `import`?

A.It only works with CommonJS modules, not with ES Modules
B.It loads the module faster because it skips dependency resolution
C.It loads the module at runtime and returns a Promise, so it can be called conditionally
D.It loads the module into global scope rather than the calling file's scope

Answer: C

import() is an expression evaluated at runtime that returns a Promise, so it can go inside a function, inside an if branch, or be called when the user performs some action. Top-level import is statically analyzed, always loads, and must sit at the top level.

References

MDN — import() (dynamic import)

Question 42Events

In what order does a DOM event travel once dispatched?

A.From the root down to the target element, then bubbling back up to the root
B.From the target element straight up to the root, with no downward phase
C.From the root down to the target and then stops, with no return trip
D.Only at the target element itself, without reaching any other element

Answer: A

Three phases: capture travels from window down to the target element, target is the element where the event occurred, then bubble rises back up. addEventListener registers in the bubble phase by default; pass { capture: true } to catch in the capture phase.

References

MDN — Event.eventPhase

Question 43Events

What is the difference between `preventDefault()` and `stopPropagation()`?

A.One cancels the event entirely, the other merely defers the handling
B.One stops the event in the bubble phase, the other in the capture phase
C.One blocks the default behaviour, the other blocks the event from travelling further
D.One is for mouse events, the other for keyboard events

Answer: C

preventDefault() blocks the browser's default behavior — a link doesn't navigate, a form doesn't submit — but the event still keeps propagating. stopPropagation() stops the event from reaching ancestor elements but the default behavior still happens. Two separate things, so call both if you need both.

References

MDN — Event.stopPropagation()

Question 44Browser

Why does `DOMContentLoaded` fire earlier than `load`?

A.Because it fires as soon as the server returns the document's first byte
B.Because it only waits for the document head, not for the body to be built
C.Because it only waits for the HTML to be parsed, not for images and sub-resources
D.Because it skips script tags, so it need not wait for code to load and run

Answer: C

Because DOMContentLoaded only waits for the HTML to be parsed and the DOM tree fully built. load also waits for all sub-resources to finish loading — images, iframes, stylesheets, fonts — so it usually arrives hundreds of milliseconds to a few seconds later.

References

MDN — DOMContentLoaded event

Question 45Objects

What limitation of `JSON.parse(JSON.stringify(obj))` does `structuredClone()` fix?

A.It preserves Date, Map, Set and handles circular references
B.It preserves the prototype chain, so the copy is of the same class as the original
C.It can also copy the source object's functions and methods
D.It copies many times faster by skipping the string conversion step

Answer: A

It preserves Date, Map, Set, RegExp, ArrayBuffer correctly and handles circular references. The old approach turned Date into a string, lost Map/Set, skipped undefined and functions, and threw on circular references.

References

MDN — structuredClone()

Question 46Strings

What does `"a-b-c".replace("-", "+")` produce?

A."a+b+c", because the method replaces every occurrence by default
B."a+b-c", because a string pattern only replaces the first occurrence
C."a-b-c", because strings are immutable so the method can change nothing
D.An error, because the first argument must be a valid regular expression

Answer: B

"a+b-c". When the first argument is a string, replace only replaces the first occurrence. To replace all, use replaceAll("-", "+") or a regex with the global flag: replace(/-/g, "+").

References

MDN — String.prototype.replace()

Question 47Numbers

What do `parseInt("12px")` and `Number("12px")` return?

A.12 and 12, because both ignore the non-numeric suffix
B.NaN and NaN, because the string contains letters and cannot be converted
C.NaN and 12, because the type-conversion function is more lenient than the parser
D.12 and NaN

Answer: D

parseInt("12px") gives 12, while Number("12px") gives NaN. parseInt reads from the start of the string and stops at the first invalid character, returning what it managed to parse. Number requires the whole string to be a valid number, and a single stray character makes it return NaN.

References

MDN — parseInt()

Question 48Scope

What is the temporal dead zone for `let` and `const`?

A.The stretch from the top of the block to the declaration line, where access throws
B.The stretch after a function ends during which its variables are not yet reclaimed
C.The region where a variable is hidden by one of the same name in an inner block
D.The region where a variable has left scope but a closure still references it

Answer: A

The range from the start of the enclosing block up to the declaration itself. During this period the variable exists (it has been hoisted) but is not yet initialized, so accessing it throws a ReferenceError. Unlike var, which is initialized to undefined, so early access does not error.

References

MDN — let: temporal dead zone

Question 49Operators

When does `a ??= b` perform the assignment?

A.When a holds any falsy value, including 0 and the empty string
B.When a has never been declared in any scope
C.Always — the two differ only in how they handle a nullish b
D.When a is null or undefined

Answer: D

Only when a is null or undefined. This is the logical assignment form of nullish coalescing: it only evaluates the right-hand side and performs the assignment when the current value is null or undefined. Unlike ||=, it preserves valid falsy values such as 0, "" and false.

References

MDN — Nullish coalescing assignment (??=)

Question 50API

Formatting currency for Vietnam — which approach?

A.A separate library for formatting dates and numbers
B.Multiplying, dividing and rounding by hand before display
C.The language's built-in locale-aware formatting API
D.Inserting grouping dots yourself with a regular expression

Answer: C

Use the built-in locale formatting API of the language. It already handles grouping separators, currency symbol placement and decimal digits per regional conventions, and requires no extra library.

References

MDN — Intl

Question 51Object

Uppercasing every value of an object while keeping keys — concise way?

A.Iterate with a loop and assign into an empty object
B.Convert to a JSON string and replace with a regular expression
C.Call a map method directly on the object
D.Convert to an array of pairs, transform, then rebuild the object

Answer: D

Convert to an array of key-value pairs, transform, then rebuild the object. The two conversion functions let you use any array method in between instead of writing a manual loop.

References

MDN — Object.fromEntries

Question 52Array

Finding a matching element from the end of an array — what do you use?

A.The search method that scans backwards from the end of the array
B.Reverse the array and use the ordinary find method
C.Filter everything and take the last element of the result
D.Use a loop counting down from the last index

Answer: A

Use the search method that iterates from the end of the array. It stops at the first matching element from the end, so you don't need to reverse the array or create any intermediate copy.

References

MDN — Array.prototype.findLast

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JavaScript quiz — 75 questions with answers and explanations

23 Beginner · 52 Intermediate

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What the JavaScript quiz covers

Array Methods9
Coercion8
Objects7
Async5
Types5
Functions4
Operators4
Scope4
Events3
Numbers3
Strings3
Browser2
Closures2
Destructuring2
Error Handling2
Event Loop2
Modules2
this2
API1
Array1
Arrays1
Classes1
Hoisting1
Object1

Want long-form answers? Read 165 JavaScript interview questions with answers — same topic, in more depth.

Beginner level — 23 JavaScript questions

Question 1Coercion

What is the core difference between `==` and `===` in JavaScript?

A.== compares by reference, === compares by value
B.There is no difference, they are just two ways to write the same operation
C.== compares with type coercion, === compares both value and type
D.=== is faster, so it always yields a different result than ==

Answer: C

References

MDN — Equality comparisons and sameness

Question 2Array Methods

What is the difference between `Array.prototype.map()` and `Array.prototype.forEach()`?

A.map() cannot access the index, but forEach() can
B.Both return a new array — there is no difference
C.forEach() mutates the original array in place, while map() does not
D.map() returns a new array of the transformed values

Answer: D

References

MDN — Array.prototype.map()

Question 3Coercion

Which of the following values is NOT falsy in JavaScript?

A.0 (zero)
B.[] (an empty array)
C."" (the empty string)
D.NaN (not a number)

Answer: B

References

MDN — Falsy

Question 4Scope

What happens when you reassign a value to a variable declared with `const`?

A.The variable takes the new value normally; const is only a naming convention
B.It only errors if the new value has a different type from the old one
C.The program throws a TypeError at runtime — const does not allow reassignment
D.The assignment is silently ignored and the variable keeps its old value

Answer: C

References

MDN — const

Question 5Strings

How do template literals (backtick strings) differ from regular quoted strings?

A.They allow embedded expressions with ${...} and multi-line strings written directly
B.${...} works in every kind of string, including single and double quotes
C.It is just another way of writing single quotes, with no extra features
D.Template literals run faster, so they should always replace normal quoted strings

Answer: A

References

MDN — Template literals

Question 6Array Methods

What does `Array.prototype.filter()` do to the original array, and what does it return?

A.It returns true/false indicating whether any element matches
B.It returns a NEW array of the elements for which the callback returned truthy; the original array is unchanged
C.It returns the FIRST element matching the condition, or undefined if there is none
D.It deletes the failing elements from the original array in place

Answer: B

References

MDN — Array.prototype.filter()

Question 7Types

What is the difference between `undefined` and `null` in JavaScript?

A.undefined only appears when the code throws an error
B.undefined is the default value when a variable or property has not been assigned
C.null means the variable has not been declared in the program
D.They are identical and interchangeable everywhere

Answer: B

References

Question 8Destructuring

Destructuring with defaults and rest — what does the following code print?

javascript

const [a, b = 5, ...rest] = [1]
console.log(a, b, rest)
const { p: renamed, q = 9 } = { p: 7 }
console.log(renamed, q)

A.1 undefined [] 7 9
B.1 5 [] undefined 9
C.1 5 undefined 7 9
D.1 5 [] 7 9

Answer: D

References

MDN — Destructuring assignment

Question 9Operators

When does `a ?? b` (nullish coalescing) give a different result from `a || b`?

A.?? falls back on every falsy value, while || only falls back on null/undefined
B.When a is null or undefined
C.When a is falsy but NOT null/undefined
D.They always give the same result, only the spelling differs

Answer: C

References

MDN — Nullish coalescing operator (??)

Question 10Coercion

The `+` and `-` operators on numeric strings — what does the following code print?

console.log('5' + 2)
console.log('5' - 2)

A.52 3
B.52 52
C.7 7
D.7 3

Answer: A

References

MDN — Addition (+)

Question 11Coercion

`parseInt` on mixed strings — what does the following code print?

console.log(parseInt('08'))
console.log(parseInt('12px'))
console.log(parseInt('px12'))

A.8 NaN NaN
B.8 12 NaN
C.8 12 0
D.0 12 NaN

Answer: B

References

MDN — parseInt()

Question 12Types

Nested `typeof` — what does the following code print?

console.log(typeof typeof 1)

A.string
B.number
C.undefined
D.object

Answer: A

References

MDN — typeof

Question 13Arrays

Assigning to an array `length` — what does the following code print?

javascript

const arr = [1, 2, 3, 4, 5]
arr.length = 2
console.log(arr)
console.log(arr[3])

A.[1, 2] undefined
B.[1, 2, 3, 4, 5] 4
C.[1, 2] 4
D.TypeError

Answer: A

References

MDN — Array.prototype.length

Question 14Types

Why does `typeof null` return `"object"`?

A.Because null inherits from Object.prototype like every other object
B.It is a bug from the first version of JavaScript, kept because fixing it would break existing code
C.Because null is a special empty object the runtime creates up front
D.Because typeof returns "object" for every value that is not a number or a string

Answer: B

References

MDN — typeof

Question 15Array Methods

What does `[10, 9, 100].sort()` return?

javascript

const nums = [10, 9, 100]
console.log(nums.sort())

A.[10, 100, 9], because by default the elements are compared as strings
B.[100, 10, 9], because the default order is descending by magnitude
C.[10, 9, 100], because an array of integers keeps its original order
D.[9, 10, 100], because the method recognises a numeric array and sorts by value

Answer: A

References

MDN — Array.prototype.sort

Question 16Array Methods

What is the difference between `find()` and `filter()`?

A.find returns the first matching element; filter returns an array of all matches
B.find mutates the original array while filter creates an entirely new one
C.find only works with arrays of numbers while filter works with any type
D.find returns the element's index while filter returns the element itself

Answer: A

References

MDN — Array.prototype.find()

Question 17Objects

What does optional chaining `?.` do when the left-hand value is `null` or `undefined`?

A.It returns null to preserve the type of the left-hand value
B.It still throws, but with a clearer message than the usual error
C.It stops and returns undefined instead of throwing
D.It automatically creates an empty object so the expression can continue

Answer: C

References

MDN — Optional chaining (?.)

Question 18Functions

When does the default parameter in `function f(a = 1)` apply?

A.When the argument passed is null or undefined
B.When the argument passed is undefined, or nothing is passed at all
C.When fewer arguments are passed than parameters were declared
D.When the argument passed is any falsy value

Answer: B

References

MDN — Default parameters

Question 19Async

What states can a Promise be in?

A.pending, fulfilled, rejected — and it transitions exactly once
B.open, closed, cancelled — matching the cancellation of a task midway
C.waiting, resolved, failed — with the ability to return to pending
D.pending, running, done — following the lifecycle of the async task

Answer: A

References

MDN — Promise

Question 20Browser

How long does data in `sessionStorage` persist?

A.Until the user logs out of their account on the site
B.Forever, until code explicitly calls a delete
C.Until the expiry date set when the data was written
D.Until the open tab is closed

Answer: D

References

MDN — Window.sessionStorage

Question 21Strings

After `let s = "abc"; s.toUpperCase()`, what is the value of `s`?

A.undefined, because the result was not assigned to any variable
B."ABC", because the method transforms the string held by the variable directly
C.A runtime error, because a method was called without using its return value
D."abc", because strings are immutable so the method returns a new string

Answer: D

References

MDN — String (immutability)

Question 22Numbers

What type does `(1.005).toFixed(2)` return?

A.A string
B.An object representing the number formatted for the locale
C.An integer, because the decimals were dropped during formatting
D.A number rounded to two decimal places

Answer: A

References

MDN — Number.prototype.toFixed()

Question 23Scope

Before ES6, what was an IIFE mainly used for?

A.Creating a private scope so variables did not leak into the global one
B.Speeding up execution by hinting to the engine that it should optimise that code
C.Allowing await at the top level of a source file
D.Guaranteeing the enclosed code runs exactly once

Answer: A

References

MDN — IIFE

Intermediate level — 52 JavaScript questions

Question 1Closures

What is a closure in JavaScript?

A.A function that can still access variables from the scope where it was defined
B.An object frozen using Object.freeze()
C.A way to call a function immediately after declaration (IIFE)
D.A function that takes no parameters

Answer: A

References

MDN — Closures

Question 2Event Loop

Microtask vs `setTimeout` — what is the output order of the following code?

console.log('A')
setTimeout(() => console.log('B'), 0)
Promise.resolve().then(() => console.log('C'))
console.log('D')

A.A B C D
B.A D B C
C.A D C B
D.A C D B

Answer: C

References

MDN — The event loop

Question 3this

How is the value of `this` determined inside an arrow function?

A.this always points to the object that called the function, like a normal function
B.You can change an arrow function's this with .bind()
C.No this of its own — it inherits this from the enclosing lexical scope
D.this is always window/globalThis inside an arrow function

Answer: C

References

MDN — Arrow function expressions

Question 4Hoisting

Hoisting with `var` — what does the following code print?

console.log(x)
var x = 5
console.log(x)

A.null 5
B.5 5
C.undefined 5
D.ReferenceError

Answer: C

References

MDN — Hoisting

Question 5Scope· select all that apply

Which of the following are correct differences between `var`, `let`, and `const`? (select all that apply)

A.var cannot be redeclared with the same name in the same scope
B.var is function-scoped, while let/const are block-scoped
C.let and const sit in the Temporal Dead Zone until they are declared
D.const prevents reassigning the variable, but the object/array it points to can still be mutated

Answer: B, C, D

References

MDN — let

Question 6Closures

Closures in a `var` loop — what does the following code print?

const fns = []
for (var i = 0; i < 3; i++) {
  fns.push(() => i)
}
console.log(fns[0](), fns[1](), fns[2]())

A.3 3 3
B.0 1 2
C.undefined undefined undefined
D.0 0 0

Answer: A

References

MDN — Closures: Creating closures in loops

Question 7Async

What is the main difference between `async/await` and Promise `.then()` chaining?

A.async/await cannot catch errors, so you must always use .catch()
B.It is a syntax layer over Promises, letting you write asynchronous code in a sequential flow
C.async/await runs synchronously and blocks the main thread until it finishes
D.Only async/await can run several tasks in parallel

Answer: B

References

MDN — async function

Question 8Coercion

`typeof null`, `NaN` and float equality — what does the following code print?

console.log(typeof null)
console.log(typeof NaN)
console.log(0.1 + 0.2 === 0.3)

A.null NaN true
B.object NaN false
C.object number true
D.object number false

Answer: D

References

MDN — typeof

Question 9Coercion

The `+` operator on two arrays — what does the following code print?

console.log([1, 2, 3] + [4, 5, 6])

A.1,2,3,4,5,6
B.[1,2,3,4,5,6]
C.1,2,34,5,6
D.NaN

Answer: C

References

MDN — Addition (+)

Question 10Objects· select all that apply

Regarding shallow copy vs deep copy of an object, select all the correct statements. (select all that apply)

A.A shallow copy only copies the top level; nested properties (child objects/arrays) still share references with the original
B.JSON.parse(JSON.stringify(obj)) is a safe deep copy for every data type
C.structuredClone(obj) is the modern standard deep copy, handling Date/Map/Set and circular references
D.Spread {...obj} and Object.assign({}, obj) both make a shallow copy, not a deep copy

Answer: A, C, D

References

MDN — structuredClone()

Question 11Array Methods

What is `Array.prototype.reduce()` used for, and what are its callback parameters?

A.Transforms each element and returns a new array of the same length
B.Sorts the array in place using a comparator
C.Drops the elements that fail a condition and returns a shorter array
D.Folds the array down to a single value

Answer: D

References

MDN — Array.prototype.reduce()

Question 12Functions· select all that apply

Regarding the `...` operator as spread and rest, select all the correct statements. (select all that apply)

A.Object spread and array spread use the same mechanism: both iterate via Symbol.iterator
B.Object spread { ...obj } only makes a shallow copy of own enumerable properties (it does not copy the prototype); nested objects/arrays still share references
C.A rest parameter must be the LAST parameter — function f(a, ...rest, b) {} is a SyntaxError
D.Spread in call/array literal expands an iterable; object literal copies own enumerable props; rest gathers leftovers

Answer: B, C, D

References

Question 13Modules

Which statement about ES Modules (`import`/`export`) is correct?

A.import can appear anywhere and runs dynamically at runtime like an ordinary function call
B.A file may have at most one export statement
C.An imported binding is a "live binding"
D.Default and named exports differ only in syntax; you can import a default with braces { }

Answer: C

References

MDN — JavaScript modules

Question 14Events

What is event delegation and what mechanism does it rely on?

A.Attach ONE listener on a parent element and rely on event bubbling to catch events from its children
B.Attach a separate listener to every child so each one handles its own events independently
C.event.target and event.currentTarget always point to the same element
D.Delegation relies on event capturing (the downward phase), which is the browser default

Answer: A

References

MDN — Event bubbling and capture

Question 15Objects· select all that apply

Regarding `Map`/`Set` versus objects/arrays, select all the correct statements. (select all that apply)

A.A Map accepts a key of any type, including an object
B.A Set can hold duplicate elements just like an array, differing only in access speed
C.A Map preserves insertion order, has .size, and is directly iterable
D.WeakMap/WeakSet accept only objects and hold weak references

Answer: A, C, D

References

Question 16Coercion

`map(parseInt)` and the index argument — what does this code print?

console.log([1, 2, 3].map(parseInt))

A.[1, 2, 3]
B.[1, 2, NaN]
C.[NaN, NaN, NaN]
D.[1, NaN, NaN]

Answer: D

References

MDN — parseInt()

Question 17Array Methods

`sort()` without a comparator — what does the following code print?

console.log([10, 1, 2, 21].sort())

A.[1, 10, 2, 21]
B.[10, 1, 2, 21]
C.[21, 10, 2, 1]
D.[1, 2, 10, 21]

Answer: A

References

MDN — Array.prototype.sort()

Question 18Functions

`typeof arguments` in a normal function — what does the following code print?

console.log(
  (function () {
    return typeof arguments
  })(),
)

A.object
B.arguments
C.array
D.undefined

Answer: A

References

MDN — The arguments object

Question 19Operators

Chained `>` comparisons — what does the following code print?

console.log(3 > 2 > 1)

A.false
B.1
C.true
D.ReferenceError

Answer: A

References

MDN — Greater than (>)

Question 20Numbers

`Math.max()` and `Math.min()` with no arguments — what does the following code print?

console.log(Math.max())
console.log(Math.min())

A.NaN NaN
B.Infinity -Infinity
C.-Infinity Infinity
D.undefined undefined

Answer: C

References

MDN — Math.max()

Question 21Coercion

Unary `+` coercing arrays and objects — what does the following code print?

console.log(+[])
console.log(+[1])
console.log(+[1, 2])
console.log(+{})

A.NaN NaN NaN NaN
B.0 1 3 0
C.0 1 NaN NaN
D.0 1 2 NaN

Answer: C

References

MDN — Unary plus (+)

Question 22Destructuring

Default parameters with `undefined` vs `null` — what does the following code print?

javascript

function pick({ a = 1, b = 2 } = {}) {
  return [a, b]
}
console.log(pick({ a: undefined, b: null }))
console.log(pick())

A.[1, null] TypeError
B.[undefined, null] [1, 2]
C.[1, 2] [1, 2]
D.[1, null] [1, 2]

Answer: D

References

MDN — Destructuring assignment (default values)

Question 23Operators

`?.` and `??` vs `||` — what does the following code print?

javascript

const user = { profile: null, age: 0 }
console.log(user.profile?.name)
console.log(user.profile?.name ?? 'guest')
console.log(user.age ?? 18)
console.log(user.age || 18)

A.TypeError guest 0 18
B.null guest 0 18
C.undefined guest 0 18
D.undefined guest 18 18

Answer: C

References

Question 24Objects

Getters and setters in an object literal — what does the following code print?

javascript

const counter = {
  count: 0,
  get double() {
    return this.count * 2
  },
  set double(value) {
    this.count = value / 2
  },
}
counter.double = 10
console.log(counter.count)
console.log(counter.double)
counter.count++
console.log(counter.double)

A.10 20 22
B.5 10 10
C.0 10 12
D.5 10 12

Answer: D

References

MDN — getter

Question 25Types

Why is `NaN === NaN` false, and how do you correctly test for NaN?

A.Because the IEEE-754 standard says NaN is not equal to itself; use Number.isNaN()
B.Because JavaScript creates a separate NaN object each time one is produced
C.Because === cannot compare the special values of the number type
D.Because NaN carries an error trace, so two NaNs have different contents

Answer: A

References

MDN — NaN

Question 26Types

What is a Symbol used for in JavaScript?

A.Creating a unique property key that cannot clash with keys chosen by others
B.Representing special characters that an ordinary string cannot express
C.Marking a property as private so nothing outside can reach it
D.Creating a constant whose value cannot be changed after initialisation

Answer: A

References

MDN — Symbol

Question 27Array Methods

How do `slice()` and `splice()` differ?

A.slice returns the removed elements while splice returns what is left
B.slice is for arrays while splice also works on strings
C.slice returns a new array and leaves the original alone; splice modifies the original in place
D.slice can only cut from the start of the array while splice can cut anywhere

Answer: C

References

MDN — Array.prototype.splice()

Question 28Array Methods

Why does `[NaN].indexOf(NaN)` give `-1` while `[NaN].includes(NaN)` gives `true`?

A.Because indexOf only works correctly with ordinary primitive values
B.Because includes converts the elements to strings before comparing
C.Because the two methods use different comparison algorithms
D.Because includes scans backwards from the end and meets NaN differently

Answer: C

References

MDN — Array.prototype.includes()

Question 29Array Methods

What combination is `flatMap()` equivalent to?

A.map() followed by flat() with infinite depth for every level of nesting
B.map() followed by flat() with a depth of 1
C.flat() followed by map() on the fully flattened array
D.filter() followed by map() to drop empty entries before transforming

Answer: B

References

MDN — Array.prototype.flatMap()

Question 30Objects

After `Object.freeze(obj)`, which operation still works?

A.Assigning a new value to a top-level property
B.Changing a property of an object nested inside obj
C.Deleting an existing property with the delete operator
D.Adding an entirely new property to the frozen object

Answer: B

References

MDN — Object.freeze()

Question 31Objects

How does a getter declared with `get` in an object literal behave?

A.It only has an effect when the object is accessed from within itself
B.It is read like an ordinary property but actually calls a function on every read
C.It is only valid when declared alongside a matching setter
D.It computes the value once at creation and stores it for subsequent reads

Answer: B

References

MDN — getter (get)

Question 32this

Passing `obj.method` as a callback loses `this` inside it. Why?

javascript

const counter = {
  count: 0,
  increment() { this.count++ },
}

setTimeout(counter.increment, 0)

A.Because the function is copied onto another object when passed as an argument
B.Because a callback always runs in strict mode, which strips this
C.Because an object's methods are not inherited through the prototype chain
D.Because this is decided by how the function is called, not by where it is defined

Answer: D

References

MDN — Function.prototype.bind()

Question 33Classes

Why must `super()` be called before using `this` in a subclass constructor?

A.Because it is a style convention that makes the code easier to read, not a requirement
B.Because without it the parent's methods would not be inherited
C.Because the subclass object is only initialised after the parent constructor has run
D.Because the call copies the parent's properties down onto the subclass

Answer: C

References

MDN — super

Question 34Functions

Why can an arrow function not use the `arguments` object?

A.Because it has no binding of its own and takes it from the enclosing function, just like this
B.Because an arrow function accepts at most a single parameter
C.Because an arrow function always runs in strict mode, which forbids that object
D.Because that object was removed entirely from newer versions of JavaScript

Answer: A

References

MDN — Arrow function expressions

Question 35Async

Why does a `try/catch` around `setTimeout` not catch an error thrown inside its callback?

A.Because catching an async error requires a Promise catch
B.Because the timer function silently swallows every error raised inside the callback
C.Because the callback runs on a separate thread, so the error cannot travel back
D.Because the callback runs on a later turn, long after the try block has ended

Answer: D

References

MDN — setTimeout()

Question 36Async

How does `for await (const x of arr)` differ from `arr.forEach(async x => ...)`?

A.The loop runs in parallel while forEach goes through elements one at a time
B.The loop waits for each element to finish before moving on; forEach waits for nothing
C.Both wait sequentially, they only differ in how they are written
D.The loop only works on arrays while forEach works on any iterable

Answer: B

References

MDN — for await...of

Question 37Event Loop

When does `setTimeout(fn, 0)` actually run the function?

A.Immediately, because the delay is set to zero milliseconds
B.Exactly 0 milliseconds after the call, regardless of synchronous code still running
C.After the current synchronous code and every pending microtask have finished
D.Before any pending Promises, because the timer queue has higher priority

Answer: C

References

MDN — setTimeout(): reasons for delays

Question 38Async

What is `AbortController` used for?

A.Automatically retrying a failed request according to a preset policy
B.Limiting how many requests may be sent concurrently to the same server
C.Cancelling an in-flight request or asynchronous task
D.Catching errors raised in a Promise so the program does not stop

Answer: C

References

MDN — AbortController

Question 39Error Handling

What does the function below return?

javascript

function f() {
  try {
    return 1
  } finally {
    return 2
  }
}

console.log(f())

A.A syntax error, because return is not allowed inside a finally block
B.2, because the return in finally overrides the value from try
C.undefined, because the two return statements conflict so nothing is returned
D.1, because the return in try executes first and therefore wins

Answer: B

References

MDN — try...catch

Question 40Error Handling

Why define a custom error class extending `Error` instead of throwing a plain object?

A.So try/catch can catch it, since a plain object is not caught by a catch block
B.To preserve the stack trace and classify errors with instanceof
C.So the error is automatically written to the monitoring system's log
D.So the error automatically becomes an HTTP response with a matching status code

Answer: B

References

MDN — Error

Question 41Modules

How does the function-form `import()` differ from a top-level `import`?

A.It only works with CommonJS modules, not with ES Modules
B.It loads the module faster because it skips dependency resolution
C.It loads the module at runtime and returns a Promise, so it can be called conditionally
D.It loads the module into global scope rather than the calling file's scope

Answer: C

References

MDN — import() (dynamic import)

Question 42Events

In what order does a DOM event travel once dispatched?

A.From the root down to the target element, then bubbling back up to the root
B.From the target element straight up to the root, with no downward phase
C.From the root down to the target and then stops, with no return trip
D.Only at the target element itself, without reaching any other element

Answer: A

References

MDN — Event.eventPhase

Question 43Events

What is the difference between `preventDefault()` and `stopPropagation()`?

A.One cancels the event entirely, the other merely defers the handling
B.One stops the event in the bubble phase, the other in the capture phase
C.One blocks the default behaviour, the other blocks the event from travelling further
D.One is for mouse events, the other for keyboard events

Answer: C

References

MDN — Event.stopPropagation()

Question 44Browser

Why does `DOMContentLoaded` fire earlier than `load`?

A.Because it fires as soon as the server returns the document's first byte
B.Because it only waits for the document head, not for the body to be built
C.Because it only waits for the HTML to be parsed, not for images and sub-resources
D.Because it skips script tags, so it need not wait for code to load and run

Answer: C

References

MDN — DOMContentLoaded event

Question 45Objects

What limitation of `JSON.parse(JSON.stringify(obj))` does `structuredClone()` fix?

A.It preserves Date, Map, Set and handles circular references
B.It preserves the prototype chain, so the copy is of the same class as the original
C.It can also copy the source object's functions and methods
D.It copies many times faster by skipping the string conversion step

Answer: A

References

MDN — structuredClone()

Question 46Strings

What does `"a-b-c".replace("-", "+")` produce?

A."a+b+c", because the method replaces every occurrence by default
B."a+b-c", because a string pattern only replaces the first occurrence
C."a-b-c", because strings are immutable so the method can change nothing
D.An error, because the first argument must be a valid regular expression

Answer: B

"a+b-c". When the first argument is a string, replace only replaces the first occurrence. To replace all, use replaceAll("-", "+") or a regex with the global flag: replace(/-/g, "+").

References

MDN — String.prototype.replace()

Question 47Numbers

What do `parseInt("12px")` and `Number("12px")` return?

A.12 and 12, because both ignore the non-numeric suffix
B.NaN and NaN, because the string contains letters and cannot be converted
C.NaN and 12, because the type-conversion function is more lenient than the parser
D.12 and NaN

Answer: D

References

MDN — parseInt()

Question 48Scope

What is the temporal dead zone for `let` and `const`?

A.The stretch from the top of the block to the declaration line, where access throws
B.The stretch after a function ends during which its variables are not yet reclaimed
C.The region where a variable is hidden by one of the same name in an inner block
D.The region where a variable has left scope but a closure still references it

Answer: A

References

MDN — let: temporal dead zone

Question 49Operators

When does `a ??= b` perform the assignment?

A.When a holds any falsy value, including 0 and the empty string
B.When a has never been declared in any scope
C.Always — the two differ only in how they handle a nullish b
D.When a is null or undefined

Answer: D

References

MDN — Nullish coalescing assignment (??=)

Question 50API

Formatting currency for Vietnam — which approach?

A.A separate library for formatting dates and numbers
B.Multiplying, dividing and rounding by hand before display
C.The language's built-in locale-aware formatting API
D.Inserting grouping dots yourself with a regular expression

Answer: C

References

MDN — Intl

Question 51Object

Uppercasing every value of an object while keeping keys — concise way?

A.Iterate with a loop and assign into an empty object
B.Convert to a JSON string and replace with a regular expression
C.Call a map method directly on the object
D.Convert to an array of pairs, transform, then rebuild the object

Answer: D

Convert to an array of key-value pairs, transform, then rebuild the object. The two conversion functions let you use any array method in between instead of writing a manual loop.

References

MDN — Object.fromEntries

Question 52Array

Finding a matching element from the end of an array — what do you use?

A.The search method that scans backwards from the end of the array
B.Reverse the array and use the ordinary find method
C.Filter everything and take the last element of the result
D.Use a loop counting down from the last index

Answer: A

Use the search method that iterates from the end of the array. It stops at the first matching element from the end, so you don't need to reverse the array or create any intermediate copy.

References

MDN — Array.prototype.findLast

25 more Advanced questions for JavaScript

Advanced goes deep on real-world scenarios and the technical pitfalls candidates actually run into. Included with Pro.

Upgrade to Pro

Beginner level — 23 JavaScript questions

What is the core difference between == and === in JavaScript?

What is the difference between Array.prototype.map() and Array.prototype.forEach()?

Which of the following values is NOT falsy in JavaScript?

What happens when you reassign a value to a variable declared with const?

How do template literals (backtick strings) differ from regular quoted strings?

What does Array.prototype.filter() do to the original array, and what does it return?

What is the difference between undefined and null in JavaScript?

Destructuring with defaults and rest — what does the following code print?

When does a ?? b (nullish coalescing) give a different result from a || b?

The + and - operators on numeric strings — what does the following code print?

parseInt on mixed strings — what does the following code print?

Nested typeof — what does the following code print?

Assigning to an array length — what does the following code print?

Why does typeof null return "object"?

What does [10, 9, 100].sort() return?

What is the difference between find() and filter()?

What does optional chaining ?. do when the left-hand value is null or undefined?

When does the default parameter in function f(a = 1) apply?

What states can a Promise be in?

How long does data in sessionStorage persist?

After let s = "abc"; s.toUpperCase(), what is the value of s?

What type does (1.005).toFixed(2) return?

Before ES6, what was an IIFE mainly used for?

Intermediate level — 52 JavaScript questions

What is a closure in JavaScript?

Microtask vs setTimeout — what is the output order of the following code?

How is the value of this determined inside an arrow function?

Hoisting with var — what does the following code print?

Which of the following are correct differences between var, let, and const? (select all that apply)

Closures in a var loop — what does the following code print?

What is the main difference between async/await and Promise .then() chaining?

typeof null, NaN and float equality — what does the following code print?

The + operator on two arrays — what does the following code print?

Regarding shallow copy vs deep copy of an object, select all the correct statements. (select all that apply)

What is Array.prototype.reduce() used for, and what are its callback parameters?

Regarding the ... operator as spread and rest, select all the correct statements. (select all that apply)

Which statement about ES Modules (import/export) is correct?

What is event delegation and what mechanism does it rely on?

Regarding Map/Set versus objects/arrays, select all the correct statements. (select all that apply)

map(parseInt) and the index argument — what does this code print?

sort() without a comparator — what does the following code print?

typeof arguments in a normal function — what does the following code print?

Chained > comparisons — what does the following code print?

Math.max() and Math.min() with no arguments — what does the following code print?

Unary + coercing arrays and objects — what does the following code print?

Default parameters with undefined vs null — what does the following code print?

?. and ?? vs || — what does the following code print?

Getters and setters in an object literal — what does the following code print?

Why is NaN === NaN false, and how do you correctly test for NaN?

What is a Symbol used for in JavaScript?

How do slice() and splice() differ?

Why does [NaN].indexOf(NaN) give -1 while [NaN].includes(NaN) gives true?

What combination is flatMap() equivalent to?

After Object.freeze(obj), which operation still works?

How does a getter declared with get in an object literal behave?

Passing obj.method as a callback loses this inside it. Why?

Why must super() be called before using this in a subclass constructor?

Why can an arrow function not use the arguments object?

Why does a try/catch around setTimeout not catch an error thrown inside its callback?

How does for await (const x of arr) differ from arr.forEach(async x => ...)?

When does setTimeout(fn, 0) actually run the function?

What is AbortController used for?

What does the function below return?

Why define a custom error class extending Error instead of throwing a plain object?

How does the function-form import() differ from a top-level import?

In what order does a DOM event travel once dispatched?

What is the difference between preventDefault() and stopPropagation()?

Why does DOMContentLoaded fire earlier than load?

What limitation of JSON.parse(JSON.stringify(obj)) does structuredClone() fix?

What does "a-b-c".replace("-", "+") produce?

What do parseInt("12px") and Number("12px") return?

What is the temporal dead zone for let and const?

When does a ??= b perform the assignment?

Formatting currency for Vietnam — which approach?

Uppercasing every value of an object while keeping keys — concise way?

Finding a matching element from the end of an array — what do you use?

25 more Advanced questions for JavaScript

Beginner level — 23 JavaScript questions

What is the core difference between == and === in JavaScript?

What is the core difference between `==` and `===` in JavaScript?

What is the difference between `Array.prototype.map()` and `Array.prototype.forEach()`?

What happens when you reassign a value to a variable declared with `const`?

What does `Array.prototype.filter()` do to the original array, and what does it return?

What is the difference between `undefined` and `null` in JavaScript?

When does `a ?? b` (nullish coalescing) give a different result from `a || b`?

The `+` and `-` operators on numeric strings — what does the following code print?

`parseInt` on mixed strings — what does the following code print?

Nested `typeof` — what does the following code print?

Assigning to an array `length` — what does the following code print?

Why does `typeof null` return `"object"`?

What does `[10, 9, 100].sort()` return?

What is the difference between `find()` and `filter()`?

What does optional chaining `?.` do when the left-hand value is `null` or `undefined`?

When does the default parameter in `function f(a = 1)` apply?

How long does data in `sessionStorage` persist?

After `let s = "abc"; s.toUpperCase()`, what is the value of `s`?

What type does `(1.005).toFixed(2)` return?

Microtask vs `setTimeout` — what is the output order of the following code?

How is the value of `this` determined inside an arrow function?

Hoisting with `var` — what does the following code print?

Which of the following are correct differences between `var`, `let`, and `const`? (select all that apply)

Closures in a `var` loop — what does the following code print?

What is the main difference between `async/await` and Promise `.then()` chaining?

`typeof null`, `NaN` and float equality — what does the following code print?

The `+` operator on two arrays — what does the following code print?

What is `Array.prototype.reduce()` used for, and what are its callback parameters?

Regarding the `...` operator as spread and rest, select all the correct statements. (select all that apply)

Which statement about ES Modules (`import`/`export`) is correct?

Regarding `Map`/`Set` versus objects/arrays, select all the correct statements. (select all that apply)

`map(parseInt)` and the index argument — what does this code print?

`sort()` without a comparator — what does the following code print?

`typeof arguments` in a normal function — what does the following code print?

Chained `>` comparisons — what does the following code print?

`Math.max()` and `Math.min()` with no arguments — what does the following code print?

Unary `+` coercing arrays and objects — what does the following code print?

Default parameters with `undefined` vs `null` — what does the following code print?

`?.` and `??` vs `||` — what does the following code print?

Why is `NaN === NaN` false, and how do you correctly test for NaN?

How do `slice()` and `splice()` differ?

Why does `[NaN].indexOf(NaN)` give `-1` while `[NaN].includes(NaN)` gives `true`?

What combination is `flatMap()` equivalent to?

After `Object.freeze(obj)`, which operation still works?

How does a getter declared with `get` in an object literal behave?

Passing `obj.method` as a callback loses `this` inside it. Why?

Why must `super()` be called before using `this` in a subclass constructor?

Why can an arrow function not use the `arguments` object?

Why does a `try/catch` around `setTimeout` not catch an error thrown inside its callback?

How does `for await (const x of arr)` differ from `arr.forEach(async x => ...)`?

When does `setTimeout(fn, 0)` actually run the function?

What is `AbortController` used for?

Why define a custom error class extending `Error` instead of throwing a plain object?

How does the function-form `import()` differ from a top-level `import`?

What is the difference between `preventDefault()` and `stopPropagation()`?

Why does `DOMContentLoaded` fire earlier than `load`?

What limitation of `JSON.parse(JSON.stringify(obj))` does `structuredClone()` fix?

What does `"a-b-c".replace("-", "+")` produce?

What do `parseInt("12px")` and `Number("12px")` return?

What is the temporal dead zone for `let` and `const`?

When does `a ??= b` perform the assignment?

What is the core difference between `==` and `===` in JavaScript?

What is the difference between `Array.prototype.map()` and `Array.prototype.forEach()`?

What happens when you reassign a value to a variable declared with `const`?

What does `Array.prototype.filter()` do to the original array, and what does it return?

What is the difference between `undefined` and `null` in JavaScript?

When does `a ?? b` (nullish coalescing) give a different result from `a || b`?

The `+` and `-` operators on numeric strings — what does the following code print?

`parseInt` on mixed strings — what does the following code print?

Nested `typeof` — what does the following code print?

Assigning to an array `length` — what does the following code print?

Why does `typeof null` return `"object"`?

What does `[10, 9, 100].sort()` return?

What is the difference between `find()` and `filter()`?

What does optional chaining `?.` do when the left-hand value is `null` or `undefined`?

When does the default parameter in `function f(a = 1)` apply?

How long does data in `sessionStorage` persist?

After `let s = "abc"; s.toUpperCase()`, what is the value of `s`?

What type does `(1.005).toFixed(2)` return?

Microtask vs `setTimeout` — what is the output order of the following code?

How is the value of `this` determined inside an arrow function?