Junior JavaScript Interview Questions (50 MCQs)

Question 1

What is the main difference between `let`/`const` and `var`?

Accepted Answer

`let`/`const` are block-scoped; `var` is function-scoped

Answer

`var` only works in strict mode

Answer

`const` makes objects deeply immutable

Answer

They have no scoping difference

Question 2

Which of the following is NOT a primitive?

Accepted Answer

`array`

Answer

`string`

Answer

`symbol`

Answer

`bigint`

Question 3

What are the results of `0 == false` and `0 === false`?

Accepted Answer

`true` and `false`

Answer

`true` and `true`

Answer

`false` and `true`

Answer

`false` and `false`

Question 4

Which method returns a new array by transforming each item?

Accepted Answer

`map`

Answer

`forEach`

Answer

`reduceRight` always

Answer

`push`

Question 5

Which character delimits a template literal?

Accepted Answer

Backtick `` ` ``

Answer

Single quote `'`

Answer

Double quote `"`

Answer

Slash `/`

Question 6

Which value is truthy in a condition?

Accepted Answer

`[]`

Answer

`0`

Answer

`""`

Answer

`null`

Question 7

What does `typeof NaN` return?

Accepted Answer

"number"

Answer

"NaN"

Answer

"undefined"

Answer

"object"

Question 8

What is the most reliable way to detect an array?

Accepted Answer

`Array.isArray(x)`

Answer

`typeof x === "array"`

Answer

`x.length > 0`

Answer

`typeof x === "object"`

Question 9

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

Accepted Answer

`undefined` means not assigned; `null` is an intentional empty value

Answer

They are identical and `===` equal

Answer

`null` only exists in TypeScript

Answer

`undefined` is an object

Question 10

What is `"5" + 3`?

Accepted Answer

`"53"`

Answer

`8`

Answer

`NaN`

Answer

TypeError

Question 11

What is the difference between `push` and `concat`?

Accepted Answer

`push` mutates the array; `concat` returns a new array

Answer

Both mutate the original array

Answer

`concat` only accepts strings

Answer

`push` returns a new array

Question 12

If `user` is `null`, what does `user?.name` return?

Accepted Answer

`undefined`

Answer

TypeError

Answer

`null`

Answer

`""`

Question 13

What does `typeof null` return?

Accepted Answer

`'object'`

Answer

`'null'`

Answer

`'undefined'`

Answer

`'number'`

Question 14

What is the result of `0.1 + 0.2 === 0.3`?

Accepted Answer

`false`

Answer

`true`

Answer

`NaN`

Answer

A `TypeError`

Question 15

What is the result of `'5' - 2`?

Accepted Answer

`3`

Answer

`'52'`

Answer

`'3'`

Answer

`NaN`

Question 16

What does `[1, 2, 3].forEach(x => x * 2)` return?

Accepted Answer

`undefined`

Answer

`[2, 4, 6]`

Answer

`[1, 2, 3]`

Answer

`6`

Question 17

What does `parseInt('42px')` return?

Accepted Answer

`42`

Answer

`NaN`

Answer

`'42'`

Answer

`0`

Question 18

What happens after `const user = { name: 'Ali' }; user.name = 'Sara';`?

Accepted Answer

`user.name` becomes `'Sara'`

Answer

A `TypeError` is thrown

Answer

The change is silently ignored

Answer

A new object is created

Question 19

What is the result of `NaN === NaN`?

Accepted Answer

`false`

Answer

`true`

Answer

`NaN`

Answer

`undefined`

Question 20

After `const arr = [1, 2, 3, 4]; arr.length = 2;`, what is `arr`?

Accepted Answer

`[1, 2]`

Answer

`[1, 2, 3, 4]`

Answer

`[3, 4]`

Answer

A `TypeError`

Question 21

Which is true about `slice` and `splice`?

Accepted Answer

`slice` doesn't mutate the array, `splice` does

Answer

Both mutate the original array

Answer

`splice` doesn't mutate, `slice` does

Answer

They only work on strings

Question 22

What does `typeof function () {}` return?

Accepted Answer

`'function'`

Answer

`'object'`

Answer

`'undefined'`

Answer

`'callable'`

Question 23

After `let s = 'cat'; s[0] = 'b';`, what is `s`?

Accepted Answer

`'cat'`

Answer

`'bat'`

Answer

`'b'`

Answer

`undefined`

Question 24

Given `function f(x = 10) { return x; }`, what are `f(undefined)` and `f(null)`?

Accepted Answer

`10` and `null`

Answer

`10` and `10`

Answer

`undefined` and `null`

Answer

`undefined` and `10`

Question 25

What does `const a = [1]; const b = a; b.push(2); console.log(a.length);` print?

Accepted Answer

`2`

Answer

`1`

Answer

`0`

Answer

It throws an error

Question 26

What does `Array.from({ length: 3 }, (_, i) => i * 2)` return?

Accepted Answer

`[0, 2, 4]`

Answer

`[2, 4, 6]`

Answer

`[undefined, undefined, undefined]`

Answer

`[3]`

Question 27

What are `[NaN].includes(NaN)` and `[NaN].indexOf(NaN)`?

Accepted Answer

`true` and `-1`

Answer

`true` and `0`

Answer

`false` and `-1`

Answer

`false` and `0`

Question 28

What does `JSON.stringify({ a: undefined, b: 1 })` return?

Accepted Answer

`'{"b":1}'`

Answer

`'{"a":undefined,"b":1}'`

Answer

`'{"a":null,"b":1}'`

Answer

A `TypeError`

Question 29

What does `setTimeout(() => console.log('A'), 0); console.log('B');` print?

Accepted Answer

`B` then `A`

Answer

`A` then `B`

Answer

Only `B`

Answer

The order is random

Question 30

What does `document.querySelector` return if nothing matches?

Accepted Answer

`null`

Answer

An empty `NodeList`

Answer

`undefined`

Answer

It throws an error

Question 31

What is the main advantage of `addEventListener` over `element.onclick = fn`?

Accepted Answer

You can register multiple handlers for the same event

Answer

It runs faster

Answer

It's the only way to catch click events

Answer

It removes the handler automatically after running

Question 32

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

Accepted Answer

The first cancels the browser's default action, the second stops event propagation

Answer

They do exactly the same thing

Answer

The first stops propagation, the second cancels the default action

Answer

Both remove the handler from the element

Question 33

After `localStorage.setItem('n', 5)`, what is `typeof localStorage.getItem('n')`?

Accepted Answer

`'string'`

Answer

`'number'`

Answer

`'object'`

Answer

`'undefined'`

Question 34

What happens when running `[].reduce((a, b) => a + b)`?

Accepted Answer

It throws a `TypeError`

Answer

It returns `0`

Answer

It returns `undefined`

Answer

It returns `NaN`

Question 35

What does `Number('')` return?

Accepted Answer

`0`

Answer

`NaN`

Answer

`undefined`

Answer

`''`

Question 36

What does `Math.max()` return with no arguments?

Accepted Answer

`-Infinity`

Answer

`0`

Answer

`Infinity`

Answer

`undefined`

Question 37

In `function f(...args) {}`, what is `args`?

Accepted Answer

A real `Array`

Answer

An array-like object like `arguments`

Answer

A `Set`

Answer

Only the first argument

Question 38

What does `for (const x of ['a', 'b']) console.log(x)` print, compared to `for...in`?

Accepted Answer

`for...of` gives values (`a`, `b`); `for...in` gives keys (`0`, `1`)

Answer

Both give values

Answer

`for...of` gives keys; `for...in` gives values

Answer

`for...of` doesn't work on arrays

Question 39

What does `switch (1) { case '1': console.log('A'); break; default: console.log('B'); }` print?

Accepted Answer

`B`

Answer

`A`

Answer

`A` and `B`

Answer

Nothing

Question 40

What does `const o = {}; o[1] = 'a'; console.log(o['1']);` print?

Accepted Answer

`'a'`

Answer

`undefined`

Answer

`null`

Answer

It throws an error

Question 41

What does `[10, 1, 2].sort()` return?

Accepted Answer

`[1, 10, 2]`

Answer

`[1, 2, 10]`

Answer

`[10, 2, 1]`

Answer

`[2, 1, 10]`

Question 42

If `foo` was never declared, what does `typeof foo` do?

Accepted Answer

Returns `'undefined'`

Answer

Throws a `ReferenceError`

Answer

Returns `'null'`

Answer

Returns `'object'`

Question 43

What is the difference between `DOMContentLoaded` and `load`?

Accepted Answer

`DOMContentLoaded` fires after HTML is parsed; `load` after all resources like images load

Answer

They always fire at the same time

Answer

`load` always fires before `DOMContentLoaded`

Answer

`DOMContentLoaded` is only for images

Question 44

What does the `defer` attribute on `<script>` do?

Accepted Answer

Downloads in parallel and runs in order after HTML parsing finishes

Answer

Runs immediately and blocks parsing

Answer

Never runs unless called manually

Answer

Runs after `load` in random order

Question 45

Why is `textContent` better than `innerHTML` for displaying user input?

Accepted Answer

It doesn't parse HTML, preventing XSS

Answer

It translates the text automatically

Answer

It's slower but prettier

Answer

It only works in old browsers

Question 46

What happens with `function f() { try { return 1; } finally { console.log('F'); } } f();`?

Accepted Answer

`F` is logged and the function returns `1`

Answer

Nothing is logged because `return` comes first

Answer

A syntax error occurs

Answer

`F` is logged and it returns `undefined`

Question 47

What does `0 && 'hello'` evaluate to?

Accepted Answer

`0`

Answer

`'hello'`

Answer

`false`

Answer

`true`

Question 48

What is `{ a: 1 } === { a: 1 }`?

Accepted Answer

`false`

Answer

`true`

Answer

A syntax error

Answer

`undefined`

Question 49

What is `'  hi  '.trim().length`?

Accepted Answer

`2`

Answer

`6`

Answer

`4`

Answer

`0`

Question 50

What does `const f = () => { a: 1 }; f();` return?

Accepted Answer

`undefined`

Answer

`{ a: 1 }`

Answer

`1`

Answer

A syntax error

JavaScript · Junior · 50

What is the main difference between `let`/`const` and `var`?