JavaScript Basics Quiz

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100% acceptance

A beginner-friendly quiz to evaluate your understanding of core JavaScript fundamentals including variables, data types, operators, and foundational syntax. This quiz builds the base for mastering more advanced JavaScript concepts.

40 Questions

~80 minutes

Question 1

What is the output of the following code?

javascript

console.log(typeof undefined)

undefined

object

string

null

Question 2

Which of the following declares a block-scoped variable?

var

let

function

define

Question 3

What will the following expression return?

javascript

"5" === 5

true

false

undefined

NaN

Question 4

What is the output of the following code?

javascript

console.log(2 + "3")

"5"

NaN

Question 5

Which statement best describes JavaScript?

A server-side language only

A compiled, strongly typed language

A dynamically typed, interpreted language

A markup language used for styling

Question 6

Which keyword declares a constant variable that cannot be reassigned?

let

const

var

static

Question 7

How do you write a template literal that outputs “Hello, John!” when name is “John”?

javascript

const name = "John"
// Fill the blank
const message = `Hello, ${_____}!`

name

"name"

${name}

Hello

Question 8

What is the default value of a variable declared with let but not assigned?

null

undefined

false

Question 9

Which array method creates a new array with results of calling a provided function on every element?

forEach()

map()

filter()

reduce()

Question 10

What will be logged?

javascript

const obj = { a: 1 }
const copy = obj
copy.a = 2
console.log(obj.a)

undefined

Throws an error

Question 11

Which of these values is falsy in JavaScript?

"0"

[]

{}

Question 12

Which statement properly checks if `person.age` exists before accessing it?

if (person.age)

if (person?.age)

if (person.age?)

if (person => age)

Question 13

What will `Array.isArray([])` return?

true

false

null

undefined

Question 14

How do you convert the string "42" to a number using shorthand syntax?

Number("42")

parseInt("42")

+ "42"

"42".toNumber()

Question 15

Which keyword is used to create a block scope?

for

let

switch

Question 16

What is the output of the following arrow function?

javascript

const add = (a, b) => a + b
console.log(add(2, 3))

undefined

NaN

Error

Question 17

Which method converts a JSON string into an object?

JSON.stringify()

JSON.parse()

JSON.object()

JSON.convert()

Question 18

What does the spread operator do in the following context?

javascript

const arr = [1, 2, 3]
const copy = [...arr]

Creates a shallow copy of arr

Creates a deep copy of arr

Converts arr to an object

Sorts arr automatically

Question 19

What will be logged?

javascript

console.log(1 == "1")

true

false

Throws TypeError

undefined

Question 20

What is the result of `typeof NaN`?

"number"

"NaN"

"undefined"

"object"

Question 21

How do you check if an array includes a specific value?

arr.has(value)

arr.includes(value)

arr.contains(value)

arr.any(value)

Question 22

What does the `trim()` method do on a string?

Removes whitespace from both ends

Removes whitespace from the start only

Removes whitespace from the end only

Removes all spaces within the string

Question 23

Which pattern describes an immediately invoked function expression?

(function() {})()

function() {}

const fn = () => {}

new Function()

Question 24

What will this code log?

javascript

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

undefined

Throws an error

Question 25

Which statement about strict mode is true?

It is enabled by default in all scripts

It prevents the use of undeclared variables

It allows duplicate parameter names

It disables all type coercion

Question 26

How do you create a Promise that resolves immediately with value 5?

new Promise(5)

Promise.resolve(5)

Promise.done(5)

Promise.then(5)

Question 27

What is logged?

javascript

let x
console.log(x === undefined)

true

false

Throws ReferenceError

null

Question 28

Which loop is best for iterating over object keys?

for (const key in obj)

for (const value of obj)

while(obj)

loop obj.keys

Question 29

How do you declare a default parameter in a function?

javascript

function greet(name = "Guest") {
  return `Hello, ${name}!`
}

Set default inside function body

Assign default in parameter list

Use name.default = value

You cannot set defaults

Question 30

What is the purpose of the `finally` block in a try/catch?

It only runs when an error occurs

It runs after try and catch regardless of the outcome

It stores the caught error

It restarts the try block

Question 31

What does `document.querySelector(".button")` return?

An array of all elements with class button

The first element with class button

An HTMLCollection of elements

null always

Question 32

Which operator is used for nullish coalescing?

Question 33

What will this log?

javascript

const value = "5"
console.log(value * 2)

"10"

NaN

Error

Question 34

Which method removes the last element from an array and returns it?

shift()

pop()

slice()

splice()

Question 35

What is the result of `[1, 2, 3].includes(2)`?

false

true

null

undefined

Question 36

Which statement about `setTimeout` is correct?

It blocks the main thread until finished

It executes the callback after a delay without blocking

It guarantees exact timing

It returns the result of the callback

Question 37

What danger exists when comparing objects directly with `===`?

It always returns true

It compares references, not structure

It converts objects to JSON

It throws a syntax error

Question 38

What is the correct way to destructure the `name` property from an object?

javascript

const user = { name: "Ada", age: 30 }
// Fill the blank
const { _____ } = user

name: userName

user.name

name

user = name

Question 39

Which statement properly creates a class with a constructor?

javascript

class Person {
  constructor(name) {
    this.name = name
  }
}

function Person(name) { this.name = name }

class Person { constructor(name) { this.name = name } }

const Person = () => { this.name = name }

class Person() { this.name = name }

Question 40

What will be logged?

javascript

const numbers = [1, 2, 3]
const [first, ...rest] = numbers
console.log(rest)

[2, 3]

[1]

[1, 2, 3]

[]

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QUIZZES IN JavaScript

Basics

Variables & Data Types

A focused JavaScript quiz dedicated to variables, scope, primitive values, reference types, and type conversion. Use this set to check how well you understand declarations, assignments, comparison rules, and everyday runtime behaviour.

Operators & Expressions

A detailed JavaScript quiz exploring arithmetic, comparison, logical, bitwise, and assignment operators through practical expressions and tricky evaluation scenarios.

Strings & String Methods

Tackle 75 JavaScript string challenges that cover formatting, slicing, searching, pattern matching, and Unicode-safe manipulation to solidify everyday text-processing skills.

Numbers & Math Operations

Master 75 JavaScript questions that cover numeric literals, parsing, formatting, math utilities, and the edge cases every engineer hits when working with numbers.

Arrays & Basic Array Methods

100

Tackle 100 JavaScript array questions that range from everyday indexing to tricky splice mutations and iteration strategies. Forty questions supply runnable code so you can reason through exact behaviour before answering.

Objects & Key/Value Manipulation

Explore 75 JavaScript object questions that cover construction, property access, mutation, introspection, cloning, and immutability. Thirty questions include runnable snippets so you can reason through exact behaviour before answering.

Control Flow (if, else, switch)

Explore 50 JavaScript control-flow questions covering branching, truthiness, equality pitfalls, nested decisions, switch mechanics, fallthrough, and short-circuit expressions. Twenty questions include runnable snippets so you can reason about exact evaluation before answering.

Functions & Parameters

Explore 50 JavaScript function questions covering declarations, expressions, arrow functions, scopes, parameters, defaults, rest arguments, return values, closures, and execution flow.

Boolean Logic & Truthy/Falsy

Explore 50 JavaScript questions covering boolean values, logical operators, comparison pitfalls, truthy/falsy coercion, short-circuit flow, optional chaining, nullish coalescing, ternaries, and real-world conditional patterns.

Type Conversion & Coercion

Explore 50 JavaScript questions that test implicit coercion, explicit casts, string and number conversions, truthy/falsy evaluations, equality quirks, arithmetic side effects, ToPrimitive algorithms, JSON behavior, and the most common pitfalls developers hit when the engine reshapes data types.

Basic Error Types (SyntaxError, TypeError)

Explore 40 JavaScript questions covering SyntaxError definitions, parsing mistakes, prevention strategies, TypeError causes, undefined versus null access issues, strict mode implications, debugging stacks, try/catch usage, and best practices for runtime resilience.

Console API (log, error, table)

Explore 40 questions that cover console.log, console.error, console.table, message formatting, styling, grouping, timing, assertions, debugging workflows, clearing output, and practical logging best practices in JavaScript.

Basic Debugging in Browser DevTools

Understand 30 questions covering how to open DevTools, inspect the DOM, use the console, set and manage breakpoints, step through code, inspect the call stack, monitor network calls, leverage overrides, analyze performance and memory, debug event listeners, and work in responsive design mode.

Math Object & Random Numbers

Explore 50 JavaScript questions covering Math constants, rounding methods, min/max, exponentiation, roots, absolute values, sign detection, trig/log functions, random generation, range helpers, conceptual seeding, randomization patterns, and performance tips for numerical code.

Array Iteration Essentials (forEach)

Cover 40 questions on how Array.forEach works, callback parameters, side effects, mutation patterns, differences versus loops and map/filter, thisArg usage, arrow callbacks, early-exit limitations, nested iterations, object handling, common patterns, debugging techniques, and best practices.

Basic Date Handling (Date object)

Answer 50 questions on JavaScript Date basics: creating instances, timestamps, extracting components, setting values, parsing strings, basic formatting, Date.now, arithmetic, time zones, UTC helpers, comparisons, ISO conversion, and common pitfalls.

Intro to DOM (querySelector & textContent)

Answer 40 questions covering DOM fundamentals, document structure, element selection with querySelector/querySelectorAll, CSS selector usage, reading and updating textContent, handling null results, safe DOM manipulation, and best practices.

Prototype & Inheritance

Tackle 75 questions covering prototype chain fundamentals, __proto__ vs prototype, constructor functions, adding and overriding prototype methods, Object.create patterns, class syntax with extends/super, distinguishing instance vs prototype data, checking inheritance relationships, understanding this, performance considerations, pitfalls, and best practices.

Classes & OOP

Work through 50 questions on class syntax, constructors, instance and static members, getters/setters, inheritance with extends/super, private fields, composition vs inheritance, using this, creating multiple instances, basic design patterns, and best practices.

Modules (import/export)

Answer 50 questions on ES modules: named vs default exports, import syntax variations, renaming, exporting functions/classes/constants, re-exporting, module scope, namespace imports, dynamic import(), browser/module loading, resolution rules, Node.js ESM/CommonJS, bundling basics, and best practices.

Error Handling (try, catch, finally)

Tackle 40 questions on why try/catch exists, how to structure catch/finally blocks, inspecting error objects, throwing custom errors, handling specific cases, nesting handlers, and modern best practices.

Asynchronous JS (Callbacks)

Answer 75 questions on asynchronous JavaScript callbacks: fundamentals, error-first patterns, execution order, callback hell, timers, DOM and I/O callbacks, higher-order functions, data passing, chaining, and modern replacements like Promises.

Promises & Promise Chaining

Tackle 40 questions about JavaScript Promises: states, creating promises, then/catch/finally, value chaining, error propagation, avoiding callback hell, Promise.all, Promise.race, and coordinating multiple async operations.

Async/Await Deep Understanding

Answer 40 questions on async/await internals: how async functions work, await behavior, try/catch error handling, sequential vs parallel flows, awaiting multiple promises, mixing with raw promises, event loop interaction, return values, pitfalls, and best practices.

Execution Context

Dive into 40 questions covering how JavaScript builds global and function execution contexts, creation and execution phases, lexical and variable environments, scope chains, hoisting, this binding, and call stack behavior.

Hoisting & Temporal Dead Zone

Tackle 40 questions covering hoisting concepts, var behavior, function declaration hoisting, function expressions, let/const characteristics, temporal dead zone pitfalls, runtime errors, and best practices.

Closures & Lexical Environment

Study 40 JavaScript questions on lexical scoping, scope chains, closure creation, accessing outer variables, state preservation, memory considerations, and avoiding accidental closures.

Scope Types (Global, Block, Function)

Learn 30 questions that analyze JavaScript global scope, function scope, block scope, var quirks, visibility rules, shadowing, scope chains, and best practices for managing identifiers.

Event Loop & Microtasks Introduction

Answer 50 questions about the JavaScript event loop, call stack, macrotasks, microtasks, promise timing, rendering, async APIs, starvation risks, and debugging workflows.

Advanced Array Methods (map, filter, reduce)

Answer 35 questions that dive into map transformations, filter logic, reduce accumulations, callback signatures, immutability, chaining, patterns, performance trade-offs, and behavior comparisons.

Spread & Rest Operator Deep Usage

Answer 35 questions exploring spread on arrays and objects, cloning behavior, combining collections, rest parameters, destructuring techniques, pitfalls, performance trade-offs, and modern practical patterns.

6 Destructuring Use Cases (Nested, Default)

Tackle 30 questions on nested object/array destructuring, defaults, renaming bindings, destructuring parameters, and skipping positions.

Template Literals Advanced (Tagged Templates)

Answer 35 questions digging into template literal basics, tagged function mechanics, string/value processing, security sanitization, custom formatting, localization, DSL construction, and performance trade-offs.

Regular Expressions Basics

Answer 35 questions covering regex literal vs constructor syntax, metacharacters, character classes, quantifiers, anchors, grouping, escaping, and JS testing/matching methods.

Shallow vs Deep Copying Objects

Answer 30 questions comparing shallow copy techniques, deep copy strategies, spread/Object.assign behavior, JSON and structuredClone approaches, reference vs value semantics, and common pitfalls like cyclic structures.

JSON Handling (stringify, parse pitfalls)

Answer 40 questions covering JSON.stringify basics, JSON.parse behavior, handling undefined and functions, circular structure errors, Date serialization, number precision, reviver/replacer usage, and safe parsing practices.

DOM Events Bubbling & Capturing

Answer 30 questions on event propagation phases, capturing basics, target behavior, bubbling mechanics, addEventListener options, stopping propagation, preventing default actions, and event delegation techniques.

Performance Optimization Techniques

Answer 35 questions on minimizing reflows, debouncing/throttling, faster loops, efficient structures, avoiding leaks, web workers, lazy loading, caching, cutting re-renders, and profiling with DevTools.

Iterators & Generators (yield, next, custom iterators)

Answer 35 questions covering iterator protocol basics, next() behavior, iterable objects, generator syntax, yield mechanics, execution control, passing values, custom iterators, lazy generator use cases, and async-iterator differences.

Async Iteration & for-await-of

Answer 35 questions covering the async iterator protocol, Symbol.asyncIterator, async generator syntax, yield vs yield*, promise handling, for-await-of mechanics, error management, streaming data scenarios, and using async generators with fetch.

Debounce & Throttle Patterns

Answer 35 questions covering debounce timing, throttle intervals, leading/trailing triggers, preventing noisy events, scroll/input use cases, manual implementations, pitfalls, and UI performance gains.

Shadow DOM & Web Components Basics

Answer 35 questions on custom element fundamentals, shadow DOM encapsulation, open vs closed roots, templates and slots, style isolation, lifecycle callbacks, reusable component design, and light/shadow DOM interaction.

Intersection Observer

Answer 35 questions covering observers and targets, threshold/rootMargin tuning, lazy-loading patterns, visibility detection, cleanup best practices, entry properties, performance wins vs scroll events, and common use cases like infinite scroll, animations, and analytics.

MutationObserver: Watching DOM Changes

Test your knowledge with 35 questions on MutationObserver—covering DOM observation, configuration options, attribute and child tracking, subtree monitoring, mutation records, cleanup, performance, and real-world use cases.

Browser Rendering Pipeline

Test your understanding of how browsers render web pages with 35 questions covering HTML/CSS/JS parsing, the Critical Rendering Path, layout, painting, compositing, and performance optimizations.

Security Basics

Test your knowledge with 40 questions on web security fundamentals—covering XSS types, prevention strategies, CSRF mechanics, and defense techniques like tokens, SameSite cookies, and Content Security Policy.

JavaScript Basics Quiz

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

Question 21

Question 22

Question 23

Question 24

Question 25

Question 26

Question 27

Question 28

Question 29

Question 30

Question 31

Question 32

Question 33

Question 34

Question 35

Question 36

Question 37

Question 38

Question 39

Question 40

About This JavaScript Basics Quiz

What You Will Learn

Why This Quiz Is Important

Other Quiz Categories

QUIZZES IN JavaScript

Basics

Variables & Data Types

Operators & Expressions

Strings & String Methods

Numbers & Math Operations

Arrays & Basic Array Methods

Objects & Key/Value Manipulation

Control Flow (if, else, switch)

Functions & Parameters

Boolean Logic & Truthy/Falsy

Type Conversion & Coercion

Basic Error Types (SyntaxError, TypeError)

Console API (log, error, table)

Basic Debugging in Browser DevTools

Math Object & Random Numbers

Array Iteration Essentials (forEach)

Basic Date Handling (Date object)

Intro to DOM (querySelector & textContent)

Prototype & Inheritance

Classes & OOP

Modules (import/export)

Error Handling (try, catch, finally)

Asynchronous JS (Callbacks)

Promises & Promise Chaining

Async/Await Deep Understanding

Execution Context

Hoisting & Temporal Dead Zone

Closures & Lexical Environment

Scope Types (Global, Block, Function)

Event Loop & Microtasks Introduction

Advanced Array Methods (map, filter, reduce)

Spread & Rest Operator Deep Usage

6 Destructuring Use Cases (Nested, Default)

Template Literals Advanced (Tagged Templates)