Question 1

What is the difference between var, let and const?

Accepted Answer

var is function-scoped and can be redeclared, while let and const are block-scoped. const prevents reassignment of the binding but does not make objects immutable.

Question 2

What is hoisting in JavaScript?

Accepted Answer

Hoisting is the behaviour where declarations are moved to the top of their scope during compilation. Function declarations are fully hoisted, var is hoisted as undefined, and let/const are hoisted but unusable until their declaration line.

Question 3

What is the difference between == and ===?

Accepted Answer

== compares values after type coercion, while === compares both value and type without coercion. In assertions and test logic always use === to avoid false positives.

Question 4

What are the data types in JavaScript?

Accepted Answer

JavaScript has seven primitive types (string, number, bigint, boolean, undefined, null, symbol) and one non-primitive type, object, which includes arrays and functions. Primitives are copied by value, objects by reference.

Question 5

Which array methods do you use most in automation scripts?

Accepted Answer

map, filter, find, some, every, reduce and forEach are the most common. They are used to transform locator text lists, validate sorting, and pick test data.

Question 6

Explain destructuring and the spread/rest operators.

Accepted Answer

Destructuring unpacks values from arrays or properties from objects into variables. Spread expands an iterable or object, and rest collects remaining items into one variable.

Question 7

What is the difference between CommonJS and ES modules?

Accepted Answer

CommonJS uses require and module.exports and loads synchronously, while ES modules use import/export and are statically analysable. Node 20+ supports both; ESM is enabled by "type": "module" or the .mjs extension.

Question 8

What is a closure and where is it useful in testing?

Accepted Answer

A closure is a function that remembers variables from its outer scope even after that scope has finished executing. In automation it is used for data generators, counters and factory functions.

Question 9

How does the this keyword work, and how do arrow functions change it?

Accepted Answer

this is determined by how a function is called: by the object before the dot, by new, or explicitly via call/apply/bind. Arrow functions do not have their own this; they inherit it lexically from the surrounding scope.

Question 10

What is a Promise and what are its states?

Accepted Answer

A Promise represents the eventual result of an asynchronous operation. It is pending, then settles as either fulfilled with a value or rejected with a reason, and the state never changes afterward.

Question 11

How does async/await work, and what happens if you forget await?

Accepted Answer

async functions always return a Promise, and await pauses that function until the Promise settles. Forgetting await in Playwright or WebdriverIO makes the test move on before the action completes, causing flaky or falsely passing tests.

Question 12

Explain the JavaScript event loop.

Accepted Answer

JavaScript runs on a single thread with a call stack; async callbacks are queued and executed by the event loop when the stack is empty. Microtasks (Promise callbacks) run before the next macrotask (setTimeout, I/O).

Question 13

How do you handle errors in async test code?

Accepted Answer

Use try/catch with await, or .catch on Promises, and rethrow when the test should fail. Custom error classes and finally blocks help with clear messages and cleanup.

Question 14

What are package.json, package-lock.json and npm ci?

Accepted Answer

package.json declares dependencies and scripts, while package-lock.json pins exact resolved versions. npm ci installs strictly from the lockfile and is the right command for CI pipelines.

Question 15

How does JavaScript usage differ between Cypress, Playwright and WebdriverIO?

Accepted Answer

Playwright and WebdriverIO use native async/await with Promises. Cypress uses a chained command queue with built-in retries, so you use .then() or aliases instead of await.

Question 16

How would you write a generic retry helper with backoff?

Accepted Answer

Wrap an async function in a loop that catches errors, waits with increasing delay, and rethrows after the maximum attempts. Use it for flaky external setup like API seeding, not to mask product bugs.

Question 17

How do classes and prototypes relate in JavaScript?

Accepted Answer

Classes are syntactic sugar over prototype-based inheritance. Methods live on the prototype object, and extends sets up the prototype chain, which is how BasePage inheritance works in page object models.

Question 18

Coding: reverse a string and check if it is a palindrome.

Accepted Answer

Normalise the string by lowercasing and removing non-alphanumerics, then compare it with its reverse. Use split/reverse/join or a two-pointer loop for O(1) extra space.

Question 19

Coding: count character frequency and find the first non-repeating character.

Accepted Answer

Build a frequency map in one pass with a Map or object, then scan the string again to find the first character with count 1. This is O(n) time.

Question 20

Coding: remove duplicates from an array and flatten a nested array.

Accepted Answer

Use new Set to remove duplicates and Array.prototype.flat(Infinity) to flatten. Be ready to implement both manually with loops or recursion.

Question 21

What is the difference between null and undefined?

Accepted Answer

undefined means a variable was declared but never assigned, or a property doesn't exist. null is an intentional 'no value' that a developer assigns explicitly.

Question 22

What are truthy and falsy values in JavaScript?

Accepted Answer

Falsy values are false, 0, -0, 0n, empty string, null, undefined and NaN; everything else is truthy, including empty arrays and objects. Conditions coerce values to boolean using these rules.

Question 23

What are template literals and tagged templates?

Accepted Answer

Template literals use backticks and allow embedded expressions with ${...} and multi-line strings. A tagged template passes the string parts and values to a function for custom processing.

Question 24

How do arrow functions differ from regular functions?

Accepted Answer

Arrow functions have shorter syntax and don't have their own this, arguments or prototype; they inherit this from the surrounding scope. Regular functions get this from how they are called and can be used as constructors.

Question 25

How do you work with objects and JSON in JavaScript?

Accepted Answer

Objects store key-value pairs; access properties with dot or bracket notation and iterate with Object.keys, values or entries. JSON.stringify converts objects to JSON text and JSON.parse turns JSON text back into objects.

Question 26

Which string methods do you use most in JavaScript test scripts?

Accepted Answer

trim, split, includes, startsWith, replace/replaceAll, slice, toLowerCase and padStart cover most needs. They help normalize UI text and parse values like prices or IDs before asserting.

Question 27

What is the difference between for...of, for...in and forEach?

Accepted Answer

for...of iterates values of iterables like arrays, strings, Maps and Sets. for...in iterates enumerable property keys, including inherited ones, so it's meant for objects. forEach calls a callback per array element but can't be stopped with break and doesn't wait for async callbacks.

Question 28

Coding: write FizzBuzz in JavaScript.

Accepted Answer

Loop from 1 to n and build the output: 'Fizz' for multiples of 3, 'Buzz' for multiples of 5, both for multiples of 15, otherwise the number. Concatenating the two words avoids an extra 15 check.

Question 29

Coding: generate Fibonacci numbers in JavaScript, iteratively and with memoization.

Accepted Answer

Iteratively, keep two variables and update them with destructuring [a, b] = [b, a + b] in O(n). Recursion with a memo cache avoids the exponential cost of naive recursion.

Question 30

Coding: check if a number is prime in JavaScript.

Accepted Answer

Return false for numbers below 2, handle 2 and even numbers, then test odd divisors up to the square root. If none divides evenly, the number is prime.

Question 31

Coding: find the largest and smallest numbers in an array without Math.max.

Accepted Answer

Start with the first element as both min and max, then loop through the rest and update each when you find a smaller or larger value. That's a single O(n) pass.

Question 32

How do you make shallow and deep copies of objects in JavaScript?

Accepted Answer

Spread ({...obj}) and Object.assign create shallow copies, so nested objects are still shared. structuredClone() makes a true deep copy and handles Dates, Maps, Sets and cycles; JSON.parse(JSON.stringify()) works only for plain JSON data.

Question 33

Explain Promise.all, Promise.allSettled, Promise.race and Promise.any.

Accepted Answer

Promise.all resolves with all results or rejects on the first failure. allSettled waits for every promise and reports each status, race settles with the first promise to settle, and any resolves with the first success, rejecting only if all fail.

Question 34

What are callbacks and what is callback hell?

Accepted Answer

A callback is a function passed to another function to run later, often when async work completes. Callback hell is deeply nested callbacks that are hard to read and handle errors in; Promises and async/await flatten that code.

Question 35

When do you use Map and Set instead of plain objects and arrays?

Accepted Answer

Use Map when keys aren't strings, when you add and remove keys often, or when insertion order and size matter. Use Set to store unique values and check membership in O(1).

Question 36

What do optional chaining (?.) and nullish coalescing (??) do?

Accepted Answer

Optional chaining returns undefined instead of throwing when a part of the chain is null or undefined. Nullish coalescing returns the right-hand value only when the left is null or undefined, unlike || which also triggers on 0 or empty string.

Question 37

What is the difference between call, apply and bind?

Accepted Answer

All three set the this value of a function. call invokes it immediately with arguments listed individually, apply invokes it with an array of arguments, and bind returns a new function with this permanently set.

Question 38

How do you structure tests in Jest, and which matchers and hooks do you use?

Accepted Answer

Group tests with describe and write cases with test or it, using expect with matchers like toBe, toEqual, toContain, toThrow and resolves/rejects. Hooks beforeAll, beforeEach, afterEach and afterAll handle setup and teardown.

Question 39

How do you mock functions and modules in Jest?

Accepted Answer

jest.fn() creates a mock function that records calls and can return fixed values. jest.mock('module') auto-mocks an entire module, and jest.spyOn wraps an existing method so you can assert on it or override its implementation.

Question 40

How do Mocha and Chai work together, and how do they differ from Jest?

Accepted Answer

Mocha is a test runner that provides describe/it and hooks, while Chai is a separate assertion library with expect, should and assert styles. Jest bundles the runner, assertions, mocking and coverage in one package.

Question 41

Coding: check whether two strings are anagrams in JavaScript.

Accepted Answer

Normalize both strings, then either sort their characters and compare, which is O(n log n), or count characters in a Map and make sure every count returns to zero, which is O(n).

Question 42

Coding: find the second largest unique number in an array in JavaScript.

Accepted Answer

Walk the array once tracking the largest and second largest; when a value beats the largest, demote the old largest to second. Ignore values equal to the largest so duplicates don't count.

Question 43

Coding: find the missing number in a sequence from 1 to n in JavaScript.

Accepted Answer

Compute the expected sum n * (n + 1) / 2 and subtract the actual sum from reduce. For multiple gaps, put the values in a Set and collect numbers from 1 to n that aren't in it.

Question 44

Predict the output order of setTimeout, Promise.then and synchronous logs.

Accepted Answer

Synchronous code runs first, then all queued microtasks (Promise callbacks, queueMicrotask, process.nextTick in Node), and only then the next macrotask such as a setTimeout callback. So promise callbacks always beat a zero-delay timeout.

Question 45

How does garbage collection work in JavaScript, and what causes memory leaks?

Accepted Answer

Engines like V8 use mark-and-sweep: objects unreachable from roots such as globals and the current stack are freed. Leaks happen when references are kept unintentionally, for example in growing global arrays, forgotten timers, event listeners or closures.

Question 46

What are iterators and generators in JavaScript?

Accepted Answer

An iterator is an object with a next() method returning { value, done }, and anything with a [Symbol.iterator] method is iterable. A generator function (function*) creates iterators easily using yield and pauses between values.

Question 47

Which ES2020–ES2024 features do you use in test code?

Accepted Answer

Commonly used ones are optional chaining and ?? (ES2020), replaceAll and Promise.any (ES2021), top-level await, Array.prototype.at and Object.hasOwn (ES2022), non-mutating toSorted/toReversed and findLast (ES2023), and Object.groupBy (ES2024).

Question 48

Coding: sort an array of numbers without using Array.prototype.sort.

Accepted Answer

Implement bubble sort with a swapped flag for early exit, or a selection sort that repeatedly moves the smallest remaining element forward. Both are O(n^2); destructuring makes swaps concise.

Question 49

Coding: implement debounce and throttle, and explain the difference.

Accepted Answer

Debounce delays a call until a quiet period has passed since the last trigger, so only the final call in a burst runs. Throttle runs at most once per interval no matter how often it's triggered.

Question 50

Coding: implement your own version of Promise.all.

Accepted Answer

Return a new Promise, resolve each input with Promise.resolve, store each result at its original index, and resolve once the completed count equals the input length. Reject immediately when any input rejects.

Interview Questions

01What is the difference between var, let and const?BeginnerCore Language

02What is hoisting in JavaScript?BeginnerCore Language

03What is the difference between == and ===?BeginnerCore Language

04What are the data types in JavaScript?BeginnerCore Language

05Which array methods do you use most in automation scripts?BeginnerArrays & Objects

06Explain destructuring and the spread/rest operators.BeginnerArrays & Objects

07What is the difference between CommonJS and ES modules?BeginnerNode.js & Tooling

08What is a closure and where is it useful in testing?IntermediateFunctions & Scope

09How does the this keyword work, and how do arrow functions change it?IntermediateFunctions & Scope

10What is a Promise and what are its states?IntermediateAsync JavaScript

11How does async/await work, and what happens if you forget await?IntermediateAsync JavaScript

12Explain the JavaScript event loop.IntermediateAsync JavaScript

13How do you handle errors in async test code?IntermediateAsync JavaScript

14What are package.json, package-lock.json and npm ci?IntermediateNode.js & Tooling

15How does JavaScript usage differ between Cypress, Playwright and WebdriverIO?IntermediateNode.js & Tooling