Question 1

Explain ES6 modules and their benefits.

Accepted Answer

ES6 modules use import/export to share code between files. Each module has its own scope â€” no global namespace pollution. Imports are static (analyzable at parse time), enabling tree-shaking. The browser loads them with . Compared to CommonJS (require), ES modules are standard, async, and support live bindings.

Question 2

Difference between default export and named export.

Accepted Answer

Named export: export const name = ...; â€” import by exact name: import { name } from "./file". Can have multiple per file. Default export: export default ...; â€” import with any name: import anything from "./file". Only one default per file. Mix both: import React, { useState } from "react" â€” React is the default, useState is named.

Question 3

Explain template literals and tagged templates.

Accepted Answer

Template literals: backticks with ${expression} interpolation and multi-line support. Tagged templates: prefix a template literal with a function â€” html`${user}` â€” the function receives the string parts and interpolated values as separate arguments. Used in styled-components, GraphQL (gql``), and SQL builders for safe, structured string construction.

Question 4

What are destructuring assignments?

Accepted Answer

Destructuring extracts values from arrays or objects into variables in one clean step. Object: const { name, age } = user â€” assigns user.name to name, user.age to age. Array: const [first, second] = arr â€” assigns by position. Supports defaults (const { name = "Guest" } = user), renaming ({ name: userName }), and nested patterns.

Question 5

Explain rest and spread operators.

Accepted Answer

Rest (...args): collects multiple elements into one â€” in function params, function sum(...numbers) collects all args into an array; in destructuring, const { a, ...rest } = obj collects remaining properties. Spread (...arr): expands an iterable â€” [...arr1, ...arr2] merges arrays; {...obj1, ...obj2} merges objects. Rest collects; spread expands.

Question 6

What are default parameters in functions?

Accepted Answer

Default parameters give function parameters a fallback value when the argument is undefined: function greet(name = "World") {}. Called as greet() â€” name is "World". Called as greet("Alice") â€” name is "Alice". Called as greet(undefined) â€” name is "World". null does NOT trigger the default; only undefined does.

Question 7

Explain arrow functions and lexical this.

Accepted Answer

Arrow functions have lexical this â€” they don't create their own this binding. They inherit this from the enclosing scope at definition time. This makes them ideal for callbacks and array methods where you want this to refer to the outer object. Regular functions have their own this determined by how they're called â€” making them right for methods and constructors.

Question 8

Difference between let, const, and var in ES6.

Accepted Answer

var is function-scoped, hoisted, and can be redeclared. const is block-scoped, must be initialized, and can't be reassigned (though object contents can change). let is block-scoped, can be reassigned but not redeclared. ES6 rule of thumb: use const by default, let when you need reassignment, never var in new code.

Question 9

Explain promises and chaining in ES6+.

Accepted Answer

Promises allow sequential async operations without callback nesting. fetch(url).then(r => r.json()).then(data => save(data)).catch(handleError) â€” each then receives the previous result. .finally() runs regardless of outcome. Promise.all([p1, p2]) runs in parallel and resolves when all resolve. Promise.race([p1, p2]) resolves when the first settles.

Question 10

Explain async/await syntax.

Accepted Answer

async marks a function as asynchronous â€” it always returns a Promise. await pauses execution until the awaited Promise resolves, returning its value. await can only be used inside async functions (or top-level in ES modules). Try/catch handles rejections. Multiple awaits in sequence are serial; use Promise.all([await1, await2]) for parallel execution.

Junior JavaScript Interview Questions

JavaScript Interview Questions & Answers

Questions

Explain ES6 modules and their benefits.

Answer