JavaScript Versions & Standards — Interview Questions & Answers

Ques: What is ECMAScript (ES)?

Ans: ECMAScript is the official standard that defines the core features, syntax, and behavior of JavaScript. JavaScript itself is an implementation of the ECMAScript standard.

Ques: Who maintains the ECMAScript standard?

Ans: It is maintained by ECMA International, under the TC39 (Technical Committee 39), which regularly proposes and approves new features for JavaScript.

Ques: What is the relationship between ECMAScript and JavaScript?

Ques: What does “ES5”, “ES6”, or “ES2020” mean?

Ans: Each term represents a version of ECMAScript.

• ES5 (2009): Introduced modern JS foundation
• ES6 (2015): Major update with new syntax and features
• Later versions (ES2016–ES2025) added yearly improvements

Ques: When was JavaScript first standardized?

Ans: JavaScript was standardized as ECMAScript 1 (ES1) in 1997 by ECMA International.

Ques: What is the purpose of ECMAScript versions?

Ans: Each version adds new language features, improves performance, and enhances developer experience while maintaining backward compatibility.

Ques: List the major ECMAScript versions with years.

Ques: What key features were introduced in ES5?

• Strict Mode ("use strict";)
• JSON support
• Array methods: forEach(), map(), filter(), reduce()
• Getters and Setters
• Object.defineProperty()
• Better error handling with try...catch

"use strict";
var obj = { name: "JS" };
Object.defineProperty(obj, "version", { value: "ES5" });

Ques: What is “Strict Mode” in ES5?

Ans: It makes JavaScript safer and cleaner by preventing undeclared variables and silent errors.

Ques: What new Array methods came in ES5?

Ans: forEach(), map(), filter(), reduce(), some(), every(), indexOf(), and lastIndexOf().

Ques: Why is ES6 considered the biggest JavaScript update?

Ans: It introduced modern syntax and features that transformed JavaScript into a more powerful, modular, and readable language.

Ques: What are key features of ES6?

• let and const
• Arrow functions
• Template literals
• Classes & Modules
• Destructuring
• Promises
• Default Parameters
• Rest & Spread operators
• for...of loop
• Symbols

Ques: What are let and const in ES6?

Ans: Block-scoped variable declarations replacing var. let allows reassignment, const does not.

Ques: What are Arrow Functions in ES6?

Ans: A shorter syntax for writing functions with lexical this binding.

const add = (a, b) => a + b;

Ques: What are Template Literals in ES6?

Ans: They allow multi-line strings and string interpolation using backticks.

`Hello, ${name}!`

Ques: What features were added in ES7?

• Exponentiation operator (*)
• Array.prototype.includes() method

2 ** 3; // 8
[1, 2, 3].includes(2); // true

Ques: What major feature did ES8 introduce?

Ans: Async/Await for easier asynchronous programming.

Ques: Name other ES8 features.

• Object.entries() and Object.values()
• String padding (padStart, padEnd)
• Object.getOwnPropertyDescriptors()

Ques: What did ES9 introduce for objects?

Ans: Object Rest/Spread syntax.

const user = { name: "John", age: 30 };
const clone = { ...user };

Ques: What is Promise.finally() used for?

Ans: It executes a block of code after a promise is settled (fulfilled or rejected).

Ques: What are major ES10 features?

• Array.flat() and Array.flatMap()
• Object.fromEntries()
• String.trimStart() / trimEnd()
• Optional catch binding

Ques: What is Optional Chaining (?.) in ES11?

Ans: It safely accesses nested object properties without causing errors.

user?.profile?.name;

Ques: What is Nullish Coalescing (??)?

Ans: It returns the right-hand value only if the left-hand is null or undefined.

const name = user.name ?? "Guest";

Ques: What is BigInt?

Ans: A new data type for representing very large integers.

const big = 12345678901234567890n;

Ques: What are Logical Assignment Operators?

Ans: Combines logical and assignment operations:

&&=, ||=, ??=

a ||= 10; // assign only if falsy

Ques: What are Numeric Separators?

Ans: They improve readability for large numbers.

const billion = 1_000_000_000;

Ques: What is Top-level Await?

Ans: Allows using await outside async functions in modules.

const data = await fetch("/api");

Ques: What are Private Class Fields?

Ans: Properties prefixed with # that are accessible only inside the class.

class User { #password = "secret"; }

Ques: What new Array methods were added in ES14?

Ans: findLast() and findLastIndex()

[1, 2, 3, 4].findLast(n => n < 3); // 2

Ques: What are some proposed ES15/16 features?

• Pipeline operator (|>)
• Records & Tuples (immutable data)
• Temporal API for better date/time handling
• Pattern Matching

Ques: What is the Pipeline Operator?

Ans: It allows chaining function calls in a readable way.

const result = "  hello world  "
  |> (s => s.trim())
  |> (s => s.toUpperCase());

Ques: How can developers use modern JS features in old browsers?

Ans: By using transpilers like Babel, which convert ES6+ code to ES5.

Ques: What is a Polyfill?

Ans: A script that adds missing features to older browsers. Example: core-js.

import "core-js/stable";

Ques: Which browsers support ES6+ features?

Ans: All modern browsers (Chrome, Edge, Firefox, Safari, Opera). IE11 supports only ES5 features.