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?

ECMAScript	JavaScript
Defines the core language specification	Implementation of ECMAScript
Does not include DOM or browser APIs	Includes browser APIs (DOM, BOM)
Examples: ES5, ES6, ES2020	Examples: Chrome V8, Node.js

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.

Version	Year	Key Focus
ES1	1997	First edition
ES3	1999	Regular expressions, try/catch
ES5	2009	Strict mode, JSON, array methods
ES6	2015	Classes, Modules, Arrow functions
ES7	2016	Exponentiation, includes()
ES8	2017	Async/Await
ES9	2018	Rest/Spread, async iteration
ES10	2019	flat(), fromEntries()
ES11	2020	Optional chaining, BigInt
ES12	2021	Logical assignment
ES13	2022	Top-level await
ES14	2023	findLast()
ES15/16	2024–25	Pipeline operator, Records & Tuples

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.