JavaScript

In JavaScript, reserved words are identifiers (like variable names or function names) that cannot be used because they are either part of the language syntax or reserved for future use.

Using these words as variable names or function names can lead to errors or unexpected behavior.

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📌 1. Reserved Keywords (Used by JavaScript)

These are actively used in JavaScript syntax and must not be used as identifiers:

Category	Reserved Words
Control Flow	if, else, switch, case, default
Loops	for, while, do, break, continue
Declarations	var, let, const, function, class, extends, super
Scope	this, return, yield
Error Handling	try, catch, throw, finally
Imports & Exports	import, export, from, as
Others	new, delete, typeof, instanceof, void, in, with, await

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📌 2. Future Reserved Words

These are reserved for future versions of JavaScript and should be avoided:

enum, implements, package, protected, interface, private, public, static

They may not throw an error now but can break your code in the future when these become official features.

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📌 3. Reserved in Strict Mode

When using "use strict"; in your JavaScript (recommended), the following words become reserved:

let, yield, eval, arguments

Strict mode has stricter parsing and error handling and doesn’t allow reassigning or using these as identifiers.

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📌 4. Literals (Also Reserved)

You cannot use the following JavaScript literals as identifiers:

null, true, false

These represent actual values in JavaScript and are not re-definable.

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📌 5. Reserved Objects & Globals (Avoid Using as Variables)

Even though not strictly “reserved,” it’s a bad idea to use names of global objects or functions like:

Array, Object, Function, Number, String, Date, Math, JSON, RegExp, Promise, Set, Map, console, window, document

Using them as variables will override built-in features, which can break your app.

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❌ Examples of Invalid Usage

let class = "MyClass"; // ❌ SyntaxError: Unexpected token 'class'

function return() { }   // ❌ Invalid function name

const new = 5;         // ❌ 'new' is reserved

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✅ Tips to Avoid Reserved Word Errors

• Stick to descriptive variable names.
• Use linters like ESLint to catch these issues early.
• Avoid using anything that “looks” like a JS keyword for your variables or functions.

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🔍 Complete List of Reserved Words

You can check the full and up-to-date list of JavaScript reserved words in the ECMAScript specification.

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