JS Reserved Words

JavaScript Reserved Words: Language Keywords & Future Identifiers

Reserved words are keywords that cannot be used as variable names, function names, class identifiers, or loop labels because they are reserved by JavaScript's grammar parser.

Reserved Word Classification

flowchart TD
    Reserved["Reserved Words"] --> Current["Current Keywords (let, const, function, class, if, return, import)"]
    Reserved --> FutureStrict["Future Reserved in Strict Mode (implements, interface, package, private, protected, public)"]
    Reserved --> NullBool["Primitive Literals (null, true, false)"]

Reserved Keywords Reference Table

Practical Code Example

// Demonstrating Reserved Words and Safe Property Access

// 1. Invalid Variable Declarations (Un-commenting throws SyntaxError)
// const class = "Math";    // SyntaxError: Unexpected token 'class'
// let function = "Test";   // SyntaxError: Unexpected token 'function'

// 2. Safe Usage as Object Property Keys
// Modern ES6 allows reserved words as unquoted object property keys
const config = {
  class: "UserAccount",  // Allowed as object key!
  default: true,         // Allowed as object key!
  delete: false          // Allowed as object key!
};

console.log(`Object property 'class': ${config.class}`);
console.log(`Object property 'default': ${config.default}`);

// 3. Safe Destructuring with Renaming
const { default: isDefaultSetting } = config;
console.log(`Renamed reserved key: ${isDefaultSetting}`);

Best Practices & Gotchas

• Do Not Use Future Reserved Words: Avoid using words like interface, package, or private as variable identifiers even in non-strict mode to ensure seamless TypeScript migration.
• Rename Reserved Imports: When importing module items named default, rename them using alias syntax (import { default as myDefault } from "./mod.js").
• Dot Property Access Allowed: Property keys like obj.delete or obj.class are valid in ES6+, but avoid them if possible for clarity.

Self-Check Challenge

How do you safely rename a default import import { default } from "./module.js"?