Imagine you’re playing football on a school field. There are some areas you just can’t enter — like the teachers’ stand or the referee’s zone. No matter how much you want to, those spaces are off-limits.

That’s exactly how reserved keywords work in JavaScript. They are special words that the language itself uses, so programmers cannot use them as names for variables, functions, or identifiers.

If you try, JavaScript will blow the whistle 🚨 and throw an error.

🏷️ What Are Reserved Keywords?

Reserved keywords are predefined words that have a fixed meaning in the JavaScript language.

For example:

let let = 5;   // ❌ Error: "let" is reserved

Here, let is already used by JavaScript to declare variables. Trying to use it as a variable name confuses the engine.

🔑 Categories of Reserved Keywords

Reserved words in JavaScript can be grouped into categories depending on how they are used.

1. Current JavaScript Keywords

These are words we use every day in modern JavaScript. They control logic, structure, and behavior.

Examples:

  • Declarations: var, let, const, function, class
  • Control flow: if, else, switch, case, default, do, while, for, break, continue
  • Error handling: try, catch, finally, throw
  • Specials: return, new, delete, typeof, instanceof, this, super

2. Future Reserved Words

ECMAScript (the standard for JavaScript) keeps aside certain words for possible future use. You shouldn’t use them either, even if they don’t do anything yet.

Examples: enum, implements, interface, package, private, protected, public, static, yield

3. Strict Mode Reserved Words

When you use strict mode ("use strict";), JavaScript becomes extra strict and protects certain words.

Reserved in strict mode: let, static, implements, interface, package, private, protected, public

4. Null, Boolean, and Literals

You also cannot use certain literal values as names:

  • null
  • true, false

🛑 Why Should You Care?

  1. Avoid Errors If you mistakenly use reserved words as identifiers, your program will crash.
const class = "JSS3"; // ❌ SyntaxError

  1. Future-Proofing Even if some reserved words aren’t active now, future JavaScript versions may use them. So avoid them entirely.

  2. Clarity Reserved keywords have strong meanings in the language. Using them wrongly makes your code confusing to read.

✅ Best Practices

  • Always choose descriptive names: e.g., use studentName instead of class.
  • Use code editors or linters (like ESLint) — they’ll highlight reserved words when misused.
  • Stay updated with ECMAScript changes, as some new reserved words might be added in future.

📝 Review – Fill in the Gaps

  1. Reserved keywords are special words that cannot be used as __ names in JavaScript.
  2. The word let is used to declare a __ in JavaScript.
  3. Using class as a variable name will cause a __ error.
  4. Words like enum, package, and public are __ reserved for future use.
  5. When "use strict"; is enabled, JavaScript blocks even more __ words.
  6. The values true, false, and null are examples of __ that cannot be used as identifiers.
  7. Reserved words like if, else, and switch are used in controlling program __.
  8. JavaScript protects keywords to avoid __ in the engine.
  9. A linter like __ can highlight misused reserved keywords.
  10. Instead of using class as a variable, a better choice would be __.
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