Reserved Words

Reserved Words

JavaScript reserves certain words for its own use. You cannot use these as variable names, function names, or identifiers. Trying to do so will throw a syntax error or cause unexpected behavior.

Common reserved words: if, else, for, while, do, switch, case, break, continue, return, function, var, let, const, class, new, this, typeof, instanceof, try, catch, finally, throw, import, export, async, await, yield, debugger, with, void, delete, in, of, null, true, false, undefined, NaN, Infinity.

Reserved Word Error

// This will cause an error:
// let if = 5;

let name = "Alice";
console.log(name);

Valid Naming Workaround

let ifCondition = true;
let className = "highlight";
let returnValue = 42;
console.log(ifCondition, className, returnValue);

Class Names and Reserved Words

// You can use reserved words as object keys
let obj = {
  class: "first",
  return: "yes",
  default: "none"
};
console.log(obj.class);
console.log(obj.return);
console.log(obj["default"]);

Reserved words are fine as object property names (with quotes if needed), just not as bare identifiers.