What is strict mode in JavaScript

Short answer

Strict mode is a restricted variant of JavaScript enabled with "use strict"; that catches common mistakes, prevents unsafe actions, and optimizes engine performance.

How it works

Add the directive at the top of a script or function:

"use strict";
x = 3.14; // ReferenceError: x is not declared

Key changes

• Assigning to undeclared variables throws a ReferenceError.
• Deleting variables, functions, or non-configurable properties throws.
• Duplicate parameter names are forbidden.
• this in standalone functions is undefined instead of the global object.
• Octal numeric literals are forbidden.

Example

function sum(a, a) { // SyntaxError in strict mode
  "use strict";
  return a + a;
}

Why it matters

Strict mode makes code more secure and predictable. ES modules are implicitly strict, so modern code often runs in strict mode by default.