Strict Mode

                'use strict';
              

Purpose

Strict mode is a restricted variant of JavaScript that catches common mistakes by throwing errors instead of silently doing something unexpected. It’s opt-in in plain scripts, but on by default in two very common places.

Syntax

'use strict';
// entire script is strict from here on

function example() {
  'use strict';
  // only this function is strict
}

The directive must be the first statement in a script or function body (comments and other directives may precede it) — written anywhere else, it’s just an unused string literal.

Where strict mode is already on

• Inside any ES module (any file loaded via import/export).
• Inside any class body.

Both of these are strict without writing 'use strict' yourself. Plain scripts and traditional function declarations outside a module are not strict unless you opt in.

Behavior

Strict mode changes several things that would otherwise fail silently:

'use strict';

undeclaredVariable = 5;
// Uncaught ReferenceError: undeclaredVariable is not defined
// (without strict mode, this silently creates a global variable instead)

• Assigning to an undeclared name throws a ReferenceError instead of creating an accidental global.
• Assigning to a read-only or non-writable property throws a TypeError instead of failing silently.
• Duplicate parameter names (function f(a, a) {}) are a SyntaxError.
• Inside a plain function called without an object (someFunction(), not obj.someFunction()), this is undefined rather than silently becoming the global object.

Common mistakes

• Assuming a modern codebase needs an explicit 'use strict' — if every file is an ES module (the default for most current tooling), it’s redundant; modules are already strict.
• Placing the directive after other code — it only takes effect as the very first statement.

Runtime notes

Strict mode is part of the ECMAScript standard, so its behavior is identical in browsers and Node.