Strict Mode

Strict mode tells JavaScript to follow safer rules.

It catches quiet mistakes and turns them into clear errors.

Think of a teacher who asks you to write carefully.

Turn it on

Put "use strict"; at the top of a file or function.

Stops accidental globals

In sloppy mode, assigning to a missing name can create a global.

In strict mode, that is an error.

Cleaner function rules

Strict mode blocks some confusing function tricks.

Duplicate parameter names are not allowed.

this stays undefined in plain functions

In a normal function call, this is undefined in strict mode.

That prevents accidental use of the global object.

Modules are strict already

JavaScript modules run in strict mode by default.

Many project files already get these safer rules.

Tip: Keep "use strict" at the top while you learn. It turns sneaky bugs into loud lessons.