What does 'use strict' do?

It opts into a stricter variant of JavaScript that turns silent mistakes into errors and removes a few legacy behaviours.

"use strict";          // whole file — must be the first statement

function f() {
  "use strict";        // or just this function
}

What changes

• Undeclared variables throw. x = 5 without a declaration is a ReferenceError instead of silently creating a global — the single most useful change.
• Silent failures become errors. Assigning to a read-only or frozen property, or to a getter-only accessor, throws instead of doing nothing.
• this is undefined in a plain function call, rather than the global object. This is what makes forgetting new fail loudly.
• Duplicate parameter names are a syntax error.
• delete on a variable is a syntax error.
• Octal literals like 010 are rejected.
• with is banned, and eval cannot introduce variables into the surrounding scope.
• Some words are reserved for future use: implements, interface, private, public, static, yield.

You are probably already in it

ES modules and class bodies are always strict, with no directive needed. Any file with import or export, and anything inside a class, already runs this way. That is why most modern code never writes the directive — and why behaviour differs when you copy a snippet from a module into a plain <script>.

Why it exists

JavaScript could not fix its early mistakes without breaking the web, so strict mode was added as an opt-in. Newer features simply made it the default.

One practical note

Bundlers can concatenate files, so a directive at the top of one file could once affect others. That is why the function-level form exists, and why relying on file-level directives in a bundle is fragile. With modules, the question does not arise.