What does "use strict" do and why should you use it?

Strict mode activates a restricted variant of JavaScript that converts silent errors into thrown errors and disables some confusing/dangerous features.

What it prevents:

• Implicit globals — x = 5 throws ReferenceError (not window.x)
• Duplicate parameter names — function fn(a, a) {} throws
• Writing to read-only properties — throws instead of silently failing
• this in standalone functions is undefined (not window)
• delete on variables/functions — throws
• Octal literals (0777) — throws

'use strict';

x = 5;             // ReferenceError — no more accidental globals
function fn(a, a) {} // SyntaxError

function test() {
  console.log(this); // undefined (not window)
}
test();

Good news: ES6 modules and classes are always in strict mode automatically. In modern code you rarely need to write 'use strict' explicitly.