JavaScript - what does 'use strict' do ?

"use strict" directive is actually a literal expression that tells JavaScript that the code should be executed in a strict mode. It was presented in ES5 (ignored by earlier versions of JavaScript).

What strict mode does:

• you cannot use undeclared variables inside it,
• it changes previously accepted "wrong syntax" into errors,
• it doesn't allow mistyping a variable name (which outside the strict mode creates a new global variable),
• any assignment to a non-writable / getter-only / non-existing property or non-existing variable/object will throw an error.

So basically strict mode makes it easier to write secure code in JavaScript.