Strict mode

JavaScript's strict mode, introduced in ECMAScript 5, is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of "sloppy mode". Strict mode isn't just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don't rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

Strict mode makes several changes to normal JavaScript semantics:

1. Eliminates some JavaScript silent errors by changing them to throw errors.

2. Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that's not strict mode.

3. Prohibits some syntax likely to be defined in future versions of ECMAScript.

See transitioning to strict mode, if you want to change your code to work in the restricted variant of JavaScript.

// Whole-script strict mode syntax
'use strict';
var v = "Hi! I'm a strict mode script!";

Strict mode for functions

Likewise, to invoke strict mode for a function, put the exact statement "use strict"; (or 'use strict';) in the function's body before any other statements.

function strict() {
  // Function-level strict mode syntax
  'use strict';
  function nested() { return 'And so am I!'; }
  return "Hi!  I'm a strict mode function!  " + nested();
}
function notStrict() { return "I'm not strict."; }