---
title: Strict Mode
---
Strict Mode was introduced in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.

Strict mode makes several changes to normal JavaScript semantics.

*   First, strict mode eliminates some JavaScript silent errors by changing them to throw errors.
*   Second, strict mode 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.
*   Third, strict mode prohibits some syntax likely to be defined in future versions of ECMAScript.

Strict mode code and non-strict mode code can coexist in a same script.

```javascript
// Non-strict code...

(function(){
    "use strict";

    // Define your library strictly...
})();

// Non-strict code...
```

## Invoking strict mode

Strict mode applies to _entire scripts_ or to _individual functions_.

**Strict mode for scripts**

```javascript
// Whole-script strict mode syntax

"use strict";
var v = "Hi!  I'm a strict mode script!";
```

**Strict mode for functions**

```javascript
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."; }
```

**Basically it helps you make fewer errors, by detecting things that could lead to breakage which are not detected normally (non-strict mode).**

_For more information check out this <a href='https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Strict_mode' target='_blank' rel='nofollow'>MDN page</a>._