---
title: Rule strict
layout: doc
---
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# Strict Mode (strict)

A Use Strict Directive at the beginning of a script or function body enables strict mode semantics:

```js
"use strict";
```

When used globally, as in the preceding example, the entire script, including all contained functions, are strict mode code. It is also possible to specify function-level strict mode, such that strict mode applies only to the function in which the directive occurs:

```js
function foo() {
    "use strict";
    return;
}

var bar = function() {
    "use strict";
    return;
};
```

## Rule Details

This rule is aimed at using strict directives effectively, and as such, will flag any unexpected uses or omissions of strict directives.

### Options

There are three options for this rule:

1. `never` - don't use `"use strict"` at all
1. `global` - require `"use strict"` in the global scope
1. `function` - require `"use strict"` in function scopes only

### "never" mode

This mode forbids any occurrence of a Use Strict Directive.

The following patterns are considered problems:

```js
/*eslint strict: [2, "never"]*/

"use strict";          /*error Strict mode is not permitted.*/

function foo() {
    "use strict";      /*error Strict mode is not permitted.*/
    return;
}

var bar = function() {
    "use strict";      /*error Strict mode is not permitted.*/
    return;
};

foo();
bar();
```

The following patterns are considered valid:

```js
/*eslint strict: [2, "never"]*/

function foo() {
    return;
}

var bar = function() {
    return;
};

foo();
bar();
```

### "global" mode

This mode ensures that all code is in strict mode and that there are no extraneous Use Strict Directives at the top level or in nested functions, which are themselves already strict by virtue of being contained in strict global code. It requires that global code contains exactly one Use Strict Directive. Use Strict Directives inside functions are considered unnecessary. Multiple Use Strict Directives at any level also trigger warnings.

The following patterns are considered problems:

```
/*eslint strict: [2, "global"]*/

"use strict";
"use strict";           /*error Multiple "use strict" directives.*/

function foo() {
    "use strict";       /*error Use the global form of "use strict".*/

    return function() {
        "use strict";   /*error Use the global form of "use strict".*/
        "use strict";   /*error Use the global form of "use strict".*/

        return;
    };
}

foo();
```

The following patterns are considered valid:

```
/*eslint strict: [2, "global"]*/

"use strict";

function foo() {
    return function() {
        return;
    };
}

foo();
```

### "function" mode (default)

This mode ensures that all function bodies are strict mode code, while global code is not. Particularly if a build step concatenates multiple scripts, a Use Strict Directive in global code of one script could unintentionally enable strict mode in another script that was not intended to be strict code. It forbids any occurrence of a Use Strict Directive in global code. It requires exactly one Use Strict Directive in each function declaration or expression whose parent is global code. Use Strict Directives inside nested functions are considered unnecessary. Multiple Use Strict Directives at any level also trigger warnings.

The following patterns are considered problems:

```
/*eslint strict: [2, "function"]*/

"use strict";           /*error Use the function form of "use strict".*/

function foo() {        /*error Use the function form of "use strict".*/
    // Missing Use Strict Directive

    return function() {
        "use strict";   // Unnecessary; parent should contain a Strict Mode Directive
        "use strict";   /*error Multiple "use strict" directives.*/

        return;
    };
}

foo();
```

The following patterns are considered valid:

```
/*eslint strict: [2, "function"]*/

function foo() {
    "use strict";

    return function() {
        return;
    };
}

(function() {
    "use strict";

    return;
}());

foo();
```

### deprecated mode (Removed)

**Replacement notice**: This mode, previously enabled by turning on the rule without specifying a mode, has been removed in ESLint v1.0. `"function"` mode is most similar to the deprecated behavior, and has been made the default if no mode is specified.

This mode ensures that all functions are executed in strict mode. A Use Strict Directive must be present in global code or in every top-level function declaration or expression. It does not concern itself with unnecessary Use Strict Directives in nested functions that are already strict, nor with multiple Use Strict Directives at the same level.

The following patterns are considered problems:

```js
// "strict": 2

function foo() {
    return true;
}
```

The following patterns do not cause a warning:

```js
// "strict": 2

"use strict";

function foo() {
    return true;
}

// ----------------

function foo() {

    "use strict";

    return true;
}

// ----------------

(function() {
    "use strict";

    // other code
}());
```

## When Not To Use It

In a codebase that has both strict and non-strict code, either turn this rule off, or [selectively disable it](https://eslint.org/docs/user-guide/configuring) where necessary. For example, functions referencing `arguments.callee` are invalid in strict mode. A [full list of strict mode differences](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode/Transitioning_to_strict_mode#Differences_from_non-strict_to_strict) is available on MDN.

## Version

This rule was introduced in ESLint 0.1.0.

## Resources

* [Rule source](https://github.com/eslint/eslint/tree/master/lib/rules/strict.js)
* [Documentation source](https://github.com/eslint/eslint/tree/master/docs/rules/strict.md)
