# strict

> Require or disallow `"use strict"` directives

Strict mode turns silent mistakes into errors. For example, assigning to a variable that was never declared throws instead of creating a global. ES modules and class bodies are always strict, so a `"use strict"` directive there does nothing. CommonJS files and plain scripts are only strict if they say so.

In a module, every `"use strict"` is reported, no matter which option you set. In other files, the option decides where the directive goes. By default a `.js` file is linted as a module and a `.cjs` file as CommonJS.

`--fix` removes directives that do nothing: in a module, in a class, in a function that's already strict, or written twice. It never adds one.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"strict" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"strict" = ["error", "safe"] # "never", "global", "function" or "safe"
```

{/* End of the generated section. */}

## Examples

Reported, because the file is a module:

{/* bad */}
```js
"use strict";

export function run() {
  save();
}
```

Reported, because a CommonJS file needs the directive at the top:

{/* bad {"filename": "server.cjs"} */}
```js
const { save } = require("./store.cjs");

save();
```

Not reported:

{/* good */}
```js
export function run() {
  save();
}
```

In a `.cjs` file:

{/* good {"filename": "server.cjs"} */}
```js
"use strict";

const { save } = require("./store.cjs");

save();
```

## Options

| Option | What it does |
| --- | --- |
| `"safe"` (default) | The same as `"global"` in a CommonJS file, and the same as `"function"` in any other script. |
| `"global"` | Require one directive at the top of the file, and report directives inside functions. |
| `"function"` | Require a directive in every outermost function, and report one at the top of the file. Use it for scripts that are joined with other scripts, where a directive at the top would apply to all of them. |
| `"never"` | Report every directive. |

With `"function"` in a `.cjs` file, functions nested in a strict function don't need their own directive:

{/* good {"filename": "widget.cjs", "options": ["function"]} */}
```js
function setup() {
  "use strict";

  function render() {
    draw();
  }
  render();
}
```

Here both the directive at the top and `setup` are reported:

{/* bad {"filename": "widget.cjs", "options": ["function"], "count": 2} */}
```js
"use strict";

function setup() {
  draw();
}
```

## When not to use it

If all your files are ES modules, there's nothing to decide. The rule then only catches a leftover `"use strict"`.