# no-undefined

> Disallow the use of `undefined` as a variable

`undefined` isn't a keyword. It's a global variable, and a function can declare a parameter or a local variable with the same name that holds something else. Code that never writes `undefined` can't be fooled by that.

The rule reports every place where `undefined` is read, assigned, or declared.

{/* 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]
"no-undefined" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad {"count": 2} */}
```js
let selected = undefined;

if (selected === undefined) {
  selected = first();
}
```

{/* bad */}
```js
function render(options, undefined) {
  return options.title;
}
```

Not reported:

{/* good */}
```js
let selected;

if (selected == null) {
  selected = first();
}

if (typeof selected === "undefined" || selected === void 0) {
  selected = first();
}
```

## When not to use it

The global `undefined` is read-only, and `no-shadow-restricted-names`, which is on by default, reports a local variable with that name. With that rule on, `x === undefined` is safe, and this rule is a matter of taste.