# no-var

> Require `let` or `const` instead of `var`

A `var` belongs to the whole function, not to the block it's written in, and you can read it before the line that declares it. `let` and `const` are scoped to their block and can't be used before they're declared, which turns a class of silent bugs into errors.

`--fix` rewrites `var` to `let` where that can't change what the code does.

This rule is on by default in TypeScript files.

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

## Configure

On by default as a warning. To turn it off:

```toml bunfig.toml
[lint.rules]
"no-var" = "off" # or "error"
```

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

## Examples

Reported:

{/* bad */}
```js
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i)); // 3, 3, 3
}
```

Not reported:

{/* good */}
```js
for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i)); // 0, 1, 2
}
```

`declare var` isn't reported, and neither is a `var` inside `declare global`, `declare module` or `declare namespace`. It runs no code. It tells TypeScript that a global exists, and only a `var` becomes a property of `globalThis` and can be declared in more than one file:

{/* good */}
```ts
declare var BUILD_ID: string;

console.log(globalThis.BUILD_ID);
```

With an ESLint config the rule reports what ESLint reports, so that a `// eslint-disable-next-line no-var` above a `declare var` is still needed: every `var`, except one directly inside `declare global`, which only ESLint 8 reports. With an oxlint config it reports what oxlint reports: `declare var`, but no `var` inside `declare global`, `declare module` or `declare namespace`.

{/* bad {"config": "oxlint"} */}
```ts
declare var BUILD_ID: string;
```