---
url: /docs/guide/usage/linter/rules/eslint/prefer-const.md
---

### What it does

Requires `const` declarations for variables that are never
reassigned after their initial declaration.

#### Ignored Files

This rule ignores `.svelte` and `.vue` files entirely. Oxlint only parses the
`<script>` blocks of these files, so a binding that the template reassigns looks
like it is never reassigned, and turning it into a `const` makes the framework
compiler fail. In Svelte the template writes through `bind:this={el}` and
`bind:value={x}`; in Vue a `<script setup>` `let` is a `setup-let` binding that
`v-model="x"` and inline handlers such as `@click="x = 1"` assign to directly.

### Why is this bad?

If a variable is never reassigned, using the `const` declaration is better.
`const` declaration tells readers, "this variable is never reassigned," reducing cognitive load and improving maintainability.

### Examples

Examples of **incorrect** code for this rule:

```js
let a = 3;
console.log(a);

let b;
b = 0;
console.log(b);

for (let i in [1, 2, 3]) {
  console.log(i);
}
```

Examples of **correct** code for this rule:

```js
const a = 0;

let a;
a = 0;
a = 1;

let a;
if (true) {
  a = 0;
}

for (const i in [1, 2, 3]) {
  console.log(i);
}
```

## Configuration

### destructuring

type: `"any" | "all"`

Configures how destructuring assignments are handled.

#### `"any"`

Warn if any of the variables in a destructuring assignment should be `const`.

#### `"all"`

Only warn if all variables in a destructuring assignment should be `const`. Otherwise, ignore them.

### ignoreReadBeforeAssign

type: `boolean`

default: `false`

If `true`, the rule will not report variables that are read before their initial assignment.
This is mainly useful for preventing conflicts with the `typescript/no-use-before-define` rule.

## How to use

## Version

This rule was added in v1.43.0.

## References