# @typescript-eslint/restrict-plus-operands

> Require both operands of `+` to be numbers, bigints, or strings

`+` accepts anything. An object or an array is turned into a string first, so `[] + 5` is `"5"` and `"User: " + user` is `"User: [object Object]"`. A `number` plus a `bigint` throws a `TypeError` when it runs.

By default this rule reports objects, arrays, symbols, `unknown`, `never`, and a `number` mixed with a `bigint`. Turn the `allow` options off to make it stricter.

`bun lint --type-aware` turns this rule on in TypeScript files.

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

## Configure

Off by default. This rule needs types, so turning it on makes `bun lint` type check your code first.

```toml bunfig.toml
[lint.rules]
"@typescript-eslint/restrict-plus-operands" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"@typescript-eslint/restrict-plus-operands" = ["error", {
  allowAny = true,
  allowBoolean = true,
  allowNullish = true,
  allowNumberAndString = true,
  allowRegExp = true,
  skipCompoundAssignments = false,
}]
```

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

## Examples

Reported:

{/* bad {"count": 3} */}
```ts
function describe(user: { name: string }, tags: string[], size: bigint) {
  const label = "User: " + user;
  const list = "Tags: " + tags;
  const next = size + 1;
}
```

Not reported:

{/* good */}
```ts
function describe(user: { name: string }, tags: string[], size: bigint) {
  const label = "User: " + user.name;
  const list = "Tags: " + tags.join(", ");
  const next = size + 1n;
}
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `allowAny` | `true` | Allow an operand of type `any`. |
| `allowBoolean` | `true` | Allow a `boolean` operand. |
| `allowNullish` | `true` | Allow an operand that can be `null` or `undefined`. |
| `allowNumberAndString` | `true` | Allow adding a `number` or a `bigint` to a `string`. |
| `allowRegExp` | `true` | Allow a `RegExp` operand. A `RegExp` plus a `number` is always reported. |
| `skipCompoundAssignments` | `false` | Don't check `+=`. |

With all the `allow` options off, both sides must have the same type:

{/* bad {"count": 3, "options": [{"allowAny": false, "allowBoolean": false, "allowNullish": false, "allowNumberAndString": false, "allowRegExp": false}]} */}
```ts
function report(count: number, done: boolean, name?: string) {
  const a = "Items: " + count;
  const b = "Done: " + done;
  const c = "Name: " + name;
}
```

`+=` is checked like `+`. This is reported unless `skipCompoundAssignments` is on:

{/* bad */}
```ts
let log = "";
log += ["started"];
```