# unicorn/no-array-callback-reference

> Disallow passing a function by name to array methods such as `map` and `filter`

`map`, `filter`, and the other array methods call their callback with three arguments: the element, its index, and the array. A function that you pass by name receives all three. If it has an optional second parameter, or gets one in a later version of its library, the index ends up there.

The rule checks `every`, `filter`, `find`, `findLast`, `findIndex`, `findLastIndex`, `flatMap`, `forEach`, `map`, `some`, `reduce`, and `reduceRight`.

{/* 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]
"unicorn/no-array-callback-reference" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad */}
```js
const sizes = ["10", "10", "10"].map(parseInt); // [10, NaN, 2]
```

{/* bad */}
```js
import { formatUser } from "./format.js";

const lines = users.map(formatUser);
```

Not reported:

{/* good */}
```js
const sizes = ["10", "10", "10"].map((text) => parseInt(text, 10)); // [10, 10, 10]
```

{/* good */}
```js
import { formatUser } from "./format.js";

const lines = users.map((user) => formatUser(user));
```

Built-in constructors such as `Boolean`, `String`, and `Number` are allowed, and so is a function made with `.bind()`:

{/* good */}
```js
const present = values.filter(Boolean);
const labels = ids.map(String);
const rows = items.map(table.render.bind(table));
```

The rule can't tell what's an array. It skips calls on a property, such as `store.users.map()`, and on common library names, such as `_`, `lodash`, `$`, `jQuery`, `React`, and `Vue`:

{/* good */}
```js
const lines = store.users.map(formatUser);
const names = _.map(users, formatUser);
```