# unicorn/no-array-fill-with-reference-type

> Disallow filling an array with one object, array, or class instance

`fill()` evaluates its argument once and puts that same value in every slot. With an object or an array, all the slots point to one object, and a change through one slot shows up in all of them.

The rule reports an object literal, an array literal, a class, and any `new` expression except `new RegExp()`. It also reports a `const` variable that's declared with one of these. Functions and regular expression literals aren't reported.

{/* 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-fill-with-reference-type" = "error" # or "warn"
```

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

## Examples

Reported:

{/* bad */}
```js
const grid = new Array(3).fill([]);

grid[0].push("x"); // [["x"], ["x"], ["x"]]
```

{/* bad */}
```js
const empty = { visits: 0 };
const week = new Array(7).fill(empty);
```

Not reported:

{/* good */}
```js
const grid = Array.from({ length: 3 }, () => []);

grid[0].push("x"); // [["x"], [], []]
```

{/* good */}
```js
const week = Array.from({ length: 7 }, () => ({ visits: 0 }));
const totals = new Array(7).fill(0);
```