Start of main content## More like this

Returns every element that exists in any of the two arrays at least once, using a provided comparator function.

- Create a
`Set`

with all values of`a`

and values in`b`

for which the comparator finds no matches in`a`

, using`Array.prototype.findIndex()`

.

```
const unionWith = (a, b, comp) =>
Array.from(
new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
);
```

```
unionWith(
[1, 1.2, 1.5, 3, 0],
[1.9, 3, 0, 3.9],
(a, b) => Math.round(a) === Math.round(b)
);
// [1, 1.2, 1.5, 3, 0, 3.9]
```

### Mapped array union

JavaScript, Array

Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.

### Array intersection based on function

JavaScript, Array

Returns the elements that exist in both arrays, using a provided comparator function.

### Array union

JavaScript, Array

Returns every element that exists in any of the two arrays at least once.