Start of main content## Recommended snippets

Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.

- Create a
`Set`

from each array to get the unique values of each one after applying`fn`

to them. - Use
`Array.prototype.filter()`

on each of them to only keep values not contained in the other.

```
const symmetricDifferenceBy = (a, b, fn) => {
const sA = new Set(a.map(v => fn(v))),
sB = new Set(b.map(v => fn(v)));
return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
};
```

```
symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
symmetricDifferenceBy(
[{ id: 1 }, { id: 2 }, { id: 3 }],
[{ id: 1 }, { id: 2 }, { id: 4 }],
i => i.id
);
// [{ id: 3 }, { id: 4 }]
```

### Mapped array difference

JavaScript, Array

Returns the difference between two arrays, after applying the provided function to each array element of both.

### 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, after applying the provided function to each array element of both.