# Mapped array symmetric difference

JavaScript, Array · Oct 22, 2020

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 }]