Does it mutate 😱

Array.prototype.flatMap

No mutation

Description

The flatMap() method of Array instances returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. It is identical to a map() followed by a flat() of depth 1 (arr.map(...args).flat()), but slightly more efficient than calling those two methods separately.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap

Example

const arr1 = [1, 2, 1];

const result = arr1.flatMap((num) => (num === 2 ? [2, 2] : 1));

console.log(result);
// Expected output: Array [1, 2, 2, 1]