Function flatMap

Calls a defined callback function on each element of an array. Then, flattens the result into a new array. This is identical to a map followed by flat with depth 1.

Use Array.flatMap if possible or polyfill globally:

Array.prototype.flatMap = function(callback) {
    return flatMap(this, callback)
}
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Example

const arr1 = [1, 2, 3, 4]

arr1.map(x => [x * 2])
// [[2], [4], [6], [8]]

flatMap(arr1, x => [x * 2])
// [2, 4, 6, 8]

// only one level is flattened
flatMap(arr1, x => [[x * 2]])
// [[2], [4], [6], [8]]
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Tutorial

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