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  1. JavaScript Objects & Arrays
  2. Introducing JavaScript objects
  3. Standard built-in methods to work with Arrays

Array.prototype.flatMap()

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Last updated 3 years ago

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The flatMap() method 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 followed by a of depth 1, but slightly more efficient than calling those two methods separately.

var new_array = arr.flatMap(function callback(currentValue[, index[, array]]) {
    // return element for new_array
}[, thisArg])

Alternative

reduce() and concat()

var arr = [1, 2, 3, 4];

arr.flatMap(x => [x, x * 2]);
// is equivalent to
arr.reduce((acc, x) => acc.concat([x, x * 2]), []);
// [1, 2, 2, 4, 3, 6, 4, 8]

Example:

let arr1 = [1, 2, 3, 4];

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

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

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