JavaScript: Array.prototype.flatMap

New in JS2019.

arrayX.flatMap(f)

Apply function to every element of the array then flatten 1 level. Return the new array. Original array is not changed.

The function f is passed 3 args:

1. currentElement
2. currentIndex
3. arrayX.

arrayX.flatMap(f, thisArg)

Use thisArg for this Binding of f. If it is not given, undefined is used.

const a1 = [[1,2],[[3],4]];
const a2 = a1.flatMap( (x => x[0]) );
console.log(a2);
// [ 1, 3 ]