Discussion on: 15 must-know JavaScript array methods in 2020

[Ibrahima Ndaw]

I've not tested with your array. You're right. Thanks for your comment.

[Michal Novák]

Sorry, but NOW it's incorrect - it was correct before. That's beacuse the flat part in flatMap() works differently, than .flat(1) alone, it only works for [[1], [2]], but not for [[1, 2]].

[[1, 2], 3, 4].flatMap(x => [x * 2])
// [NaN, 6, 8]

[[1, 2], 3, 4].map(x => [x * 2]).flat(1)
// [NaN, 6, 8]

See? flatMap will put the NaN there as well, try it for yourself.

I'd say it's either a bug in the implementation or on MDN description:

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It is identical to a map() followed by a flat() of depth 1, but flatMap() is often quite useful, as merging both into one method is slightly more efficient.

and later:

The flatMap method is identical to a map followed by a call to flat of depth 1.

Example on MDN:

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]]

Which translates to this one-liner:

[1, 2, 3, 4].map(x => [x * 2]).flat(1);
// [2, 4, 6, 8]

and not the other way around:

[1, 2, 3, 4].flat(1).map(x => [x * 2]);
// [[2], [4], [6], [8]]