Let's start with map

In JavaScript arrays a have a built-in function called map. It takes a function and uses that function to operate on each item in the array. Something like this.

let arr = [1, 2, 3]

let result = arr.map((item) => item * 2);

result is then an array with each item from arr multiplied by two.

[2, 4, 6]

Another way to work with map

You can also use map to create a data structure inside each array element! Let's look at this example.

let arr = [1, 2, 3]

let result = arr.map((item) => [item, item * 2]);

In this case, our result is this.

[[1, 2],[2, 4],[3, 6]]

It's common to want to flatten this.

So you have

let arr = [1, 2, 3]

let result = arr.map((item) => [item, item * 2]).flat();

[1, 2, 2, 4, 3, 6]

But not with flatMap!

With the addition of flatMap this becomes simpler!

let arr = [1, 2, 3]

let result = arr.flatMap((item) => [item, item * 2]);

[1, 2, 2, 4, 3, 6]

It's the same thing.

Important

The default argument for flat is 1, NOT Infinity. This is important because flatMap is the same way. Let's look at an example.

let arr = [1, 2, 3]

let result = arr.flatMap((item) => [item, [item]]);

[ 1, [1], 2, [2], 3, [3] ]

The array is not fully flattened because flatMap only flattens one level.

Conclusion

flatMap is a great built-in function for this popular pattern! Check it out and let me know what you think.