Laurie

Posted on Jun 25, 2019 • Originally published at tenmilesquare.com

Understanding Array.prototype.flatMap

#javascript #webdev #learning #productivity

Last week we talked about the new flat method available in ES2019.

This week, we're going to dive into flatMap!

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.

Top comments (16)

Tyler V. (he/him) • Jun 25 '19

Is there a way to pass Infinity into flatmap()? Or would you need to do a Array.flatMap(itme => item).flat(Infinity) roundabout to get that effect? (I'm not even sure if this would be particularly useful in any scenario)

Laurie • Jun 25 '19

Not that I've seen. I think the way to do it would be the second example you listed.

Tanmay Rajani • Jun 26 '19

If I have more than one operations to do on an array, I subconsciously go to reduce everytime. It's better than remembering these separate functions I think. Not sure if it's the best option complexity-wise..

Laurie • Jun 26 '19

I think it depends. The Mozilla docs actually mention that for large arrays that option isn’t as efficient.

developer.mozilla.org/en-US/docs/W...

Tanmay Rajani • Jun 26 '19

Makes sense. Thanks for sharing!

lesha 🟨⬛️ • Jun 26 '19

I really hate the fact that libs like lodash are so much more useful than standard library and so happy that some of these features are making their way to standard library.

Hope they add mapValues/mapKeys/pickBy next. Having map/filter equivalents for objects is really convenient

Adam DuQuette • Jun 26 '19

Code school instructor here, I'd love to see a real-world example in your posts. The simple example is great for the intro, but having a more robust example to solidify might help. I am constantly getting that type of request from students, so I've gotten in the habit of trying to incorporate them during lecture.

That is to say, why are flat() and flatMap() being introduced? Why should a junior developer be excited about these?

Thanks : )