What's the equivalent of Array.map for ES6 Map?

Question

So I can call map() on an array to iterate through it and return another array, like so:

[1,2,3].map( (value, index) => value * index )

What if I want to do the same over a Map object?

var myMap = new Map();
myMap.set(0, 1);
myMap.set(1, 2);
myMap.set(2, 3);

This isn't valid:

myMap.map( (value, index) => value * index )

There's a forEach but that's not the same.

Is there a quick shorthand way to generate an array from a Map, like map() does on an array?

– hopefully that sentence makes sense.

What do you expect it to return? You accept a key and an index, but return only a scalar value. Is it supposed to be an array as a result? — zerkms, Aug 28 '15 at 05:49
@zerkms same as the array example – iterates through each element of the map and returns an array of the computed values from the mapped function. resulting array should have the same number of elements as the original map. — Ben, Aug 28 '15 at 05:50

score 10 · Accepted Answer · answered Aug 28 '15 at 05:51

10

If the result must be an array then you can use the Array.from() and Map.prototype.entries():

Array.from(myMap.entries()).map(([key, value]) => key * value)

alternatively you could spread the iterator:

[...myMap.entries()].map(...)

answered Aug 28 '15 at 05:51

zerkms

Both seem to work well. The second option is pretty shorthand too :) – Ben Aug 28 '15 at 05:55
2

Map.map should totally be in the specs – Ben Aug 28 '15 at 05:57
@Ben it's not obvious what type should be returned then - another map or an array. – zerkms Aug 28 '15 at 05:58
Good point. Could either return an array with implicit index or a map with the original keys.. – Ben Aug 28 '15 at 06:17
They could also add `mapValues`, that keep the same key but map the values. – skylve Sep 07 '16 at 20:06
@skylve unless it requires lodash. – zerkms Sep 07 '16 at 20:11

1 Answers1