Why does ES6's Map.forEach iterate with 'value, key' instead of 'key, value'?

Question

As shown on MDN, Map's forEach callback is called with value first, and then key. E.g.:

map.forEach(function(value, key, map) { ... })

Seems like key, value is a lot more common than value, key. Even the Map constructor expects an array of [key, value] pairs.

I usually need the value only, and would end up writing something like .map(function(_, value) { ... }); — Matteo Tassinari
– Matteo Tassinari, Commented Oct 13, 2015 at 19:11
I don't now if it is more 'common', e.g. docs.angularjs.org/api/ng/function/angular.forEach — Betty St
– Betty St, Commented Oct 13, 2015 at 19:11
fyi Array.prototype.amp is described in ECMA-262 Edition 5 ecma-international.org/ecma-262/5.1/#sec-15.4.4.19 — Andreas Louv
– Andreas Louv, Commented Oct 13, 2015 at 19:12
I think the reason is that it better aligns with forEach method of the Array. — Sergey Rybalkin
– Sergey Rybalkin, Commented Oct 13, 2015 at 19:13
This was probably chosen for consistency with Array.prototype.forEach, whose callback function takes its parameters in the order value, index, where map items (obviously) are accessed by key rather than by index. — GOTO 0
– GOTO 0, Commented Oct 13, 2015 at 19:20

Mr. Llama · Accepted Answer · 2015-10-13 19:11:38Z

17

It's probably just for laziness sake. Most forEach loops will only care about the value itself. By supplying it as the first parameter, you can construct a function that only accepts one parameter:

map.forEach(function (value) { /* do something with value */; })

Instead of

map.forEach(function (unused, value) { /* do something with value */; })

answered Oct 13, 2015 at 19:11

Mr. Llama

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3 Comments

loganfsmyth Over a year ago

Or even map.forEach(val => val) vs map.forEach((key, val) => val)

ericsoco Over a year ago

Hm, yeah, that makes sense. It bit me though when I first used it recently, hence the SO question...

sehrgut Over a year ago

Yeah, by analogy with Array.map, whose callback takes value, index, array, it makes a lot of sense.

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