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 mon than value, key. Even the Map constructor expects an array of [key, value] pairs.

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 mon than value, key. Even the Map constructor expects an array of [key, value] pairs.

• javascript
• dictionary
• foreach
• ecmascript-6

• 1 I usually need the value only, and would end up writing something like .map(function(_, value) { ... }); – Matteo Tassinari Commented Oct 13, 2015 at 19:11
• I don't now if it is more 'mon', e.g. docs.angularjs/api/ng/function/angular.forEach – Betty St Commented Oct 13, 2015 at 19:11
• fyi Array.prototype.amp is described in ECMA-262 Edition 5 ecma-international/ecma-262/5.1/#sec-15.4.4.19 – Andreas Louv Commented Oct 13, 2015 at 19:12
• 2 I think the reason is that it better aligns with forEach method of the Array. – Sergey Rybalkin Commented Oct 13, 2015 at 19:13
• 4 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 Commented Oct 13, 2015 at 19:20

1 Answer 1

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 */; })