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.

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 */; })