Make _.each/_.forEach support ES6 Set/Map

[benjycui]

IMHO, with the approaching of ES6, it is time to support new features. But I am not sure if you want it :-)
See also: #2147.

[coveralls]

Coverage increased (+0.01%) to 96.31% when pulling 9cd7d8d on benjycui:enhance-each into d04b7e9 on jashkenas:master.

[jdalton]

@megawac can you make the coveralls less chatty (there's an option in its config on the coveralls site).

@benjycui I don't think it's time to support new forms of iteration just yet. For example IE 11 doesn't support new Set([1, 2, 3]) for assigning values to a Set. I'm also not a fan of duck typing for iteration or adding more overloaded functionality to methods.

The easiest way to deal with this in your ES6 code is to convert your sets/maps to an array with Array.from or something like:

_.each([...set], function() {});

[benjycui]

@jdalton yep, you are right. We can convert Set/Map with ....

But for Map, this solution is inconsistent with the default behaviour of _.each:

Each invocation of iteratee is called with three arguments: (element, index, list). If list is a JavaScript object, iteratee's arguments will be (value, key, list).

var map = new Map([[1, 'one'], [2, 'two'], [3, 'three']]);
_.each([...map], function (kv) {  // I think this arguments list should be: `(value, key, list)`
  console.log(kv[0], ': ', kv[1]);
});

Do you think that consistency is worthy of 2 lines of source code? :-)

[jdalton]

But for Map, this solution is inconsistent with the default behaviour of _.each:

If you want the familiar arg sig then you can do
_.each(_.object([...map]), function() {})

Do you think that consistency is worthy of 2 lines of source code? :-)

It's not just 2 lines, you're essentially buying into the whole duck type thing which then requires modifying all other methods. Duck typing adds a bit of a wild card to the API too as you don't know how the duck typed method will really act. In this case you probably want a specialized method instead of ones designed for arrays and objects.

[jridgewell]

Do you think that consistency is worthy of 2 lines of source code? :-)

I agree with @jdalton here. Adding this means adding it to all collection-iteration functions. Additionally, this doesn't fix the arg signature for Set, since it passes (value, value, set) to the forEach callback.

[jdalton]

Closing for now.

[benjycui]

Thank you for your explanation :-)