Is anyone else concerned about .toArray() working differently in the new version? I would need to change 100s of places to call .toList().toArray() instead of just .toArray() like in 3.x.

#1340 indicates that this is likely to be a change that sticks. The justification is to more generally make the behavior of Immutable.Map more closely match that of native JS Map, where tuples are the norm for how to represent key value pairs. For example:

const nativeMap = new Map([['key', 'val']]);
// Map(1) {"key" => "val"}
Array.from(nativeMap);
// [["key", "val"]]

From that PR, the recommended code transform is x.toArray() -> x.valueSeq().toArray() (which will be more efficient than x.toList().toArray()).

This is definitely a big breaking change and should be loudly called out in the final release notes for 4 stable.

Ok :-( I suppose if React accepted the results of a call like

<div>
  {Map({
    name: 'Andy'
  }).map((item, key) => <div key={key}>Something</div>)}
</div>

Then it would be no problem. We could just remove the code :)

About the toArray in the first post, I think it is caused by the following code in src/CollectionImpl.js:

mixin(Collection, {
  // ### Conversion to other types

  toArray() {
    assertNotInfinite(this.size);
    const array = new Array(this.size || 0);
    const useTuples = isKeyed(this);
    let i = 0;
    this.__iterate((v, k) => {
      // Keyed collections produce an array of tuples.
      array[i++] = useTuples ? [k, v] : v;
    });
    return array;
  },

The question is that shall we fix that part of code? Personally I think the immutable v-4's result seems more sensible. @acusti

The change from Map.toArray => [...values] to [...[key, value]] is more familiar to how ES6 Map works and I think it's change that should stay.

The case of changing toArray() to toList().toArray(), could be fixed via some sort of codemod?

Sounds good. Should I close this?

@richtera yes, please close this