1285453 - for-of of Objects is not described

Status: RESOLVED FIXED

(Developer Documentation Graveyard :: JavaScript, defect, P5)

Description

[ron]

:: Developer Documentation Request

      Request Type: New Documentation
     Gecko Version: unspecified
 Technical Contact: 

:: Details

All kinds of specific classes (iterators) and arrays are shown in examples. But how for-of behaves on plain objects is not demonstrated. Is there a way to get both key and value in the iteration? In other words: how does Object iterate?

Comment 1

[Tooru Fujisawa [:arai]]

for-of iterates over "iterable object" [1], so plain Object without iterable protocol is not iterable with for-of (it will just throw an exception)
Object.entries [2][3] will help you iterating over plain object's key-value pairs, but it's still experimental and not-available on some browsers.

maybe we could add an example with minimal iterable object there.

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#iterable
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/entries
[3] https://tc39.github.io/ecma262/#sec-object.entries

Comment 2

[Tooru Fujisawa [:arai]]

Added an example with iterable protocol.

  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of#Iterating_over_other_iterable_objects

does it make sense?  or do you need any other example?

Comment 3

[ron]

Great, thanks for the quick clarification and doc improvement. I was under the false assumption that normal Object instances had a default iterator, allowing one to simply for-of on { a: 1, b: 2 }. I wonder why that isn't possible in the first place though, it would be a very nice default.

Comment 4

[Tooru Fujisawa [:arai]]

adding default iterator will break existing web apps, as it could be observable via property access in other places.
When Object.entries becomes non-draft spec (maybe next year?) and available on all browsers, you can iterator with the following code:

  for (let [key, value] of Object.entries(obj)) ...