Stage 2 Draft / January 30, 2020

Object iteration (alternate spec text)

1Object.iterateKeys( O )

When the iterateKeys function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Return CreateObjectIterator(obj, key).

2Object.iterateValues( O )

When the iterateValues function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Return CreateObjectIterator(obj, value).

3Object.iterateEntries( O )

When the iterateEntries function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Return CreateObjectIterator(obj, key+value).

4Object Iterator Objects

An Object Iterator is an object, that represents a specific iteration over some specific instance object. There is not a named constructor for Object Iterator objects. Instead, Object Iterator objects are created by calling certain methods of the Object global.

4.1CreateObjectIterator ( obj, kind )

Several methods of the Object global return Iterator objects. The abstract operation CreateObjectIterator with arguments obj and kind is used to create such iterator objects. It performs the following steps:

  1. Let iterator be ObjectCreate(%ObjectIteratorPrototype%, « [[IteratedObject]], [[ObjectNextIndex]], [[ObjectIterationKind]] »).
  2. Set iterator.[[IteratedObject]] to obj.
  3. Set iterator.[[ObjectNextIndex]] to 0.
  4. Set iterator.[[ObjectIterationKind]] to kind.
  5. Return iterator.

4.2The %ObjectIteratorPrototype% Object

The %ObjectIteratorPrototype% object:

  • has properties that are inherited by all Object Iterator Objects.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %IteratorPrototype%.
  • has the following properties:

4.2.1%ObjectIteratorPrototype%.next ( )

  1. Let O be the this value.
  2. If Type(O) is not Object, throw a TypeError exception.
  3. If O does not have all of the internal slots of a Object Iterator Instance (4.3), throw a TypeError exception.
  4. Let obj be O.[[IteratedObject]].
  5. Let index be O.[[ObjectNextIndex]].
  6. Let itemKind be O.[[ObjectIterationKind]].
  7. If obj is undefined, return CreateIterResultObject(undefined, true).
  8. Let items be ? EnumerableOwnProperties(obj, kind).
  9. Let numItems be the number of elements of items.
  10. NOTE: numItems must be redetermined each time this method is evaluated.
  11. Repeat, while index is less than numItems,
    1. Let i be the Record that is the value of items[index].
    2. Set index to index + 1.
    3. Set O.[[ObjectNextIndex]] to index.
    4. Return CreateIterResultObject(i, false).
  12. Set O.[[IteratedObject]] to undefined.
  13. Return CreateIterResultObject(undefined, true).

4.2.2%ObjectIteratorPrototype% [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "Object Iterator".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

4.3Properties of Object Iterator Instances

Object Iterator instances are ordinary objects that inherit properties from the %ObjectIteratorPrototype% intrinsic object. Object Iterator instances are initially created with the internal slots described in Table 1.

Table 1: Internal Slots of Object Iterator Instances
Internal Slot Description
[[IteratedObject]] The object that is being iterated.
[[ObjectNextIndex]] The integer index of the next enumerable element to be examined by this iterator.
[[ObjectIterationKind]] A String value that identifies what is returned for each element of the iteration. The possible values are: key, value, key+value.

ACopyright & Software License

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© 2020 Jonathan Keslin

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