Stage 0 Draft / October 23, 2025

Proposal Iterator Join

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1 Iterator.prototype.join ( separator )

This method converts the values of the iterator to Strings, and then concatenates these Strings, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

It performs the following steps when called:

  1. Let O be the this value.
  2. If O is not an Object, throw a TypeError exception.
  3. Let iterated be the Iterator Record { [[Iterator]]: O, [[NextMethod]]: undefined, [[Done]]: false }.
  4. If separator is undefined, then
    1. Let sep be ",".
  5. Else,
    1. Let sep be Completion(ToString(separator)).
    2. IfAbruptCloseIterator(sep, iterated).
  6. Set iterated to ? GetIteratorDirect(O).
  7. Let R be the empty String.
  8. Let first be true.
  9. Repeat,
    1. Let value be ? IteratorStepValue(iterated).
    2. If value is done, return R.
    3. If first is true, then
      1. Set first to false.
    4. Else,
      1. Set R to the string-concatenation of R and sep.
    5. If value is neither undefined nor null, then
      1. Let S be Completion(ToString(value)).
      2. IfAbruptCloseIterator(S, iterated).
      3. Set R to the string-concatenation of R and S.

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