In plenary, some argued for passing a second index parameter to the Array.prototype-like methods' callbacks. I'm not entirely opposed to this but I'm unconvinced at the moment that this is an improvement. Also, I am really uninterested in adding separate "indexed" versions of each helper, as was mentioned. Here's how I see the pros/cons of each approach.

Motivation for not passing an index

• index is not useful for lookaround indexing pattern
  • besides, we will have better patterns for this that don't involve indexing, such as zipping
• indices are relative to in-progress iteration, not some indexable structure
  • it's easy to change the base of these indices when introducing a drop/filter/etc earlier in the chain
• avoids the common class of error that includes passing parseInt to Array.prototype.map
• if we don't continue passing index for non-Array.prototype methods (like zipWith, takeWhile, tap), we create a weird divide
• matches for-of not providing a counter
• implementations don't need to keep a counter

Motivation for passing an index

• familiarity from Array.prototype methods
  • though, no indexable third parameter passed, so easy to misuse for indexing an original indexable/iterable structure
• convenient: don't need to add an indexed() to the chain
• no intermediate tuples to create and GC
• 1 less method on Iterator.prototype

/cc participants: @js-choi @waldemarhorwat @ljharb @bakkot @erights @rbuckton @littledan