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I think it's a convention taken from math notation conventions.
I is short for "index" for a traditional for loop for mapping over an array and looking up by index. J comes after I and is used for nested loops so it doesn't shadow the outer I.
Is it for “index” or “iterator”?
index and jindex
jndex
yes
I believe index for the classical need to iterate through an array. E.g.
for (i = 0; I <= arr.length; i++) { var thing = arr[i] ... }
So to me it stands for "index" for array lookup.
Before map and iterators were implemented in a lot of languages, this was the defacto way to iterate a list. At least this is how I learned it in java/c back in the day. Nowadays I think most OOP languages including java have implemented the "for ... in ..." Syntax or similar which deprecates this convention.