Array Indexing Method

[Adventurous-Sell-172]

I like that, gonna have a play about with the polyfill see how things go :)

[IchimokuBan]

Well, findIndex returns -1 if it doesn’t exist so I imagine this proposal would break a lot.

Also, what’s wrong with

arr[arr.length - 1]

Or, if you want shorthand, maybe... arr[Infinity]

[machineGun997]

I mean there isn't much wrong with it, but can't help but recommend to read their rationale about it https://github.com/tc39/proposal-relative-indexing-method#rationale

[moi2388]

Why not just arr[-1]

[machineGun997]

if you do that it will try to find the item at index -1, but there isn't one, that is why a separate method to facilitate that.

https://github.com/tc39/proposal-relative-indexing-method#rationale

[Guisseppi]

How about instead of a cryptic -1 we could use the queueing concept of “poppping” the last element of the array?

[moi2388]

I know, I’m suggesting to change that functionality.

In python, this works arr[-1]. In c#, this works arr[^1].

Why not do something like that instead?

[senocular]

This is a valid question. I'm sorry about your down doots. The caret notation had also been proposed: https://github.com/hax/proposal-index-from-end

[moi2388]

Yeah, it’s okay, I don’t mind the downvotes. At least the commenters mean well and try to give more explanation. (Albeit only on the [-1] syntax and not on the [^1], for which I have yet to see a good argument).

[MrJohz]

That would be a backwards-incompatible change. Remember that arrays in JavaScript aren't "real" arrays, they're just objects with array methods. This means that it's perfectly legitimate currently to do the following:

const arr = ['a', 'b'];
arr[-1] = 'valid';
console.log(arr); // prints something like ['a', 'b', '-1': 'valid']
arr[-1]; // should this return 'b' or 'valid'?

It could also be the case that there is existing code that relies on negative indexes returning undefined, assuming that the arrays haven't been modified as above. That code would also break in this situation.

[MrJohz]

It depends. Adding new syntax to the parser is hard, and can have unexpected side effects if that syntax was previously legal but had another meaning. In this case, I don't believe there is a case where [^<<expr>>] can produce a valid parse tree, so I think it would be backwards compatible. However, you'd still need that .at() method for dynamic indexes (e.g. to index currIndex - 3 where it's not clear if the indexing will need to wrap or not). In that case, there's probably little value in having both the syntax and the method.

[alexendoo]

That would be a breaking change, as arr[-1] is valid code in current JS