Support `Array.prototype.at`

[bpasero]

There is a new method being added for Array: at

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at

I think typing this in a way that undefined is returned would allow a gradual adoption of the noUncheckedIndexedAccess rule:

at(index: number): T | undefined;

[MartinJohns]

I think typing this in a way that undefined is returned would allow a gradual adoption of the noUncheckedIndexedAccess rule:

I don't see how this is related to noUncheckedIndexedAccess. The function returns an explicit undefined if the index does not exist.

Also, please note that there is an issue template for library changes: lib_change.md

[bpasero]

I don't see how this is related to noUncheckedIndexedAccess.

Being in a codebase that would produce hundreds of compile errors when noUncheckedIndexedAccess is enabled, adopting Array.at would allow to gradually adopt this to the point where noUncheckedIndexedAccess could then be enabled by default.

I notice #36554 exists, should I merge my issue into there?

[SimonSchick]

You can always just add ! null assertions to your array indexing, and gradually remove it, at shouldn't be used as a migration strategy imo.

[bpasero]

👎 For me, using ! is like using any and I stay away from it unless there is really no other way.

[FelipeSharkao]

The biggest thing of Array.prototype.at is the possibility to use negative indexes. array.at(-1) is way clearer than array[array.length - 1].

[orta]

We chatted in the design meeting about this #45551, and agreed with @bpasero that it's a good call (and we debated techniques for typing the minus numbers but don't have an instant answer yet)

Also linking to #44656 which this should probably be a part of

[Ethan-G]

For impacts: I went to use .at() today only to discover it's not supported yet.

[jonlepage]

ho i lost so many time to try understand !

So It not yet added ! ?

[MartinJohns]

Also linking to #44656 which this should probably be a part of

@orta You probably mean #44571.