Add get/set for TypedArrays.

[derekdreery]

This provides the ability to do

let value = array[idx];
array[idx] = value;

for typed arrays. They are named with an underscore so thay do not clash with the set method, which does something different.

[alexcrichton]

Thanks for the PR! This feels a bit odd though with the trailing underscores? Is there perhaps a different way we could expose this?

[derekdreery]

Hmm, looking at the mdn page for indexed collections, it seems that currently the two main types for this are Array and the various typed arrays. So we can either bikeshed some names like get_array or get_indexed etc., or have a trait called IndexedArray (bikesheddable) with methods get and set. The js_sys::Array type just has the get and set methods already defined, but we can't do that on typed arrays because the set method means something else. Unfortunately, the typed arrays don't inherit from Array, so we can't just deref to Array either.

I don't think there is an obvious optimal solution, but I think personally I'd go for something like get_array_like, with documentation explaining the situation (e.g. saying

equivalent to val = arr[idx] on the javascript side

). I'll implement this in the PR, but am happy to change it to something else if preferred.

[Pauan]

I'd be in favor of get_index and set_index.

[derekdreery]

Actually thinking about it again, I agree.

[alexcrichton]

Thanks!