[JS] Document the .at() proposal

[chrisdavidmills]

Acceptance criteria

1. • The listed features are documented sufficiently on MDN
2. • BCD is updated
3. • Interactive example and data repos are updated if appropriate
4. • The content has been reviewed as needed

Features to document

1. .at(). From some simple testing, it looks like this is supported in Chrome Canary (89) and Firefox Nightly (85), but not release versions yet (see https://bugzilla.mozilla.org/show_bug.cgi?id=1681371, for example). I still think it is worth documenting; I reckon we need pages for:

• String.prototype.at()
• Array.prototype.at()
• TypedArray.prototype.at()
• Anything else?

Related Gecko bugs

1. • Implement the .at() proposal

For folks helping with Firefox-related documentation features — make sure above AC have been done, but also:

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
• Add entry to Firefox experimental features page if feature is not yet enabled in release

[Rumyra]

Just reminding myself to update this page as well https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/@@unscopables

[Rumyra]

• Interactive examples for array, string & typedarray: see Adding at() examples interactive-examples#1753

This PR isn't passing test and I'm not sure why 🤔

• New docs
  • array
  • string
  • typed array
• Updating existing object pages

For above draft PR is here #1216

• BCD

I've added a PR here mdn/browser-compat-data#8721 however I used the proposal url as there is no spec url at this time... tests have not passed because of this and am unsure how to proceed

I'll do these once above are done/resolved

• @@unscopables
• Experimental features page

[Rumyra]

Added note to experimental features #1525

[Rumyra]

Added to @@unscopables #1526

[Rumyra]

content PR #1216

[Rumyra]

Apart from review of the content and dev-doc-complete flag, my todo lists are done for this @chrisdavidmills

[chrisdavidmills]

Yay, set to dev-doc-complete. I'll ask Florian about what to do for the spec thing.

[Rumyra]

I believe this is done \o/ everything should be merged... I'm moving it 🆒

[chrisdavidmills]

Woo hoo!