Syntax highlighting breaks on optional chaining ?. or nullish coalescing operator ??

[rixo]

When JS is used as the lang for a Svelte component script, either explicitly (<script lang="js">) or implicitly (<script>), presence of ?. or ?? in the script breaks syntax highlighting or the part of the code after the script (with the markup apparently highlighted with JS rules).

For example, this snippet breaks:

<script>
  const x = a?.b
</script>

<div>Hello</div>

Or this one:

<script>
  const x = a ?? b
</script>

<div>Hello</div>

Oh? This even breaks in Github! What grammar are they using?

But with our new grammar this works (not on Github -- what the hell??):

<script lang="ts">
  const x = a ?? b
</script>

<div>Hello</div>

I have confirmed that changing source.js injections to source.ts fixes the problem, so that seems to indicate that the problem originates in the language-javascript grammar. Or maybe an incompatibility between something it contains and Atom's (and Github's?) implementation of TextMate grammars.

So an easy fix would be to change all injections of source.js to source.ts in our grammar. language-typescript is a core grammar bundled with Atom, so everyone should have it. But this fix would mean JS only users would noticeably get a different highlighting of their JS Svelte components and their standard .js files.

Would that be acceptable, at least as a temporary quick fix?