Implement optional chaining and nullish coalescing

a?.b → a && a.b Ok or fork?

[alangpierce]

Yep, agreed! I’ve been thinking about this more over the last few days since the TS release, and I’ll reopen this issue and repurpose it to built-in support in Sucrase. I’ll add some more detail when I get a chance.

Chrome Canary now has optional chaining available by default, and in Chrome Beta it’s available behind the “Experimental JavaScript” flag, so it’s already reasonable to use it in development with Sucrase (which will emit it unchanged) for running code in the browser. My team runs tests in Node, though, and it looks like Node 14 (to be released next year) will be the first version with support. We also lag behind the latest Node, so it will be a while before we have native support for the syntax in tests. It seems like a high-enough value syntax that it can be an exception to the “Sucrase is hesitant to implement upcoming JS features” rule.

Sucrase transforms require a lot more thought and care than Babel transforms since Sucrase makes the parser as minimal as possible and transforms code in a left-to-right scan, so it’s tricky, but I think there will be a way to make it work.

[alangpierce]

I worked through the details and different options by writing up a technical plan at https://github.com/alangpierce/sucrase/wiki/Sucrase-Optional-Chaining-and-Nullish-Coalescing-Technical-Plan . I’ll get started on the implementation soon.