Thread by @jorendorff

I have learned the most wonderful thing about parsing JS.

The other day I was chatting with @loganfsmyth about JS parsers. What if, I said, we made a fast JS parser that could compile to wasm? Could any existing JS-ecosystem projects use that? What about Babel? It must have a parser of some kind. And he's like “ok, SO...”

It turns out Babel has to support not only the syntax of JS—which is already pretty special—but all the syntax of TypeScript, Flow, and JSX. Flow in particular is… a little odd!

var foo = doThing<Foo>(x);

is valid JS code and valid Flow code. But with two different meanings! In Flow, it's a call to a generic function. Bahaha. Only if it appears after a `// @flow` comment, though 😂

there's a COMMENT
that SWITCHES THE GRAMMAR

It gets even better when you cross the streams, use two extensions at once :D

<@loganfsmyth> e.g. `var foo = <T>(a = 1 + 2 + 3, b = a)`, you can't know if that's the first piece of a Flow generic arrow function, or a JSX element with some text until you find a `</T>` or a `=>`

unbounded lookahead IN THE TOKENIZER lolololol 🤣 💃💃💃

Because I work inside a browser engine, I miss all this exciting nonsense going on so close by, right in the same ecosystem! It's amazing.