@Pike, here's the finished PR migrating @fluent/syntax to TypeScript. Would you have a moment to take a look at it? I expect fluent-syntax/src/ast.ts will be the most interesting, but I'm looking for general feedback too. Thanks!

Thanks for the review, @Pike!

One thing I only realized in this patch, is the use of .ts files, but imports from .js files. That puzzled me, though VS Code doesn't have a problem resolving those.

TS doesn't touch thte from part of imports. Including .js makes the generated output (in esm/) work in <script type=module> situations. Node as well as Rollup would also accept extensionless paths, which is what most JS/TS projects generally do. FWIW I think this is a bad practice because it precludes browser compatibility from the get go.

Generally, does the introduction of [].includes change the browser compat matrix? Asking for Pontoon.

Good question. Array.prototype.includes was added in Firefox 43 (we support 52+) and Chrome 47 (we support 55+). Old Edge and Safari should also work.

And there are a few more left-over checks on node.type, I wonder if you want to convert them to instanceof checks, too?

Yeah, I think so. instanceof helps TS infer types, which improves the quality of the code and the editing experience.

And then maybe this one would be good to go through a feedback loop of a Pontoony, I just noticed that I don't know how they construct Fluent instances in the rich editor.

Good idea, I'll reach out.

I updated the PR to address your comments. I'd like to look again at the index signatures of Visitor and Transformer. I think the way I declare them right now would make it hard for consumers using TypeScript to keep state on the instances of subclasses. I'll fix this tomorrow.

I relaxed the index signature, documented both the Visitor and the Transformer, and also explicitly listed all possible visit${node} methods to help implementors get their type right.

I've verified in a local instance that @fluent/syntax built from this PR works in Pontoon. I'll merge and release on Monday.