Design Meeting Notes, 8/11/2020

[DanielRosenwasser]

/// Comments

(10 minutes)

#39930

• Idea is using single-line 3-slash/triple-slash /// comments instead of /**
• We might be the biggest processor of JSDoc
• JSDoc also has a /// comment plugin supported since 2011.
• Orta submitted an issue, JSDoc team more or less said "won't add it to core because VS Code won't support it" (!!!???)
• Community Support
  • Deno wanted to add it for itself.
  • A couple of votes in TSDoc, team agrees it's a better design.
• Why are we leading on this one?
  • Not one specific
• Need a thing on TSServer to specify preferences
• What about /// <reference /> comments?
  • Scanner throws out directive comments.
• What about directive comments like /** @jsx */?
  • But ts-check is single line?
  • No, some directives require multiline comments.
    • We're just copying Babel's logic there.
    • Don't change it.
• TmLanguage needs updates
• Need community updates
  • JSDoc
  • TSDoc
  • WebStorm
  • Closure
  • Others? (sorry!)
• What about XML comments? C# converts might expect XML.
  • Probably won't do this.
• Multiple ways to do something is not ideal.
  • Also, lint rules can help pick up slack here.
• Work with other stakeholders in community.

WASM Imports

• Example with AssemblyScript
  • Not showing Rust because their WASM support is super good.
  • WasmPack generates foo.wasm.d.ts
• What happens with numbers?
  • i32, i64, f32, f64
  • Proposals for expanded types.
  • But all of them are just number to JavaScript, and those numbers get coerced.
• Implementation takes the WASM and generates a virtual .d.ts file that gets type-checked against.
• How do you report errors on these?
  • Unclear.
• Go to definition?
  • Virtual file is the definition.
  • But also, can read DWARF info to get the real source in some cases.
• Meta-point: our APIs were hard to work with.
  • readFile gets a string. WASM is not a string!
  • readFileBuffer 😣
• Also problems with incremental updates.
• Unclear whether this should just be a standalone tool or an integrated thing.
• No reason it can't start out as a build tool.
• Definitely want to continue exploring this space.

#39784

Recursive Conditional Types

• If you want to write the Awaited type today, you can only do it with indexed access type hacks

• But it is ugly.

  • But people are writing this in Stack Overflow answers.

• We even use the same trick for Array.prototype.flat()

• When we started, we intentionally limited recursive conditional type by forcing eager evaluation.

  • But could still use indexed access type trick. In the last few years, we've been hardening the compiler against run-away recursion. In a sense, these wacky indexed access types gave us early feedback on how to implement conditional recursive types correctly.

• So adding recursive conditional types is trivial - we more or less just stop eager evaluation of any given branch of a conditional type. We now we defer until instantiation time.

• Awaited can be written as

  type Awaited<T> =
    T extends null | undefined ? T : // for non-strictNullChecks
    T extends PromiseLike<infer U> ? Awaited<U> :
    T

• The thing about Awaited is that every method that passes the await-ed value ought to use Awaited - but that breaks a ton of stuff!

• Let's get a PR out and get some community feedback!

Remove Optionality for Promise's resolve callback

#39817

• Found some issues where in JavaScript, expecting some optionality
• Unclear what behavior should be.