Design Meeting Notes, 4/16/2024

[DanielRosenwasser]

The configDir Configuration Variable

#57485

• Configuration variable for directory of the current tsconfig.json
  • We are leaning towards configDir
  • We are also looking towards using ${ and } as the delimiters.
    • This is awful because lint rules.
• < and > avoid conflicts, but we kinda like.
• Why configDir? Why not projectDir?
  • Shrug.
• We like that it matches VS Code's style and matches intuitions JS template strings.

Correctly Check this Type Predicates

#57341

• We were not correctly checking this type predicates.

• This PR actually performs the checking.

• Unfortunate that this breaks:

  interface Animal {
    isDog() this is Dog;
  }
  
  interface Dog {
    isDog(): true;
  }

• Kind of weird that someone wrote this

  interface Animal {
    isDog() this is Dog;
  }
  
  interface Dog {
    isDog(): this is never;
  }

  • But this is wrong - type predicates have to be invariant.
    • Wait really?
    • Yes really.
    • Why?
    • Because type predicates returning false implies that the type should be narrowed to the negative cases.
    • If you had Poodle say this is Poodle, then returning false implies that you don't have a Dog if you have a BorderCollie.

• Is there a similar problem for assertion function?

• This seems correct.

Perf Suites Benchmarking Additions/Modifications

microsoft/typescript-benchmarking#32

• We are trying to fix up our perf suite to get representative tests.
• Also, moving away from internal testing. Huge pain to maintain.
• Need to put out a call for test cases.
  • Medium-sized (compiles between 10s to 30s)
  • Modern (works on the latest version)
  • Exhibits usage of diverse features
    • Module modes (nodenext, bundler, preserve)
    • JSX
    • Decorators
    • ...

noCheck (or skipCheck?)

• Will we be adding this as a --build option in the future?
• We will probably need it for ourselves.
• However, if you run this from the command line, there's no way to know how to do this incrementally.
  • So you'd have to have 2 tsconfig.jsons for each project.
• What exaclty is going wrong?
  • tsc -b --skipCheck outputs all .d.ts files.
  • Then when you run tsc -b, all the outputs are newer so it doesn't try to rebuild!
• We would need different metadata to be stored to make this work.
• Also: likely won't work with noEmitOnError.
  • Or at least - it may work awkwardly with noEmitOnError.
• On the CLI, we don't feel like the UX makes sense yet, but we want to add this internally.