Design Meeting Notes, 4/2/2024

[DanielRosenwasser]

Set configurable: true in Downlevel Targets

#57784

• Jest, the testing framework, wants to be able to mock out modules in a way that works for basic CJS modules, but not for ESM-style CJS modules.
• Why wouldn't we do this?
  • esbuild, the bundler, does not do this.
  • ESM itself doesn't allow this, so it's strange to change our emit to do this.
• Jest and the like mock ESM in a different mechanism though.
• What does Babel do?
  • Babel makes them non-configurable.
• Same with Rollup.
• So we'd be the only one doing this?
  • Yes.
• Why does Jest need them to be configurable? Can't they just return a new object?
  • Possibly? Some stuff around objects. Maybe Jest should use a Proxy?
• Switching your TS emitter from TS itself to esbuild or Babel or whatever would break your tests. Kind of a footgun!
• If every other compiler works this way, it seems like testing libraries should find an alternative solution.
• Conclusion: we will not make this change.

Remove FlowNode and FlowFlags from Public API

#58036

• We want to change the control flow graph (Monomorphic flow nodes #57977) but it would kind of be a breaking change.
• We expose these types publicly, but nobody has a public way to access them without unsafely access the internals.
• Only known use is utilities for visualizing the control flow graph.
• Do it.

Types as Configuration

#58025

• This is only viable because we now have enough literal-i-ness in the type system.
• One of the concerns we have is that the semantics of TypeScript change from version-to-version.
• This is in an uncanny valley of "a superset of JSON but also a subset of valid TypeScript".
• But broadly, there's a good way to reframe what we want: imports and spreads.
• Let's find a way to bring that into tsconfig.json.