bun-types: infer strict Subprocess from Bun.spawn() options

[paperdave]

This refactors the types around Bun.spawn to properly give strict types, easing development with TypeScript.

Before and After

This is done by changing Subprocess<IO> to Subprocess<In, Out, Err>, where those three parameters are the values passed. In my opinion this also makes that interface easier to use, since you can do

function readData(process: Subprocess<any, "pipe", any>) {
  process.stdin  // FileSink | undefined
  process.stdout // ReadableStream
  process.stdout // ReadableStream | number | undefined
}

function writeData(process: Subprocess<"pipe">) {
  process.stdin  // FileSink
  process.stdout // ReadableStream | number | undefined
  process.stdout // ReadableStream | number | undefined
}

function doSomething(process: Subprocess) {
  process.stdin  // FileSink | undefined
  process.stdout // ReadableStream | number | undefined
  process.stdout // ReadableStream | number | undefined
}

and then it only accepts the process when it was spawned with stdout: "pipe"

Transformations from the Readable and Writable (unions of allowed options) to the properties that you see on Subprocess and SubprocessSync are in the following utility types:

• ReadableToIO
• ReadableToSyncIO
• WritableToIO
• OptionsToSubprocess
• OptionsToSyncSubprocess

These are all located in the SpawnOptions namespace.

[paperdave]

i realized my commit introduced a lot more failing type tests, just fixed those.

i also added ts-ignore to this line to fix the test error "Export declarations are not permitted in a namespace.", considering that it works fine in the intellisense. if i shouldn't have done this i can remove that.

the remaining tsd errors are from child_process and tcp stuff.

[paperdave]

One unideal thing ive discovered is that these two types should be the same and assign to one another, but do not:

• Subprocess<"ignore", "ignore", "ignore">
• Subprocess<"inherit", "inherit", "inherit">

not sure off the top of my head how that would be fixed. do we want that to be something?

---

another idea i had would be to add some helper types for certain types of subprocesses, which you may see where i spotted the above

type ReadableSubprocess = Subprocess<any, 'pipe', 'pipe'>;
type WritableSubprocess = Subprocess<'pipe', any, any>;
type PipeSubprocess = Subprocess<'pipe', 'pipe', 'pipe'>;
type NullSubprocess = Subprocess<'ignore' | 'inherit', 'ignore' | 'inherit', 'ignore' | 'inherit'>;

should i throw these extra types in for ease of use?

[paperdave]

about the ignore inherit thing, one solution could be to change Subprocess to accept either a strict string like "pipe" | "fd" | "null" -> Subprocess<"null", "pipe", "pipe"> for the three ways the resulting io property can be.
other idea would be to pass those types directly like Subprocess<undefined, ReadableStream<Buffer>, ReadableStream<Buffer>>; i like the strings approach a bit more, as it's a more concise when you hover over a subprocess variable, and its much less to type.

might try that out on tuesday or wednesday when i'm free

[colinhacks]

Gorgeous ❤️