Promise chains are flattened

[wesleytodd]

async function execThing (args) {
  const cmd = await getCmd()
  return execa(cmd, args)
}

const proc = await execThing(['--version'])
proc.stdout.pipe(process.stdout) // fails because proc.stdout is a string

Since a promise resolved with another promise will wait for the underlying promise to resolve, we are unable to pass the return of an execa call from a async function or resolve a promise with it. In this case it waits to unravel the promise chain.

In this example it crashes because proc.stdout is a string because the promise chain has be resolved. I am not sure how we would resolve this with just the current api surface. It seems like the most simple solution would be to add an option extendPromise: false to disable this behavior.

Is there a way to get this behavior with the current API I am not seeing? Thanks for the help in advance 😄

[ehmicky]

We previously discussed with @sindresorhus that the fact that the returned value is both a child process and a promise might be an issue, for several reasons including your case.

Our conclusion was that changing it was unfortunately too much of a breaking change at this point. @sindresorhus Please let us know if I got this wrong.

A workaround is to wrap the return value in a plain object:

async function execThing (args) {
  const cmd = await getCmd()
  const proc = execa(cmd, args)
  return { proc }
}

const { proc } = await execThing(['--version'])
proc.stdout.pipe(process.stdout)

[wesleytodd]

I guess I will have to change the API exposed in my use case. If there is an ongoing discussion on this I would love to put in a vote that this be reconsidered if there is a reasonable proposal for alternate behavior. Do you have a link to there it was discussed?

[ehmicky]

@wesleytodd Absolutely. The discussion was in #351, feel free to jump in.