@codemodsquad/asyncify

Transforms promise chains into async/await. I wrote this to refactor the 5000+ .then/.catch/.finally calls in the sequelize codebase. This is slightly inspired by async-await-codemod, but written from scratch to guarantee that it doesn't change the behavior of the transformed code, and keeps the code reasonably tidy.

Usage

npx @codemodsquad/asyncify path/to/your/project/**/*.js

This command just forwards to jscodeshift, you can pass other jscodeshift CLI options.

Support table

	asyncify
Converts .then	✅
Converts .catch	✅
Converts .finally	✅
Renames identifiers in handlers that would conflict	✅
Converts promise chains that aren't returned/awaited into IIAAFs	✅
Converts return Promise.resolve()/return Promise.reject()	✅
Removes unnecessary Promise.resolve() wrappers	✅
Warns when the original function could return/throw a non-promise	Planned
Refactoring/inlining handlers that contain conditional returns
All but one if/else/switch branch return	✅
All branches return, even nested ones	✅
All but one nested if/else/switch branch return	🚫
More than one if/else/switch branch doesn't return	🚫
Return inside loop	🚫

Warnings

Comments can sometimes get deleted due to an impedance mismatch between @babel and recast ASTs. If you use the --commentWorkarounds=true option it will try to prevent more comments from getting deleted but it sometimes causes an assertion to fail in recast.

There are a few edge cases where asyncify produces funky output. It's intended to not break any existing behavior (I know of no cases where it does, and I have fixed several such issues) but sometimes the output will be be semantically wrong even if it behaves correctly. For example, I've seen a case where doing an async operation several times in a row:

it('test', () => {
  const doSomething = () => {
    // ...
  }

  return doSomething()
    .then(doSomething)
    .then(doSomething)
})

Gets converted to:

it('test', async () => {
  const doSomething = () => {
    // ...
  }
  await doSomething(await doSomething(await doSomething()))
})

This works even though it initially seems like it wouldn't and is obviously not what you want:

it('test', async () => {
  const doSomething = () => {
    // ...
  }
  await doSomething()
  await doSomething()
  await doSomething()
})

Although I could possibly fix this for cases where it's easy to determine that the function has no parameters, there could be cases where it's impossible to determine whether the identifier doSomething is even a function or whether it has parameters.

Disabling recast workaround

At the time I wrote asyncify, there were some show-stopping bugs in old version of recast that jscodeshift depended on. To avoid this problem, asyncify parses with a newer version of recast in its own dependencies, instead of parsing with the jscodeshift API. The author of putout has asked to be able to parse with the injected jscodeshift API for performance, so you can access that version of the jscodeshift transform as:

import transform from '@codemodsquad/asyncify/noRecastWorkaround'

Or there are two ways you can do it when running via jscodeshift:

jscodeshift -t path/to/asyncify/noRecastWorkaround.js
jscodeshift -t path/to/asyncify/index.js --noRecastWorkaround=true