Promisify breaks resolve

[reggi]

I have an example of tests passing (can't publish at the moment) with require.resolve and with resolve as a callback, but when I wrap resolve in const resolveAsync = Promise.promisify(resolve) for some reason the tests that worked are failing. I've also implemented my own wrapped promise and some of tests pass, and others fail, specifically cases where there's a package.json resolve.

Any ideas why this package is un-promisify-able, and potential ways around it?

[ljharb]

What promisify implementation are you using?

Since https://npmjs.com/util.promisify is built into node, that’s really the only implementation that matters.

If resolve can’t promisify by default, we could maybe add a custom promisify symbol.

Does it work with the canonical implementation?

[reggi]

I've been using Bluebird to promisify, but even an initial check here.

➜  ~ trymodule util.promisify resolve
Gonna start a REPL with packages installed and loaded for you
'util.promisify' was already installed since before!
'resolve' was already installed since before!
Package 'util.promisify' was loaded and assigned to 'util_promisify' in the current scope
Package 'resolve' was loaded and assigned to 'resolve' in the current scope
REPL started...
> require.resolve('./Desktop/jest-example/index.js')
'/Users/treggi/Desktop/jest-example/index.js'
> util.promisify(resolve)('./Desktop/jest-example/index.js').then(console.log)
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }
> (node:86567) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Cannot find module './Desktop/jest-example/index.js' from 'internal'
(node:86567) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Running node 9.5

[ljharb]

Unhandled promise rejection (rejection id: 1): Error: Cannot find module './Desktop/jest-example/index.js' from 'internal' - it seems like it's properly rejecting. Try adding .catch(e => console.error(e))?

[reggi]

Right above it I'm showing that the same path resolves with native resolve.

…

On Feb 22, 2018 8:42 PM, "Jordan Harband" ***@***.***> wrote: Unhandled promise rejection (rejection id: 1): Error: Cannot find module './Desktop/jest-example/index.js' from 'internal' - it seems like it's properly rejecting. Try adding .catch(e => console.error(e))? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#151 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AASHXnSxljP8-9qXB79SUBqt8WVYjfEnks5tXhdvgaJpZM4SP8to> .

[ljharb]

What happens with a non-promisified version of this module, however?

[reggi]

@ljharb It works perfectly fine

Let me explain this example:

➜  ~ trymodule util.promisify resolve
Gonna start a REPL with packages installed and loaded for you
'util.promisify' was already installed since before!
'resolve' was already installed since before!
Package 'util.promisify' was loaded and assigned to 'util_promisify' in the current scope
Package 'resolve' was loaded and assigned to 'resolve' in the current scope
REPL started...

Regular require.resolve works, and resolves this path.

> require.resolve('./Desktop/jest-example/index.js')
'/Users/treggi/Desktop/jest-example/index.js'

The resolve module callback returns the correct resolved path.

> resolve('./Desktop/jest-example/index.js', (err, value) => console.log(value))
undefined
> /Users/treggi/Desktop/jest-example/index.js

Node 9's native util.promisify returns a promise, calls it's .catch and logs the error.

> util.promisify(resolve)('./Desktop/jest-example/index.js').then(console.log).catch(e => console.log(e))
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }
> { Error: Cannot find module './Desktop/jest-example/index.js' from 'internal'
    at /Users/treggi/.trymodule/node_modules/resolve/lib/async.js:61:35
    at load (/Users/treggi/.trymodule/node_modules/resolve/lib/async.js:80:43)
    at onex (/Users/treggi/.trymodule/node_modules/resolve/lib/async.js:105:17)
    at /Users/treggi/.trymodule/node_modules/resolve/lib/async.js:26:73
    at FSReqWrap.oncomplete (fs.js:166:21) code: 'MODULE_NOT_FOUND' }

The util.promisify module returns a promise, calls it's .catch and logs the error.

> util_promisify(resolve)('./Desktop/jest-example/index.js').then(console.log).catch(e => console.log(e))
Promise {
  <pending>,
  domain:
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }
> { Error: Cannot find module './Desktop/jest-example/index.js' from 'internal'
    at /Users/treggi/.trymodule/node_modules/resolve/lib/async.js:61:35
    at load (/Users/treggi/.trymodule/node_modules/resolve/lib/async.js:80:43)
    at onex (/Users/treggi/.trymodule/node_modules/resolve/lib/async.js:105:17)
    at /Users/treggi/.trymodule/node_modules/resolve/lib/async.js:26:73
    at FSReqWrap.oncomplete (fs.js:166:21) code: 'MODULE_NOT_FOUND' }

[reggi]

Any idea what's going on here @ljharb?

[ljharb]

@reggi what happens if you call the promisified resolve with an empty object as the second argument?

[ljharb]

After looking into this a bit more, the issue is that the assumption that "promisify will work on everything" is flawed; it won't.

Using util.promisfy.custom, we could provide a custom implementation that would work with the native one (but not with bluebird) - but then this module would have to add a dependency on util.promisify and/or restrict which node versions it runs on.

In other words, I think the proper solution is to wrap resolve in new Promise if you want to promisify it.