Node: util.promisify does not work as expected with object methods

[simone-sanfratello]

• Version: 12.11.1 and 13.1.0
• Platform: Linux * 4.15.0-66-generic #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem: util

Hi all

I'm having an issue with util.promisify, trying to apply it to an object method, as follow

const net = require('net')
const util = require('util')

async function start () {
  try {
    const server = net.createServer()
    const listen = util.promisify(server.listen)
    await listen(9123)
    console.log('started')
  } catch (error) {
    console.error(error)
  }
}

start()

output is

TypeError: Cannot read property '_handle' of undefined
    at Server.listen (net.js:1383:12)
    at internal/util.js:277:30
    at new Promise (<anonymous>)
    at internal/util.js:276:12
    at start (/home/simone/Desktop/util-promisify.js:8:11)
    at Object.<anonymous> (/home/simone/Desktop/util-promisify.js:15:1)
    at Module._compile (internal/modules/cjs/loader.js:945:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:962:10)
    at Module.load (internal/modules/cjs/loader.js:798:32)
    at Function.Module._load (internal/modules/cjs/loader.js:711:12)

because in net.js:1383 there is

if (this._handle) {

while util.promisify do

original.call(this, ...args, (err, ...values) => {

so this at execution of listen function is no more its own instance, but a new context.

Reading the documentation, this is not reported, so I'm opening this issue - that I suppose to be a bug.

In my opinion, a possible solution could be adding an optional arg instance to util.promisify, like

util.promisify(original, [instance])

then, after checked that instance is an object and instance[original] exists, could be

original.call(instance || this, ...args, (err, ...values) => {

it could be called as

util.promisify(server.listen, server)

and works as aspected.

If you agree with this solution, I'd be glad to do that.

[lpinca]

You can use Function.prototype.call() on the function returned by util.promisify().

await listen.call(server, 9123);

or better use events.once(emitter, name) for your example.

[tniessen]

Regarding your proposed solution: That would mean that you would have to create a new promisified function for each function and for each instance. I think @lpinca's suggestion is generally preferrable over that.

[simone-sanfratello]

Please note that I'm not asking for help to solve my problem, I already did by writing my own promisify function.

I'm reporting that, imho, util.promisify does not work as aspected and there is no information about that in the documentation.

[lpinca]

util.promisify does not work as aspected.

Why not? You are promisifying a function on net.Server prototype and call that function without context. It's the same of doing:

const listen = server.listen;

listen(9123, callback);

[tniessen]

I agree with @lpinca, it is working as expected, that is just how JavaScript works.

Please note that I'm not asking for help to solve my problem, I already did by writing my own promisify function.

You can do that, or you can just use bind if you really want to make your promisifed functions specific to instances:

util.promisify(server.listen.bind(server))

[simone-sanfratello]

In case, I prefer this solution

    const server = net.createServer()
    const listen = util.promisify(server.listen.bind(server))
    await listen(9123)

Don't you think this should be in the documentation?

[tniessen]

That seems to be exactly what I suggested 👍

[simone-sanfratello]

Submitting in the same time :)

[tniessen]

Don't you think this should be in the documentation?

It is just how JavaScript works, but since it is apparently causing confusion, I would be okay with briefly mentioning this in the documentation.

[simone-sanfratello]

Thank you

[jordanbtucker]

You'd probably want something like this.

const server = net.createServer()
server.listen(9123)
await events.once(server, 'listening')

Otherwise, your app might hang if the listening event never fires due to an error, e.g. something's already listening on the same port.