Zoltan Zoltan - 5 months ago 34
Node.js Question

How to promisify Node's child_process.exec and child_process.execFile functions with Bluebird?

I'm using the Bluebird promise library under Node.js, it's great! But I have a question:

If you take a look at the documentation of Node's child_process.exec and child_process.execFile you can see that both of these functions are returning a ChildProcess object.

So what's the recommended way to promisify such functions?

Note that the following works (I get a Promise object):

var Promise = require('bluebird');
var execAsync = Promise.promisify(require('child_process').exec);
var execFileAsync = Promise.promisify(require('child_process').execFile);

But how can one get access to the original return value of the original Node.js functions? (In these cases I would need to be able to access the originally returned ChildProcess objects.)

Any suggestion would be appreciated!


Here is an example code which is using the return value of the child_process.exec function:

var exec = require('child_process').exec;
var child = exec('node ./commands/server.js');
child.stdout.on('data', function(data) {
console.log('stdout: ' + data);
child.stderr.on('data', function(data) {
console.log('stderr: ' + data);
child.on('close', function(code) {
console.log('closing code: ' + code);

But if I would use the promisified version of the exec function ( execAsync from above ) then the return value will be a promise, not a ChildProcess object. This is the real problem I am talking about.


It sounds like you'd like to return two things from the call:

  • the ChildProcess
  • a promise that resolves when the ChildProcess completes

So "the recommended way to promisify such functions"? Don't.

You're outside the convention. Promise returning functions are expected to return a promise, and that's it. You could return an object with two members (the ChildProcess & the promise), but that'll just confuse people.

I'd suggest calling the unpromisified function, and creating a promise based off the returned childProcess. (Maybe wrap that into a helper function)

This way, it's quite explicit for the next person who reads the code.

Something like:

var Promise = require('bluebird');
var exec = require('child_process').execFile;

function promiseFromChildProcess(child) {
    return new Promise(function (resolve, reject) {
        child.addListener("error", reject);
        child.addListener("exit", resolve);

var child = exec('ls');

promiseFromChildProcess(child).then(function (result) {
    console.log('promise complete: ' + result);
}, function (err) {
    console.log('promise rejected: ' + err);

child.stdout.on('data', function (data) {
    console.log('stdout: ' + data);
child.stderr.on('data', function (data) {
    console.log('stderr: ' + data);
child.on('close', function (code) {
    console.log('closing code: ' + code);