[FIXED] How to use node's child_process.exec() with promises

Issue

I try to execute long processes sequentially with node.js (docker exec commands).

I do:

const childProcess = require('child_process');

const execWithPromise = async command => {
    return new Promise(async resolve => {
        const process = childProcess.exec(command);

        process.on('exit', err => resolve(err));
        process.on('close', err => resolve(err));
    });
};

const run = async () => {
    await execWithPromise('/usr/local/bin/docker exec -i -t cucumber node long-running-script.js');
    await execWithPromise('/usr/local/bin/docker exec -i -t cucumber node long-running-script.js');
};

run();

But the promise is resolved immediately with a result of 1. In both cases. The command runs on the commandline just fine.

Why is it returning immediately?

Solution

child_process.exec expects a callback as the second or third argument. It doesn't return a promise. You have a few choices depending on your use case and version of node.

Use a callback and return the resolve.

return new Promise(async resolve => {
     childProcess.exec(command, (err, stout, sterr) {
        resolve(err ? sterr : stout)
      }
  });

Use spawn instead (keeping most of your code)

const execWithPromise = async command => {
    return new Promise(async (resolve, reject) => {
        const process = childProcess.spawn(command);
        process.on('data', data => resolve(data));
        process.on('error', err => reject(err));
        process.on('close', err => reject(err));
    });
};

Use execSync with try catch

return new Promise(async (resolve, reject) => {
    try {
        resolve(childProcess.execSync(command));
    } catch(error) {
      reject(error) 
    }
});

Answered By - Peter Grainger

Answer Checked By - Robin (PHPFixing Admin)