Send NodeJS child process stdout to Express response object

Question

I currently pipe the stdout of a child process to an (Express) response object like this:

try{
    let proc = spawn(exe, args);
    proc.stdout.pipe(response);
    let exitCode = await ending.completion(proc);
}
finally {
    // my own clean up goes here (e.g. releasing locks)
}

where ending.completion is defined as

async function(process){
    return new Promise(function(resolve,reject){
        let done = false;
        process.on('exit', (code, signal)=>{
            if(!done){
                done = true;
                if(code !== null){
                    resolve(code);
                } else {
                    reject(signal);
                }
            }
        });
        process.on('error', (err)=>{
            if(!done){
                done = true;
                reject(err);
            }
        });
    });
};

This generally works fine except for the fact that proc internally does a lot of waiting (on the download/transfer) before it can complete/return. I would prefer allowing it to dump its output into a buffer, which then trickles into the response object as the download proceeds. How can I introduce such a buffer?

(A second (and I think related) problem is that if the download is aborted client-side, proc never completes but keeps waiting on its stdout.)

Answer 1

Looks like exec method does exactly what you need.

Spawns a shell then executes the command within that shell, buffering any generated output.

Use it instead of spawn. Note that unlike spawn, exec expects the whole command (with all the args) as a single string. Here is example from an officials documentation:

const util = require('util');
const exec = util.promisify(require('child_process').exec;

async function lsExample() {
  const { stdout, stderr } = await exec('ls');
  console.log('stdout:', stdout);
  console.error('stderr:', stderr);
}
lsExample();