Node.js: Object value as stdout of child_process.exec

Question

Node.js: Object value as stdout of child_process.exec

I'm new to Node and stumbling on some of the nonblocking elements of it. I'm trying to create a object and have one of the elements of it being a function that returns the stdout of a child_process.exec, like so:

var exec = require('child_process').exec;
var myObj = {};

myObj.list = function(){
  var result;
  exec("ls -al", function (error, stdout, stderr) {
     result = stdout;
  });
  return result;
}
console.log('Ta da : '+myObj.list);

I figure that myObj.list is returning result before it is set as stdout, but I can't figure out how to make it wait or do a callback for it. Thanks for your help!

node.js
nonblocking
child-process

Answer 1

You can't directly return the value as it's not going to be available for a bit. So instead of a return value you need to use a callback which means turning the calling code inside out a bit.

var exec = require('child_process').exec;
var myObj = {};

myObj.list = function(callback){
  var result;
  exec("ls -al", function (error, stdout, stderr) {
     callback(stdout);
  });
  // No return at all!
}
// Instead of taking a return we pass a callback
// which receives the value and carries on our computation.
myObj.list(function (stdout) {
   console.log('Ta da : '+ stdout);
});

In real code you'd probably want to have your callback take an error as its first argument, you don't have to but it's the normal way things are done in Node.JS.