Promises returning undefined in node.js

Question

I'm kind of new to node.js/JavaScript so bear with me. I'm implementing a node module named module.js like this:

function run(cmd, arg) {
    var exec = require('child_process').exec;

    return new Promise(function(resolve, reject) {
        exec(cmd + ' ' + arg, function(error, stdout, stderr) { 
            if (error) 
                reject(error);
            else 
                resolve(stdout); 
        });
    });
}

function getCon1() {
    run('ls', '')
        .then(function(result) {
            return result;
        }, function(err) {
            console.log(err);
    });
}
exports.getCon1 = getCon1;

function getCon2() {
    run('ls', '-a')
        .then(function(result) {
            return result;
        }, function(err) {
            console.log(err);
    });
}
exports.getCon2 = getCon2;

And my main.js, where I'm using my module, looks like this:

var mod = require('./module');

console.log(mod.getCon1());
console.log(mod.getCon2());

My problem is that my two getter function return undefined in my main.js. For me, this is kind of irritating because I thought thats exactly what promises are for: The getter functions return in then() when the run() function completes.

Can anybody help me out here real quick? Thank you!

They return undefined because they don't have return statements. Promises don't make asynchronous code synchronous, they just give you an object you can pass around and attach callbacks to. — Quentin, Jan 14 '16 at 21:35
You're not returning anything from `getCon1` or `getCon2`, so of course that logs `undefined`. You should return the promise instead, like `function getCon1() { return run('ls', ''); }`. Then you can do `mod.getCon1().then( function ( result ) { console.log( result ); } );` outside the module. — Paul, Jan 14 '16 at 21:37
@Paulpro Thanks for you answer! This is working but I want to do more in `getCon1()`, kinda like this: `function getCon1() { run('ls', '') .then(function(result) { if (result.search(/\bstring I#n lokking for in output\b/) > -1) { return 'found string'; } }); }` — Since001, Jan 14 '16 at 21:55
@Since001 Your should still put `return` in front of `run('ls', '').then( ... )`, that way you are still returning a promise and whatever you return from inside the `then` will be available to the caller when they do `mod.getCon1().then( ...`. — Paul, Jan 14 '16 at 22:35
@Since001 Promise are chainable, because `then` returns a new promise, so you can do `run('ls','').then( function ( result ) { return next_value or promise_of_next_value } ).then( function ( next_value ) { return third_value or promise } ).then(...).then(...)`. In your case at some point in the chain you want to return from `getCon1` so that the rest of the chain can exist outside your module. You can have any number of `.then`s inside `getCon1`, but once you're using promises you have to stay in them, so you'll need to use `.then` outside the module too after calling `getCon1`. — Paul, Jan 14 '16 at 22:46
@Paulpro Oh my god, now I get it! Feeling a little bit stupid right now ;) Thank you very much! — Since001, Jan 14 '16 at 23:53

Promises returning undefined in node.js

0 Answers0