Nice To Debug and Understand

// Legacy function with callbacks
function doShit(key, callback) {
    if(key == 1) {
        callback(null, 'SUCCESS');
    } else {
        callback(new Error('Not one'), null);
    }
}

// Consuming Legacy code
doShit(1, (error, val) => {
    if(error)
      console.log("Error occured and while doing shit. " + error);
    else
      console.log("Shit is done, horray :D.  " + val)
});

// Wrapping doShit function with Promise API
function doShitWithPromise(key) {
    return new Promise((resolve, reject) => {
        doShit(key, (error, val) => {
            if(error) reject(error);
            else resolve(val);
        });
    });
}

// Consuming new doShit function style with promise
var shitPromise = doShitWithPromise(2)
    .then((val) => console.log("Shit is done, horray :D.  " + val))
    .catch((error) => console.log("Error occured and while doing shit. " + error));