JavaScript promise example

function add(xPromise,yPromise) {
    // `Promise.all([ .. ])` takes an array of promises, and returns a new promise that waits on them all to finish
    return Promise.all( [xPromise, yPromise] )

    // when that promise is resolved, let's take the received `X` and `Y` values and add them together.
    .then( function(values){
        // `values` is an array of the messages from the previously resolved promises
        return values[0] + values[1];
    });
}

// `fetchX()` and `fetchY()` return promises for their respective values, which may be ready
add( fetchX(), fetchY() )
// we get a promise back for the sum of those two numbers. now we chain-call `then(..)` to wait for the resolution of that returned promise.
.then( function(sum){
    console.log( sum ); // that was easier!
});

function fetchX(){
	return new Promise(function(resolve,reject){
		resolve(5);
	});
}


function fetchY(){
	return new Promise(function(resolve,reject){
		resolve(6);
	});
}