3

I have two asynchronous functions that need to be executed sequentially (the second one depends on the first one's result), and once they've executed, I need to use the results of both functions. In a "classical" callback based pattern, that's pretty easy:

http://jsfiddle.net/D6m8V/

function getOriginalString(callback) {
    callback("An original string");
}

function uppercase(x, callback) {
    callback(x.toUpperCase());
}

getOriginalString(function(orig) {
    uppercase(orig, function(upper) {
        console.log(orig, upper); //Access to both results
    });
});

I'm trying to redo this using Q promises. Here's what I got:

http://jsfiddle.net/3x6v3/2/

function getOriginalString() {
    var deferred = Q.defer();
    deferred.resolve("An original string");
    return deferred.promise;
}

function uppercase(x) {
    var deferred = Q.defer();
    deferred.resolve(x.toUpperCase());
    return deferred.promise;
}

getOriginalString()
.then(uppercase)
.then(function(result) {
    console.log(result);    
})
.done();

The problem is that I only have access to the result from the last function call, uppercase. I need to have access to the result from both functions, like in the callback based example.

What's the proper way of doing this with promises?

Alex Turpin
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1 Answers1

4

One option would be to use Q.all (Promise.all in ES6 promises and other libraries like Bluebird)

getOriginalString().then(function(orig){
    return Q.all([orig,uppercase(x)]);
}).spread(function(orig,upper){
    console.log(orig,upper);
});

You can of course still use a closure like the callback version (with nesting, or an outer scope).

Also, just to make sure, you don't need to promisify any function that doesn't make an I/O or otherwise asynchronous call.

Benjamin Gruenbaum
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