avoiding nested callbacks with promises

Question

I'm still new to using Promise APIs and I'm struggling how to avoid deeply nested Promise chains, which as far as I understand, are one of the benefits to using Promises. Using the following pseudo code as an example, how do you avoid nesting Promises when the subsequent ones rely on the context of prior ones?

function loadDependency1() {
    // return a promsise to load the first dependency
}

function loadDependency2(dependency1) {
    // return a promise to load the second dependency, which relies on the first dependency
}

function loadDependency3(dependency2) {
    // return a promise to load the third dependency, which relies on the second dependency
}

function doWork(dependency1, dependency2, dependency3) {
    // finally have all the things necessary to do work
}

// load all the dependencies and eventually doWork
loadDependency1().then(function(dependency1) {
    return loadDependency2(dependency1).then(function(dependency2) {
        return loadDependency3(dependency2).then(function(dependency3) {
            doWork(dependency1, dependency2, dependency3);
        });
    });
});

Answer 1

When you return a promise from then, it will resolve when that promise resolves:

So, if the next one only needs the previous one:

loadDependency1().then(function(dependency1) {
    return loadDependency2(dependency1);
}).then(function(dependency2) {
     return loadDependency3(dependency2);
}).then(function(dependency3) {
    doWork(dependency3);
});

Works if you need the third dependency.

If the dependencies are not dependent of each other:

Promise.all([loadDependency1(),loadDependency2(),loadDependency3])
.spread(function(dep1,dep2,dep3){
    doWork(dep1,dep2,dep3);
});

If you want to keep 'state' across the promise chain and are using a modern promise library such as Bluebird you can do:

loadDependency1().bind({}).then(function(dependency1) {
    this.dep1 = dependency1;
    return loadDependency2(dependency1);
}).then(function(dependency2) {
     this.dep2 = dependency2;
     return loadDependency3(dependency2);
}).then(function(dependency3) {
    doWork(this.dep1, this.dep2, dependency3);
});

If you're not (and you really should be :) ) You can .all you way around it:

loadDependency1().then(function(dependency1) {
    return [loadDependency2(dependency1),dependency1];
}).spread(function(dependency2,dep1) {
     return [loadDependency3(dependency2),dependency2,dep1];
}).spread(function(dependency3,dependency2,dependency1) {
    doWork(dependency1, dependency2, dependency3);
});