How do I access previous promise results in a .then() chain?

Break the chain

When you need to access the intermediate values in your chain, you should split your chain apart in those single pieces that you need. Instead of attaching one callback and somehow trying to use its parameter multiple times, attach multiple callbacks to the same promise – wherever you need the result value. Don’t forget, a promise just represents (proxies) a future value! Next to deriving one promise from the other in a linear chain, use the promise combinators that are given to you by your library to build the result value.

This will result in a very straightforward control flow, clear composition of functionalities and therefore easy modularisation.

function getExample() {
    var a = promiseA(…);
    var b = a.then(function(resultA) {
        // some processing
        return promiseB(…);
    return Promise.all([a, b]).then(function([resultA, resultB]) {
        // more processing
        return // something using both resultA and resultB

Instead of the parameter destructuring in the callback after Promise.all that only became avail­able with ES6, in ES5 the then call would be replaced by a nifty helper method that was provided by many promise libraries (Q, Bluebird, when, …): .spread(function(resultA, resultB) { ….

Bluebird also features a dedicated join function to replace that Promise.all+spread combination with a simpler (and more efficient) construct:

return Promise.join(a, b, function(resultA, resultB) { … });

Leave a Comment