How Can I Access Intermediate Promise Results in a Flat Promise Chain?

Accessing Intermediate Promise Results in a Flat Promise Chain

In order to retrieve intermediate promise results within a flat promise chain, it is necessary to break the chain apart into separate segments.

Promise Combinators

Instead of relying on the parameter of a single callback to obtain intermediate values, it is recommended to utilize promise combinators to create the desired composite value. This approach ensures a clear and structured control flow, making modularization straightforward.

Example

Consider the following example:

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
    });
}

In this example, the promise combinator Promise.all is used to aggregate the results of a and b. The callback that follows Promise.all can then access and utilize both resultA and resultB to construct the composite value.

Helper Methods

Libraries like Q, Bluebird, and when provide helper methods such as .spread to simplify the processing of multiple promise results in ES5.

…
return Promise.all([a, b]).then(function(results) {
    results.spread(function(resultA, resultB) { … });
});

Promise.join

Bluebird offers a dedicated Promise.join function as a more efficient alternative to the Promise.all and .spread combination.

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

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