How Can I Access Intermediate Promise Results in JavaScript Promise Chains?

Managing Intermediate Results in Promise Chains

Problem:

When creating promise chains, it can be challenging to access intermediate promise results within the final callback. In the following example, how can we access the result of promiseA within the callback for promiseB?

function getExample() {
    return promiseA(…).then(function(resultA) {
        // Processing
        return promiseB(…);
    }).then(function(resultB) {
        // How can we access resultA here?
    });
}

Solution: Breaking the Chain

To access intermediate results in the promise chain, it's best to break the chain into separate components. This allows you to attach multiple callbacks to the same promise, accessing the resolved value when needed. Here's an improved version of the code:

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

This approach utilizes the Promise.all() method to create an array of all the promises in the chain. We then chain another then() callback to the Promise.all() result, passing a function that receives an array of the resolved values. This allows us to easily access both resultA and resultB within the final callback.

Alternate Solutions:

In ES5, you can use the .spread() method to achieve similar functionality:

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

Bluebird provides a dedicated join() method that simplifies this process further:

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

By breaking the chain and using the appropriate promise combinators, you can elegantly access and manipulate intermediate promise results, ensuring a clear and efficient code structure.

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