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

Accessing Intermediate Promise Results in a Promise Chain

In a promise chain, each callback receives the result of the preceding promise. However, accessing intermediate results within the chain can be challenging.

Breaking the Chain

To access intermediate results, consider splitting the chain into smaller segments. Instead of attaching callbacks to a single promise, attach multiple callbacks to the same promise. This provides access to the result values whenever needed.

Promise Combinators

Promise libraries provide combinators such as Promise.all(), which combines multiple promises into a single promise that resolves with an array containing the results of each promise.

Example

Suppose you have promises A and B, and you want to access the result of promise A in the callback for promise B:

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 ES6, you can use array destructuring to access the results in the final callback. In ES5, you can use the .spread() helper method or Promise.join() for a cleaner syntax.

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