How Can I Access Previous Promise Results in a JavaScript .then() Chain?

Accessing Previous Promise Results in a .then() Chain

In the pursuit of structuring code using promises, a "flat promise chain" comprising several .then() callbacks may be created. However, accessing intermediate promise results within this chain can be challenging since they are not typically in scope for subsequent callbacks.

Breaking the Chain

To access intermediate values, consider breaking the chain into smaller pieces. Instead of attaching a single callback and attempting to utilize its parameter repeatedly, attach multiple callbacks to the same promise where each result value is required. By incorporating promise combinators provided by the library, the result value can be constructed.

This approach simplifies control flow, enhances composition, and facilitates modularization.

Modified Code 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
  });
}

ES5 alternative:

…
return a.then(function(resultA) {
  return b.then(function(resultB) {
    // more processing
    return // something using both resultA and resultB
  });
});

Bluebird solution:

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

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