How to chain a variable number of promises in Q, in order?

Question

I have seen Chaining an arbitrary number of promises in Q ; my question is different.

How can I make a variable number of calls, each of which returns asynchronously, in order?
The scenario is a set of HTTP requests, the number and type of which is determined by the results of the first HTTP request.

I'd like to do this simply.

I have also seen this answer which suggests something like this:

var q = require('q'),
    itemsToProcess =  ["one", "two", "three", "four", "five"];

function getDeferredResult(prevResult) {
  return (function (someResult) {
    var deferred = q.defer();
    // any async function (setTimeout for now will do, $.ajax() later)
    setTimeout(function () {
      var nextResult = (someResult || "Initial_Blank_Value ") + ".." + itemsToProcess[0];
      itemsToProcess = itemsToProcess.splice(1);
      console.log("tick", nextResult, "Array:", itemsToProcess);
      deferred.resolve(nextResult);
    }, 600);

    return deferred.promise;
  }(prevResult));
}

var chain = q.resolve("start");
for (var i = itemsToProcess.length; i > 0; i--) {
    chain = chain.then(getDeferredResult);
}

...but it seems awkward to loop through the itemsToProcess in that way. Or to define a new function called "loop" that abstracts the recursion. What's a better way?

Answer 1

There's a nice clean way to to this with [].reduce.

var chain = itemsToProcess.reduce(function (previous, item) {
    return previous.then(function (previousValue) {
        // do what you want with previous value
        // return your async operation
        return Q.delay(100);
    })
}, Q.resolve(/* set the first "previousValue" here */));

chain.then(function (lastResult) {
    // ...
});

reduce iterates through the array, passing in the returned value of the previous iteration. In this case you're returning promises, and so each time you are chaining a then. You provide an initial promise (as you did with q.resolve("start")) to kick things off.

At first it can take a while to wrap your head around what's going on here but if you take a moment to work through it then it's an easy pattern to use anywhere, without having to set up any machinery.