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.