javascript - How can I make a waterfall Q promises? - Stack Overflow

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let's take this as an example:

• I have 3 urls in an array urls
• require function returns a promise which just makes an $http call

this is a working code, but as the array can be '1 to n' this is obviously not what I want. I need the 3 require as a waterfall, not in parallel. in the last promise, I need to resolve a final promise which is the var deferred.

require(urls[0]).then(function () {                            
    require(urls[1]).then(function () {                                
        require(urls[2]).then(function () {                                    
            deferred.resolve();
        });
    });
})

this approach is not working, because this will do all the $http calls in parallel.

var promises = [];
angular.forEach(urls, function (value) {
    promises.push(require(value));
});
$q.all(promises).then(function () {
    deferred.resolve();
});

is there a nice way to do this with a for/cycle?

let's take this as an example:

• I have 3 urls in an array urls
• require function returns a promise which just makes an $http call

this is a working code, but as the array can be '1 to n' this is obviously not what I want. I need the 3 require as a waterfall, not in parallel. in the last promise, I need to resolve a final promise which is the var deferred.

require(urls[0]).then(function () {                            
    require(urls[1]).then(function () {                                
        require(urls[2]).then(function () {                                    
            deferred.resolve();
        });
    });
})

this approach is not working, because this will do all the $http calls in parallel.

var promises = [];
angular.forEach(urls, function (value) {
    promises.push(require(value));
});
$q.all(promises).then(function () {
    deferred.resolve();
});

is there a nice way to do this with a for/cycle?

• javascript
• angularjs
• promise
• q
• deferred

Share Improve this question Follow asked Apr 30, 2015 at 19:43 Luca TrazziLuca Trazzi 1,26011 gold badge1313 silver badges3232 bronze badges

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3 Answers 3

Sorted by: Reset to default 6

Create a function to handle the iterations:

function go (urls) {
    if (urls[0]) {
      require(urls[0]).then(function () {
          go(urls.slice(1));
      });
    }
}

go(urls);

Here is an excellent blog post: http://www.codeducky/q-serial/

I will share only the part that is relevant.

First we define this helper method:

function serial(tasks) {
  var prevPromise;
  angular.forEach(tasks, function (task) {
    //First task
    if (!prevPromise) { 
      prevPromise = task(); 
    } else {
      prevPromise = prevPromise.then(task); 
    }
  });
  return prevPromise;
}

Then we use it.

serial([
  function() { return require(urls[0]) },
  function() { return require(urls[1]) },
  function() { return require(urls[2]) }
]).then(function () {
    deferred.resolve();
});

Just to offer another way, there's a "one-line" solution to this without having to make another method:

return promises.reduce($q.when, promises[0]);

See demo here: http://plnkr.co/edit/xlGxYj57lzXdAMM5Iv6s?p=preview (I changed the default $q.when to something else to show handling each promise.)

Update: Made the plunker more representative of OP's scenario.

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