async.waterfall无法正常工作

Question

I'm using async.waterfall for the first time and I'm running into some trouble.

Here are the two functions I'm trying to call:

function generateImageURL(data, callback){
   // ... xhr stuff
    xhr.onload = function () {
        callback(data, JSON.parse(xhr.responseText).data.link);
    }
  // ... more xhr stuff
    xhr.send(fd);
}

and

function generateCoordinates(data, url, callback){
    console.log("CALLED"); // never gets called
    navigator.geolocation.getCurrentPosition(function(p){
        data.image_url = url;
        data.coordinates = [p.coordinates.longitude, p.coordinates.latitude];
        callback(data);
    });
}

My waterfall function looks like this:

 async.waterfall([
        generateImageURL.bind(this, data),
        generateCoordinates
    ], function(err, result){

    });

I want to pass in data from the outer scope into generateImageURL and then pass that data on towards generateCoordinates along with the url . The callback from generateCoordinates should call the anonymous function.

My issue is that generateCoordinates is never being called. Even though I call it within generateImageURL .

Answer 1

From the docs :

Each function is passed a callback(err, result1, result2, ...) it must call on completion. The first argument is an error (which can be null) and any further arguments will be passed as arguments in order to the next task.

and

If any of the tasks pass an error to their own callback, the next function is not executed.

Your callback is callback(data, JSON.parse(xhr.responseText).data.link); , you pass data as the err argument, therefore it's interpreted as an error and your next function is not called. You should see it if you console.log(err) in your final callback.

Call your callback as callback(null, data, JSON.parse(xhr.responseText).data.link);