JavaScript Promise gets stuck at resolve

Question

I have an async function that I do multiple times (2), but for some reason the promise get stuck at resolve. It executes resolve(), but doesn't do anything.

Here's the function (basically creates a blob from an URL):

function getBlobAt(url, callback) {
console.log("New getBlobAt Call");
return new Promise(function(resolve) {
    console.log("getBlobAt Promise Created");
    window.URL = window.URL || window.webkitURL;
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url, true);
    xhr.responseType = 'blob';
    xhr.onload = function () {
        console.log("getBlobAt promise completed, resolving...");
        var burl = window.URL.createObjectURL(this.response);
        console.log("getBlobAt promise completed, resolving2...");
        resolve(burl);
        console.log("getBlobAt promise completed, resolving3...");
        //window.URL.revokeObjectURL(burl); //DO THIS WHEN LOADING NEW SONG
    };
    console.log("getBlobAt xhr.send() and callback");
    xhr.send();
    callback(xhr);
    //return xhr;
});
}

And here's the task map:

            var taskstrings = [endmp3, albumart];
            var getBlobTasks = taskstrings.map(function (xurl, i) {
                return function () {
                    console.log("Running a getBlobTask");
                    return getBlobAt(xurl, function (xhr) {
                        console.log("getBlobTask callback");
                        if (g == 0) { //First one is always the mp3 link
                            current_xhr = xhr;
                        } else {
                            current_xhr_album = xhr;
                        }
                    }).then(function (res) {
                        console.log("getBlobTask complete - returning!");
                        if (g == 0) { //First one is always the mp3 link
                            current_blob = res;
                        } else {
                            current_blob_album = res;
                        }
                        return res;
                    });
                };
            });

            var q = getBlobTasks[0](); // start the first one
            for (var i = 1; i < getBlobTasks.length; i++) q = q.then(getBlobTasks[i]);
            q.then(function (result) {
                //Both globs have been returned
                console.log("All getBlobTasks completed!");
                setPlayer(current_blob, current_blob_album);
                target.attr('class', 'glyphicon glyphicon-pause');
            });

It gets stuck at the first resolve(). This is the console output:

Running a getBlobTask
New getBlobAt Call
getBlobAt Promise Created
getBlobAt xhr.send() and callback
getBlobTask callback
getBlobAt promise completed, resolving... 
getBlobAt promise completed, resolving2...
getBlobAt promise completed, resolving3...

Answer 1

You have a g variable you check against 0 but you did not define g anywhere.

The reason you did not see this is that native (or jQuery) promises do not automatically track possibly unhandled rejections.

You can check for errors by appending a .catch to the end of the chain and see if anything went wrong.

q.then(function (result) {
    ...
}).catch(function(e){
     console.log(e.message);
     console.log(e.stack);
});

Which would have shown you the issue.

Alternatively, use a stronger library like Bluebird that will alert you in the case of unhandled rejections.