//files is an array of local files from a <input type="file">
for (var i = 0, f; f = files[i]; i++) {
     var imageFile = files[i];
     var response = uploadImageAsPromise(imageFile);
}
//after completing all uploads, execute some code here
console.log("Finished uploading all files");

function uploadImageAsPromise(imageFile) {
    var image_id = new Date().valueOf();
    var storageRef = firebase.storage().ref().child(image_id + "");
    return new Promise(function (resolve, reject) {
        //Upload file
        var task = storageRef.put(imageFile);

        //Update progress bar
        task.on('state_changed',
            function progress(snapshot) {
                var percentage = snapshot.bytesTransferred / snapshot.totalBytes * 100;
            },
            function error(err) {
                console.log(err);
            },
            function complete() {
                task.snapshot.ref.getDownloadURL().then(function (downloadURL) {
                    var picture = downloadURL;
                    console.log("Finished uploading file: " + image_id);
                });
            }
        );
    });
}