fetching mp3 file from MeteorJS and trying to convert it into a Blob so that I can play it - meteor

Question

am playing around with downloading and serving mp3 files in Meteor.
I am trying to download an MP3 file (https://www.sample-videos.com/audio/mp3/crowd-cheering.mp3) on my MeteorJS Server side (to circumvent CORS issues) and then pass it back to the client to play it in a AUDIO tag.
In Meteor you use the Meteor.call function to call a server method. There is not much to configure, it's just a method call and a callback.
When I run the method I receive this:
content:
"ID3���#K `�)�<H� e0�)������1������J}��e����2L����������fȹ\�CO��ȹ'�����}$A�Lݓ����3D/����fijw��+�LF�$?��`R�l�YA:A��#�0��pq����4�.W"�P���2.Iƭ5��_I�d7d����L��p0��0A��cA�xc��ٲR�BL8䝠4���T��..etc..", data:null,
headers: {
accept-ranges:"bytes",
connection:"close",
content-length:"443926",
content-type:"audio/mpeg",
date:"Mon, 20 Aug 2018 13:36:11 GMT",
last-modified:"Fri, 17 Jun 2016 18:16:53 GMT",
server:"Apache",
statusCode:200
which is the working Mp3 file (the content-length is exactly the same as the file I write to disk on the MeteorJS Server side, and it is playable).
However, my following code doesn't let me convert the response into a BLOB:
```
MeteorObservable.call( 'episode.download', episode.url.url ).subscribe( ( result: any )=> {
console.log( 'response', result);
let URL = window.URL;
let blob = new Blob([ result.content ], {type: 'audio/mpeg'} );
console.log('blob', blob);
let audioUrl = URL.createObjectURL(blob);
let audioElement:any = document.getElementsByTagName('audio')[0];
audioElement.setAttribute("src", audioUrl);
audioElement.play();
})
When I run the code, the Blob has the wrong size and is not playable
Blob(769806) {size: 769806, type: "audio/mpeg"}
size:769806
type:"audio/mpeg"
__proto__:Blob
Uncaught (in promise) DOMException: Failed to load because no supported source was found.
On the backend I just run a return HTTP.get( url ); in the method which is using import { HTTP } from 'meteor/http'.
I have been trying to use btoa or atob but that doesn't work and as far as I know it is already a base64 encoded file, right?
I am not sure why the Blob constructor creates a larger file then the source returned from the backend. And I am not sure why it is not playing.
Can anyone point me to the right direction?

Answer 1

Finally found a solution that uses request instead of Meteor's HTTP:
First you need to install request and request-promise-native in order to make it easy to return your result to clients.
$ meteor npm install --save request request-promise-native
Now you just return the promise of the request in a Meteor method:
server/request.js
import { Meteor } from 'meteor/meteor'
import request from 'request-promise-native'
Meteor.methods({
getAudio (url) {
return request.get({url, encoding: null})
}
})
Notice the encoding: null flag, which causes the result to be binary. I found this in a comment of an answer related to downloading binary data via node. This causes not to use string but binary representation of the data (I don't know how but maybe it is a fallback that uses Node Buffer).
Now it gets interesting. On your client you wont receive a complex result anymore but either an Error or a Uint8Array which makes sense because Meteor uses EJSON to send data over the wires with DDP and the representation of binary data is a Uint8Array as described in the documentation.
Because you can just pass in a Uint8Array into a Blob you can now easily create the blob like so:
const blob = new Blob([utf8Array], {type: 'audio/mpeg'})
Summarizing all this into a small template if could look like this:
client/fetch.html
<template name="fetch">
<button id="fetchbutton">Fetch Mp3</button>
{{#if source}}
<audio id="player" src={{source}} preload="none" content="audio/mpeg" controls></audio>
{{/if}}
</template>
client/fetch.js
import { Template } from 'meteor/templating'
import { ReactiveVar } from 'meteor/reactive-var'
import './fetch.html'
Template.fetch.onCreated(function helloOnCreated () {
// counter starts at 0
this.source = new ReactiveVar(null)
})
Template.fetch.helpers({
source () {
return Template.instance().source.get()
},
})
Template.fetch.events({
'click #fetchbutton' (event, instance) {
Meteor.call('getAudio', 'https://www.sample-videos.com/audio/mp3/crowd-cheering.mp3', (err, uint8Array) => {
const blob = new Blob([uint8Array], {type: 'audio/mpeg'})
instance.source.set(window.URL.createObjectURL(blob))
})
},
})

Answer 2

Alternative solution is adding a REST endpoint *using Express) to your Meteor backend.
Instead of HTTP we use request and request-progress to send the data chunked in case of large files.
On the frontend I catch the chunks using https://angular.io/guide/http#listening-to-progress-events to show a loader and deal with the response.
I could listen to the download via
this.http.get( 'the URL to a mp3', { responseType: 'arraybuffer'} ).subscribe( ( res:any ) => {
var blob = new Blob( [res], { type: 'audio/mpeg' });
var url= window.URL.createObjectURL(blob);
window.open(url);
} );
The above example doesn't show progress by the way, you need to implement the progress-events as explained in the angular article. Happy to update the example to my final code when finished.
The Express setup on the Meteor Server:
/*
Source:http://www.mhurwi.com/meteor-with-express/
## api.class.ts
*/
import { WebApp } from 'meteor/webapp';
const express = require('express');
const trackRoute = express.Router();
const request = require('request');
const progress = require('request-progress');
export function api() {
const app = express();
app.use(function(req, res, next) {
res.header("Access-Control-Allow-Origin", "*");
res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
next();
});
app.use('/episodes', trackRoute);
trackRoute.get('/:url', (req, res) => {
res.set('content-type', 'audio/mp3');
res.set('accept-ranges', 'bytes');
// The options argument is optional so you can omit it
progress(request(req.params.url ), {
// throttle: 2000, // Throttle the progress event to 2000ms, defaults to 1000ms
// delay: 1000, // Only start to emit after 1000ms delay, defaults to 0ms
// lengthHeader: 'x-transfer-length' // Length header to use, defaults to content-length
})
.on('progress', function (state) {
// The state is an object that looks like this:
// {
// percent: 0.5, // Overall percent (between 0 to 1)
// speed: 554732, // The download speed in bytes/sec
// size: {
// total: 90044871, // The total payload size in bytes
// transferred: 27610959 // The transferred payload size in bytes
// },
// time: {
// elapsed: 36.235, // The total elapsed seconds since the start (3 decimals)
// remaining: 81.403 // The remaining seconds to finish (3 decimals)
// }
// }
console.log('progress', state);
})
.on('error', function (err) {
// Do something with err
})
.on('end', function () {
console.log('DONE');
// Do something after request finishes
})
.pipe(res);
});
WebApp.connectHandlers.use(app);
}
and then add this to your meteor startup:
import { Meteor } from 'meteor/meteor';
import { api } from './imports/lib/api.class';
Meteor.startup( () => {
api();
});