buffer.js

class Buffer {

  constructor(context, urls) {
    this.context = context;
    this.urls = urls;
    this.buffer = [];
  }

  loadSound(url, index) {
    let request = new XMLHttpRequest();
    request.open('get', url, true);
    request.responseType = 'arraybuffer';
    let thisBuffer = this;
    request.onload = function() {
      thisBuffer.context.decodeAudioData(request.response, function(buffer) {
        thisBuffer.buffer[index] = buffer;
        updateProgress(thisBuffer.urls.length);
        if(index == thisBuffer.urls.length-1) {
          thisBuffer.loaded();
        }
      });
    };
    request.send();
  };

  loadAll() {
    this.urls.forEach((url, index) => {
      this.loadSound(url, index);
    })
  }

  loaded() {
    // what happens when all the files are loaded
  }

  getSoundByIndex(index) {
    return this.buffer[index];
  }

}