daniellmb/downlinkMax

/**
 * @file downlinkMax is 0.26KB Network Information API downlinkMax polyfill micro-library.
 * @author Daniel Lamb <dlamb.open.source@gmail.com>
 */
function downlinkmax() {
  var limitless = Infinity,
    nav = navigator,
    speed,

  // deal with vendor prefixing and fallback if not supported
    connection = nav.connection || nav.mozConnection || nav.webkitConnection || {
      // API not supported
      downlinkMax: limitless
    };

  // check that the API doesn't already support downlinkMax
  if (!('downlinkMax' in connection)) {

    // assume NOT W3C Editor's Draft 09 October 2014
    // check if API supports bandwidth
    if ('bandwidth' in connection) {

      // assume W3C Working Draft 29 November 2012
      // standardize connection.bandwidth value by converting megabytes per second (MB/s) to megabits per second (Mbit/s)
      connection.downlinkMax = connection.bandwidth * 8;
    } else {

      // assume W3C Working Draft 07 June 2011
      // convert connection.type value to approximate downlink values
      // speed estimate is based on the median downlink value for common devices in megabits per second (Mbit/s)
      switch (connection.type) {
      case 'none':
        speed = 0;
        break;
      case '2g':
        speed = 0.134;
        break;
      case 'bluetooth':
      case 'cellular':
        speed = 2;
        break;
      case '3g':
        speed = 8.95;
        break;
      case '4g':
        speed = 100;
        break;
      case 'ethernet':
        speed = 550;
        break;
      case 'wifi':
        speed = 600;
        break;
      // other, unknown etc.
      default:
        speed = limitless;
        break;
      }
      connection.downlinkMax = speed;
    }
  }

  // return the maximum downlink speed
  return connection.downlinkMax;
}