Retina checking function

onRetina.js

  export default function onRetina() {
    if (window.matchMedia) {
      var mq = window.matchMedia('only screen and (-webkit-min-device-pixel-ratio: 2),' +
                                 'only screen and (   min--moz-device-pixel-ratio: 2),' +
                                 'only screen and (     -o-min-device-pixel-ratio: 2/1),' +
                                 'only screen and (        min-device-pixel-ratio: 2),' +
                                 'only screen and (                min-resolution: 192dpi),' +
                                 'only screen and (                min-resolution: 2dppx)');
      return mq ? mq.matches : false
    }
  }