判断是否是高清屏设备

isRetina.js

var isRetina = function(){
    var mediaQuery = "(-webkit-min-device-pixel-ratio: 1.5),\
                      (min--moz-device-pixel-ratio: 1.5),\
                      (-o-min-device-pixel-ratio: 3/2),\
                      (min-resolution: 1.5dppx)";

    if (window.devicePixelRatio > 1)
      return true;

    if (window.matchMedia && window.matchMedia(mediaQuery).matches)
      return true;

    return false;
};