获取设施像素比

对于更多日常应用的公共类的操作方法，能够关注下小滑轮网站 http://www.feiaci.com/#/self/...

/** * 获取设施像素比 * @returns {number} * @example * // window.navigator.appVersion(5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1) * getPixelRatio(); * // => 2 */function getPixelRatio() {  let ctx = document.createElement('canvas').getContext('2d');  let dpr = window.devicePixelRatio || 1;  let bsr = ctx.webkitBackingStorePixelRatio ||      ctx.mozBackingStorePixelRatio ||      ctx.msBackingStorePixelRatio ||      ctx.oBackingStorePixelRatio ||      ctx.backingStorePixelRatio || 1;  return dpr / bsr;}