【前端开发】移动端适配方案js，rem单位转换，640设计稿20px=1rem

! function() {
    var style = document.createElement("STYLE"),
        docEl = document.documentElement,
        header = document.getElementsByTagName("HEAD")[0],
        viewport = document.createElement("meta"),
        dpr = 0,
        isAndroid = navigator.appVersion.match(/android/gi),
        isIPhone = navigator.appVersion.match(/iphone/gi),
        baseDpr = "devicePixelRatio" in window ? devicePixelRatio : 1,
        dpr = baseDpr;
    dpr = isIPhone ? devicePixelRatio >= 3 && (!dpr || dpr >= 3) ? 1 : devicePixelRatio >= 2 && (!dpr || dpr >= 2) ? 1 : 1 : 1, scale = 1 / dpr, docEl.setAttribute("data-dpr", dpr), docEl.setAttribute("data-device-type", isIPhone ? "iphone" : isAndroid ? "android" : "other"), viewport.name = "viewport", viewport.content = "initial-scale=" + scale + ", maximum-scale=" + scale + ", minimum-scale=" + scale + ", user-scalable=no", header.appendChild(viewport);
    var width = document.documentElement.clientWidth,
        height = document.documentElement.clientHeight;
    isIPhone || screen.availWidth == width || (width = screen.availWidth / baseDpr, height = screen.availHeight / baseDpr);
    var portrait = Math.min(width, height) / 320 * 10,
        landscape = Math.max(width, height) / 568 * 17.75;
        if(portrait > 18){
            portrait = 18;
        }
        if(landscape > 18){
            landscape = 18;
        }
    style.innerHTML = "@media screen and (orientation:portrait) { html,body { font-size: " + portrait + "px!important; } }
@media screen and (orientation:landscape) { html,body { font-size: " + portrait+ "px!important; } }", header.appendChild(style) }();