移動端rem字體適配JS

// 『REM』手機屏幕適配，兼容更改過默認字體大小的安卓用戶
function adapt(designWidth, rem2px) { 
//  designWidth：'設計圖寬度'   1rem==rem2px+'px'                     
    var d = window.document.createElement('div');
    d.style.width = '1rem';
    d.style.display = "none";
    var head = window.document.getElementsByTagName('head')[0];
    head.appendChild(d);
    var defaultFontSize = parseFloat(window.getComputedStyle(d, null).getPropertyValue('width'));
    d.remove();
    document.documentElement.style.fontSize = window.innerWidth / designWidth * rem2px / defaultFontSize * 100 + '%';
    var st = document.createElement('style');
// 適應橫屏、豎屏
    var portrait = "@media screen and (min-width: " + window.innerWidth + "px) {html{font-size:" + ((window.innerWidth / (designWidth / rem2px) / defaultFontSize) * 100) + "%;}}";
    var landscape = "@media screen and (min-width: " + window.innerHeight + "px) {html{font-size:" + ((window.innerHeight / (designWidth / rem2px) / defaultFontSize) * 100) + "%;}}"
    st.innerHTML = portrait + landscape;
    head.appendChild(st);
    return defaultFontSize
};
var defaultFontSize = adapt(640, 100);