Js延迟加载的好处

function loadScript(url, callback) {
    var script = document.createElement("script");
    script.type = "text/javascript";
    if (script.readyState) {  // IE
        script.onreadystatechange = function () {
            if (script.readyState === "loaded" || script.readyState === "complete") {
                script.onreadystatechange = null;
                callback();
            }
        };
    } else {  // 其他浏览器
        script.onload = function () {
            callback();
        };
    }
    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}

window.addEventListener('scroll', function () {
    if (window.innerHeight + window.pageYOffset >= document.body.offsetHeight - 500) {  // 距离底部 500px 时加载
        loadScript('your-script.js', function () {
            // 脚本加载完成后的操作
        });
    }
});