动态插入 JS 文件

function loadScript(url, callback) {

    var script = document.createElement("script");
    script.type = "text/javascript";

    if(script.readyState) {
        script.onreadystatechange = function() {
            if(script.readyState == "loaded" || script.readyState == "complete") {
                script.onreadystatechange = null;
                callback();
            }
        };
    } else {
        script.onload = function() {
            callback();
        };
    }

    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}

loadScript("forms.js", function() {
    console.log("加载完成！");
})