动态加载脚本

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 {
        script.onload = function() { //其他浏览器
            callback();
        };
    }
    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}

用法如下：

loadScript("file1.js", function() {
    console.log("File is loaded!");
});
loadScript("file1.js", function() {
    loadScript("file2.js", function() {
        loadScript("file3.js", function() {
            console.log("All files are loaded!");
        });
    });
});