延迟加载javascript，也就是页面加载完成之后再加载javascript，也叫on demand（按需）加载，一般有一下几个方法：
1. DOM
head append script tag
window.onload = function() {
setTimeout(function(){
// reference to
var head = document.getElementsByTagName('head')[0];
// a new CSS
var css = document.createElement('link');
css.type = "text/css";
css.rel = "stylesheet";
css.href = "new.css";
// a new JS
var js = document.createElement("script");
js.type = "text/javascript";
js.src = "new.js";
// preload JS and CSS
head.appendChild(css);
head.appendChild(js);
// preload image
new Image().src = "new.png";
}, 1000);
};
2. document.write
function include(script_filename) {
document.write('<' + 'script');
document.write(' language="javascript"');
document.write(' type="text/javascript"');
document.write(' src="' + script_filename + '">');
document.write('');
}
var which_script = '1.js';
include(which_script);
3. Iframe
和第一种类似，但是script tag是放到iframe的document里面。
window.onload = function() {
setTimeout(function(){
// create new iframe
var iframe = document.createElement('iframe');
iframe.setAttribute("width", "0");
iframe.setAttribute("height", "0");
iframe.setAttribute("frameborder", "0");
iframe.setAttribute("name", "preload");
iframe.id = "preload";
iframe.src = "about:blank";
document.body.appendChild(iframe);
// gymnastics to get reference to the iframe document
iframe = document.all ? document.all.preload.contentWindow : window.frames.preload;
var doc = iframe.document;
doc.open(); doc.writeln(""); doc.close();
// create CSS
var css = doc.createElement('link');
css.type = "text/css";
css.rel = "stylesheet";
css.href = "new.css";
// create JS
var js = doc.createElement("script");
js.type = "text/javascript";
js.src = "new.js";
// preload CSS and JS
doc.body.appendChild(css);
doc.body.appendChild(js);
// preload IMG
new Image().src = "new.png";
}, 1000);
};
4. Iframe static page
直接把需要加载东西放到另一个页面中
window.onload = function() {
setTimeout(function(){
// create a new frame and point to the URL of the static
// page that has all components to preload
var iframe = document.createElement('iframe');
iframe.setAttribute("width", "0");
iframe.setAttribute("height", "0");
iframe.setAttribute("frameborder", "0");
iframe.src = "preloader.html";
document.body.appendChild(iframe);
}, 1000);
};
5. Ajax eval
用ajax下载代码，然后用eval执行
6. Ajax Injection
用ajax下载代码，建立一个空的script tag，设置text属性为下载的代码
7. async 属性(缺点是不能控制加载的顺序)
感谢原著作者，谢谢
