javascript中延迟加载的7种方法实现_Javascript

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 <head>
        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

<script language="javascript" type="text/javascript">
        function include(script_filename) {
            document.write('<' + 'script');
            document.write(' language="javascript"');
            document.write(' type="text/javascript"');
            document.write(' src="' + script_filename + '">');
            document.write('</' + 'script' + '>');
        }

        var which_script = '1.js';
        include(which_script);
        </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("<html><body></body></html>"); 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);
};