懒加载javascript

脚本1：

$.getScript = function(url,callBACk,cachE){
   $.ajax({
      type: "GET",url: url,success: callBACk,dataType: "script",cache: cache
   });
};

SCRIPT2：

function require(file,callBACk) {
    var script = document.getElementsByTagName('script')[0],newjs = document.createElement('script');

    // IE
    newjs.onreadystatechange = function () {
        if (newjs.readyState === 'loaded' || newjs.readyState === 'complete') {
            callBACk();
        }
    };

    // others
    newjs.onload = function () {
        callBACk();
    };

    newjs.src = file;
    script.parentNode.insertBefore(newjs,script);
}

document.getElementById('id').onclick = function () {
    require('ondemand.js',function () {
        extraFunction('loaded from the parent page');
        document.body.appendChild(document.createTextNode('done!'));
    });
};

script3：

$L = function (c,d) {
    for (var b = c.length,e = b,f = function () {
            if (!(this.readyState
                    && this.readyState !== "complete"
                    && this.readyState !== "loaded")) {
                this.onload = this.onreadystatechange = null;
                --e || d()
            }
        },g = document.getElementsByTagName("head")[0],i = function (h) {
            var a = document.createElement("script");
            a.async = true;
            a.src = h;
            a.onload = a.onreadystatechange = f;
            g.appendChild(a)
        }; b;) i(c[--b])
};