JavaScript如何在延迟加载脚本准备好时捕获事件？

Question

I have a lazy loading JavaScript file, how can I catch the event when the class in the file is ready for usage? I only need to load this script in a specific case. Therefor it is not loaded via onload but in a if clause.

The lazy loading code I took from here: http://friendlybit.com/js/lazy-loading-asyncronous-javascript/

if (externalClassRequired) {
    var s   = document.createElement('script');
    s.type  = 'text/javascript';
    s.async = true;
    s.src   = 'http://yourdomain.com/script.js';
    var x   = document.getElementsByTagName('script')[0]
    x.parentNode.insertBefore(s, x);

    // When do I know when the class named "geo" is available?
}

---

Update:
Sorry guys, I totally forgot about Ajax! :) I was so focused on my problem that I didn't saw the obviously solution by @Tokimon. The simplest solution via jQuery would then be:

$.getScript('http://yourdomain.com/script.js', function() {
  // callback or use the class directly
});

Answer 1

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

Answer 2

As it requires a HTTP request, you should be able to put an onload event on it.

s.onload = function() {
  // do something
}

In 'Even Faster Web Sites' Steve Souders offer a solution, which can be seen here .

Answer 3

If I understand what you want to accomplish correctly. This is why callback methods exist.

Explained pretty well here Getting a better understanding of callback functions in JavaScript

Answer 4

Or you could get the script via ajax. Then you are sure that the script is loaded when the ajax succes event is fired :)

Answer 5

var onloadCallback = function() {
    alert('onload');
}

var s   = document.createElement('script');
s.type  = 'text/javascript';
s.async = true;
s.onreadystatechange = function () { // for IE
  if (this.readyState == 'complete') onloadCallback();
};
s.onload = onloadCallback; // for other browsers
s.src   = 'http://yourdomain.com/script.js';
var x   = document.getElementsByTagName('script')[0]
x.parentNode.insertBefore(s, x);   

You could also use an XMLHttpRequest to load the file.