How to know if JQuery has finished loading

This question already has an answer here:

How do I include a JavaScript file in another JavaScript file? 51 answers

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You will have to write code to detect when a dynamically loaded script is loaded and unfortunately, it works slightly different in some older browsers so it isn't as simple as it could be. Here's a good reference article on how to do that: http://www.ejeliot.com/blog/109

Here's some of the code from that article:

function loadScript(sScriptSrc, oCallback) {

    var oHead = document.getElementsByTagName('head')[0];
    var oScript = document.createElement('script');

    // make sure callback isn't run more than once
    function runCallback() {
        if (oCallback) {
            oCallback();
            oScript.onload = oScript.onreadystatechange = null;
            oCallback = null;
        }
    }

    oScript.type = 'text/javascript';
    // most browsers
    oScript.onload = runCallback;
    // IE 6 & 7
    oScript.onreadystatechange = function() {
        if (this.readyState === 'complete') {
            runCallback();
        }
    }
    oScript.src = sScriptSrc;
    oHead.appendChild(oScript);
}

Alternatively, you could use one of the tiny libraries that does this work for you. You can see a list of some of those here: http://microjs.com/#loader. Though I'm not sure you want to use yet another library to assist with loading your primary library.

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尝试onload事件：

 //.....
 script.onload = function(){
     alert('jQuery is loaded succesfully').
 };
 script.src = "https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js";

---

Kudos for jfriend's answer. Callbacks are sexy.

Second crack at it.

function loadJQ(callback) {
  if(!window.jQuery) {
    // Create jQuery script element.
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.src = 'path/to/jquery.js';
    document.body.appendChild(script);

    script.onload = function(){ callback(jQuery); };
    // IE 6 & 7 ala jfriend00
    script.onreadystatechange = function() {
      if (this.readyState == 'complete') callback(jQuery);
    }
  } else {
    callback(jQuery);
  }
}

Invocation

loadJQ(function($){
  alert($ === jQuery); // true. jquery is loaded.
});

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