javascript - How can I load jQuery if it is not already loaded?

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javascript - How can I load jQuery if it is not already loaded?

posted Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

javascript - How can I load jQuery if it is not already loaded?

I have a initializor.js that contains the following:

if(typeof jQuery=='undefined')
{
    var headTag = document.getElementsByTagName("head")[0];
    var jqTag = document.createElement('script');
    jqTag.type = 'text/javascript';
    jqTag.src = 'jquery.js';
    headTag.appendChild(jqTag);
}

I am then including that file somewhere on another page. The code checks if jQuery is loaded, and if it isn't, adds it to the Head tag.

However, jQuery is not initializing, because in my main document, I have a few events declared just to test this. I also tried writing some jQuery code below the check, and Firebug said:

"jQuery is undefined".

Is there a way to do this? Firebug shows the jquery inclusion tag within the head tag!

Also, can I dynamically add code into the $(document).ready() event? Or wouldn't it be necessary just to add some Click events to a few elements?

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1 Reply

深蓝 · Answer 1 · 2021-10-17T01:06:43+0000

jQuery is not available immediately as you are loading it asynchronously (by appending it to the <head>). You would have to add an onload listener to the script (jqTag) to detect when it loads and then run your code.

e.g.

function myJQueryCode() {
    //Do stuff with jQuery
}

if(typeof jQuery=='undefined') {
    var headTag = document.getElementsByTagName("head")[0];
    var jqTag = document.createElement('script');
    jqTag.type = 'text/javascript';
    jqTag.src = 'jquery.js';
    jqTag.onload = myJQueryCode;
    headTag.appendChild(jqTag);
} else {
     myJQueryCode();
}

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javascript - How can I load jQuery if it is not already loaded?

javascript - How can I load jQuery if it is not already loaded?

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