Edit masterpage, need to add jQuery - how to ensure it does not duplicate?

Question

SharePoint 2010 Server. I edited section of my masterpage like this(in sharepoint designer, not programmatically):

    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script> 

But how can I ensure that I don't connect jQuery twice? Something like this

    <script>if (typeof jQuery == 'undefined') { 
    // jQuery is not loaded} 
    else {
    // jQuery is loaded
    }
    </script>

but how can I dynamically load jQuery from here? Or if I edit .masterpage, I should look by myself if there is no jQuery already added?

Answer 1

Here is script you can use to dynamically load jQuery if it not present.

// Only do anything if jQuery isn't defined
if (typeof jQuery == 'undefined') {
    if (typeof $ == 'function') {
        // warning, global var
        thisPageUsingOtherJSLibrary = true;
    }
    function getScript(url, success) {
        var script = document.createElement('script');
        script.src = url;
        var head = document.getElementsByTagName('head')[0],done = false;
        // Attach handlers for all browsers
        script.onload = script.onreadystatechange = function() {
            if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {
                done = true;                    
                // callback function provided as param
                success();                  
                script.onload = script.onreadystatechange = null;
                head.removeChild(script);                   
            };          
        };          
        head.appendChild(script);       
    };      
    getScript('http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js', function() {       
        if (typeof jQuery=='undefined') {           
            // Super failsafe - still somehow failed...         
        } else {            
            // jQuery loaded! Make sure to use .noConflict just in case
            fancyCode();                
            if (thisPageUsingOtherJSLibrary) {
                // Run your jQuery Code
            } else {
                // Use .noConflict(), then run your jQuery Code
            }           
        }       
    });     
} else { // jQuery was already loaded       
    // Run your jQuery Code
};

http://css-tricks.com/snippets/jquery/load-jquery-only-if-not-present/

Answer 2

The follwoing code will try to load JQuery from Google CDN. If it fails, then it will look for a local copy of JQuery:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js">
</script> 
<script>
window.jQuery || document.write('<script src="/_layouts/15/JQuery/jquery-1.7.1.min.js" type="text/javascript"></script>');
</script>