Check if jQuery.js is loaded

This is the very basics of any programming language, checking if some class, method, variable or property does already exist. In our case the programming environment is JavaScript and the object we are checking for existence is jQuery() / $() function.

This method is not limited to jQuery only, you can check for any other variable or function in your javascript.

Anyway, jQuery() or $() functions will only be defined if they are already loaded into the current document. So to test if jQuery is loaded we can use 2 methods.

Method 1:

if (jQuery) {  
    // jQuery is loaded  
} else {
    // jQuery is not loaded
}

Method 2:

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

NOTE:
Here we are checking for jQuery function being defined or not. This is a safe way to check for jQuery library being loaded. In case you are not using any other javascript libraries like prototype.js or mootools.js, then you can also check for $ instead of jQuery.

You can also check if particular jQuery plugin is loaded or not.

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