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JavaScript: The Definitive Guide, 6th Edition by David Flanagan

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Extending jQuery with Plug-ins

jQuery is written so that it is easy to add new functionality. Modules that add new functionality are called plug-ins, and you can find many of them at http://plugins.jquery.com. jQuery plug-ins are just ordinary files of JavaScript code, and to use them in your web pages, you just include them with a <script> element as you would any other JavaScript library (you must include plug-ins after you include jQuery itself, of course).

It is almost trivially easy to write your own jQuery extensions. The trick is to know that jQuery.fn is the prototype object for all jQuery objects. If you add a function to this object, that function becomes a jQuery method. Here is an example:

jQuery.fn.println = function() {
    // Join all the arguments into a space-separated string
    var msg = Array.prototype.join.call(arguments, " ");
    // Loop through each element in the jQuery object
    this.each(function() {
        // For each one, append the string as plain text, then append a <br/>.
        jQuery(this).append(document.createTextNode(msg)).append("<br/>");
    });
    // Return the unmodified jQuery object for method chaining
    return this;
};

With that jQuery.fn.println function defined, we can now invoke a println() method on any jQuery object like this:

$("#debug").println("x = ", x, "; y = ", y);

It is common practice to add new methods to jQuery.fn. If you find yourself using the each() method to “manually” iterate through the elements in a jQuery object and perform some kind of operation on them, ...

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