How to Create a Basic Plugin

Sometimes you want to make a piece of functionality available throughout your code; for example, perhaps you want a single method you can call on a jQuery selection that performs a series of operations on the selection. Maybe you wrote a really useful utility function that you want to be able to move easily to other projects. In this case, you may want to write a plugin.

How jQuery Works 101: jQuery Object Methods and Utility Methods

Before we write our own plugins, we must first understand a little about how jQuery works. Take a look at this code:

                
                              $("a").css( "color", "red" );

This is some pretty basic jQuery code, but do you know what's happening behind the scenes? Whenever you use the $ function to select elements, it returns a jQuery object. This object contains all of the methods you've been using (css(), click(), etc.), and all of the elements that fit your selector. The jQuery object gets these methods from the $.fn object. This object contains all of the jQuery object methods, and if we want to write our own methods, it will need to contain those as well.

Additionally the jQuery utility method $.trim() is used above to remove any leading or trailing empty space characters from the user input. Utility methods are functions that reside directly on $ function itself. You may occasionally want to write a utility method plugin when your extension to the jQuery API does not have to do something to a set of DOM elements you've retrieved.

Basic Plugin Authoring

Let's say we want to create a plugin that makes text within a set of retrieved elements green. All we have to do is add a function called greenify to $.fn and it will be available just like any other jQuery object method.

                
                              $.fn.greenify = function() {
                            
                                this.css( "color", "green" );
                            
                              }
                            
                              $("a").greenify();  // makes all the links green

Notice that to use css(), another method, we use this, not $( this ). This is because our greenify function is a part of the same object as css().

Chaining

This works, but there's a couple of things we need to do for our plugin to survive in the real world. One of jQuery's features is chaining, when you link five or six actions onto one selector. This is accomplished by having all jQuery object methods return the original jQuery object again (there are a few exceptions: width() called without parameters returns the width of the selected element, and is not chainable). Making our plugin method chainable takes one line of code:

                
                              $.fn.greenify = function() {
                            
                                this.css( "color", "green" );
                            
                                return this;
                            
                              }
                            
                              $("a").greenify().addClass("greenified");

Note that the notion of chaining is not applicable to jQuery utility methods like $.trim().

Protecting the $ Alias and Adding Scope

The $ variable is very popular among JavaScript libraries, and if you're using another library with jQuery, you will have to make jQuery not use the $ with jQuery.noConflict(). However, this will break our plugin since it is written with the assumption that $ is an alias to the jQuery function. To work well with other plugins, and still use the jQuery $ alias, we need to put all of our code inside of an Immediately Invoked Function Expression, and then pass the function jQuery, and name the parameter $:

                
                              (function ( $ ) {
                            
                                $.fn.greenify = function() {
                            
                                  this.css( "color", "green" );
                            
                                  return this;
                            
                                }
                            
                                $.ltrim = function( str ) {
                            
                                  return str.replace(/^\s+/, '');
                            
                                }
                            
                                $.rtrim = function( str ) {
                            
                                  return str.replace(/\s+$/, '');
                            
                                }
                            
                              }( jQuery ));

In addition, the primary purpose of an Immediately Invoked Function is to allow us to have our own private variables. Pretend we want a different color green, and we want to store it in a variable.

                
                              (function ( $ ) {
                            
                                var shade = "#556B2F";
                            
                                $.fn.greenify = function() {
                            
                                  this.css( "color", shade );
                            
                                  return this;
                            
                                }
                            
                              }( jQuery ));

Minimizing Plugin Footprint

It's good practice when writing plugins to only take up one slot within $.fn. This reduces both the chance that your plugin will be overridden, and the chance that your plugin will override other plugins. In other words, this is bad:

                
                              (function( $ ) {
                            
                                $.fn.openPopup = function() {
                            
                                  // Open popup code
                            
                                };
                            
                                $.fn.closePopup = function() {
                            
                                  // Close popup code
                            
                                };
                            
                              }( jQuery ));

It would be much better to have one slot, and use parameters to control what action that one slot performs.

                
                              (function( $ ) {
                            
                                $.fn.popup = function( action ) {
                            
                                  if ( action === "open") {
                            
                                    // Open popup code
                            
                                  } if ( action === "close" ) {
                            
                                    // Close popup code
                            
                                  }
                            
                                };
                            
                              }( jQuery ));

Using the `each()` Method

Your typical jQuery object will contain references to any number of DOM elements, and that's why jQuery objects are often referred to as collections.
If you want to do any manipulating with specific elements (e.g. getting data an attribute, calculating specific positions) then you need to use each() to loop through the elements.

                
                              $.fn.myNewPlugin = function() {
                            
                                return this.each(function() {
                            
                                  // do something to each element here
                            
                                });
                            
                              };

Notice that we return the results of each() instead of returning this. Since each() is already chainable, it returns this, which we then return. This is a better way to maintain chainability than what we've been doing so far.

Accepting Options

As your plugins get more and more complex, it's a good idea to make your plugin customizable by accepting options. The easiest way do this, especially if there are lots of options, is with an object literal. Let's change our greenify plugin to accept some options.

                
                              (function ( $ ) {
                            
                                $.greenify = function( options ) {
                            
                                  // This is the easiest way to have default options.
                            
                                  var settings = $.extend({
                            
                                    "color": "#556B2F",  // These are the defaults
                            
                                    "background-color": "white"
                            
                                  }, options );
                            
                                  // Greenify the collection based on the settings variable
                            
                                  return this.css({
                            
                                    "color": settings.color,
                            
                                    "background-color": settings.background-color
                            
                                  });
                            
                                };
                            
                              }( jQuery ));

Example usage:

                
                              $("div").greenify({
                            
                                "color": "orange"
                            
                              });

The default value for color of #556B2F gets overridden by $.extend() to be orange.

Putting It Together

Here's an example of a small plugin using some of the techniques we've discussed:

                
                              (function( $ ){
                            
                                $.fn.showLinkLocation = function() {
                            
                                  return this.filter("a").each(function() {
                            
                                    $( this ).append( " (" + $( this ).attr("href") + ")" );
                            
                                  });
                            
                               };
                            
                              }( jQuery ));
                            
                               // Usage example:
                            
                               $("a").showLinkLocation();

This handy plugin goes through all anchors in the collection and appends the href attribute in brackets.

                
                                <!-- Before plugin is called: -->
                              
                              <a href="page.html">Foo</a>
                            
                                <!-- After plugin is called: -->
                              
                              <a href="page.html">Foo [page.html]</a>

Our plugin can be optimized though:

                
                              (function( $ ){
                            
                                $.fn.showLinkLocation = function() {
                            
                                  return this.filter("a").append(function() {
                            
                                    return " (" + this.href + ")";
                            
                                  });
                            
                                };
                            
                              }( jQuery ));

We're using the append() method's capability to accept a callback, and the return value of that callback will determine what is appended to each element in the collection. Notice also that we're not using the attr method to retrieve the href attribute, because the native DOM API gives us easy access with the aptly named href property.

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