Getting started with your first jQuery plugin

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by Jonathan Fielding | August 2013 | Open Source Web Development

In this article by Jonathan Fielding, the author of Instant jQuery Boilerplate for Plugins, we will have a look at creating our first plugin that manipulates the shape of the div element. You will be familiarized with both with jQuery plugin development and the jQuery Boilerplate template.

Let us write our first plugin. For this recipe, we will look at how we can create a plugin that manipulates the shape of a div element.

We will achieve this by writing some HTML to declare a shape and a button, declaring each shape in the CSS, and then using the JavaScript to toggle which shape is shown by toggling the CSS class appended to it. Writing this plugin should help you familiarize yourself both with jQuery plugin development and the jQuery Boilerplate template.

(For more resources related to this topic, see here.)

Getting ready

Before we dive into our development, we need to have a good idea of how our plugin is going to work. For this, we will write some simple HTML to declare a shape and a button. Each shape will be declared in the CSS, and then we will use the JavaScript to toggle which shape is shown by toggling the CSS class appended to it. The aim of this recipe is to help you familiarize yourself both with jQuery plugin development and the jQuery Boilerplate template.

How to do it

  1. Our first step is to set up our HTML. For this we need to open up our index.html file. We will need to add two elements in HTML: shape and wrapper to contain our shape. The button for changing the shape element will be added dynamically by our JavaScript. We will then add an event listener to it so that we can change the shape. The HTML code for this is as follows:

    <div class="shape_wrapper"> <div class="shape"> </div> </div>

    This should be placed in the div tag with class="container" in our index.html file.

  2. We then need to define each of the shapes we intend to use using CSS. In this example, we will draw a square, a circle, a triangle, and an oval, all of which can be defined using CSS. The shape we will be manipulating will be 100px * 100px. The following CSS should be placed in your main.css file:

    .shape{ width: 100px; height: 100px; background: #ff0000; margin: 10px 0px; } .shape.circle{ border-radius: 50px; } .shape.triangle{ width: 0; height: 0; background: transparent; border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 100px solid #ff0000; } .shape.oval{ width: 100px; height: 50px; margin: 35px 0; border-radius: 50px / 25px; }

  3. Now it's time to get onto the JavaScript. The first step in creating the plugin is to name it; in this case we will call it shapeShift. In the jQuery Boilerplate code, we will need to set the value of the pluginName variable to equal shapeShift. This is done as:

    var pluginName = "shapeShift"

  4. Once we have named the plugin, we can edit our main.js file to call the plugin. We will call the plugin by selecting the element using jQuery and creating an instance of our plugin by running .shapeShift(); as follows:

    (function(){
    $('.shape_wrapper').shapeShift();
    }());

    For now this will do nothing, but it will enable us to test our plugin once we have written the code.

  5. To ensure the flexibility of our plugin, we will store our shapes as part of the defaults object literal, meaning that, in the future, the shapes used by the plugin can be changed without the plugin code being changed. We will also set the class name of the shape in the defaults object literal so that this can be chosen by the plugin user as well. After doing this, your defaults object should look like the following:

    defaults = {
    shapes: ["square", "circle", "triangle", "oval"],
    shapeClass: ".shape"
    };

  6. When the .shapeShift() function is triggered, it will create an instance of our plugin and then fire the init function. For this instance of our plugin, we will store the current shape location in the array; this is done by adding it to this by using this.shapeRef = 0. The reason we are storing the shape reference on this is that it attaches it to this instance of the plugin, and it will not be available to other instances of the same plugin on the same page.
  7. Once we have stored the shape reference, we need to apply the first shape class to the div element according to our shape. The simplest way to do this is to use jQuery to get the shape and then use addClass to add the shape class as follows:

    $(this.element).find(this.options.shapeClass).addClass(this.
    options.shapes[this.shapeRef]);

  8. The final step that we need to do in our init function is to add our button to enable the user to change the shape. To do this, we simply append a button element to the shape container as follows:

    $(this.element).append('<button>Change Shape</button>');

  9. Once we have our button element, we then need to add the shape reference, which changes the shape of the elements. To do this we will create a separate function called changeShape. While we are still in our init function, we can add an event handler to call the changeShape function onto the button. For reasons that will become apparent shortly, we will use the event delegation format of the jQuery. on() function to do this:

    $(this.element).on('click','button',this.changeShape);

  10. We now need to create our changeShape function; the first thing we will do is change this function name to changeShape. We will then change the function declaration to accept a parameter, in this case e.

    The first thing to note is that this function is called from an event listener on a DOM element and therefore this is actually the element that has been clicked on. This function was called using event delegation; the reason for this becomes apparent here as it allows us to find out which instance of the plugin belongs to the button that has been clicked on. We do this by using the e parameter that was passed to the function. The e parameter passed to the function is the jQuery event object related to the click event that has been fired. Inside it, we will find a reference to the original element that the click event was set to, which in this case is the element that the instance of the plugin is tied to. To retrieve the instance of the plugin, we can simply use the jQuery.data() function. The instance of the plugin is stored on the element as data using the data key plugin_pluginName, so we are able to retrieve it the same way as follows:

    var plugin = $(e.delegateTarget).data("plugin_" + pluginName);

  11. Now that we have the plugin instance, we are able to access everything it contains; the first thing we need to do is to remove the current shape class from the shape element in the DOM. To do this, we will simply find the shape element then look up in the shapes array to get the currently displayed shape, and then use the jQuery.removeClass function to remove the individual class.

    The code for doing this starts with a simple jQuery selector that allows us to work with the plugin element; we do this using $(plugin.element). We then look inside the plugin element to find the actual shape. As the name of the shape class is configurable, we can read this from our plugin option; so when we are finding the shape we use .find(plugin.options.shapeClass). Finally we add the class; so that we know which shape is next, we look up the shape class from the shapes array stored in the plugin options, selecting the item indicated by the plugin.shapeRef. The full command then looks as follows:

    $(plugin.element).find(plugin.options.shapeClass).
    removeClass(plugin.options.shapes[plugin.shapeRef]);

  12. We then need to work out which is the next shape we should show; we know that the current shape reference can be found in plugin.shapeRef, so we just need to work out if we have any more shapes left in the shape array or if we should start from the beginning. To do this, we look at the value of plugin.shapeRef and compare it to the length of the shapes array minus 1 (we substract 1 because arrays start at 0); if the shape reference is equal to the length of the shapes array minus 1, we know that we have reached the last shape, so we reset the plugin.shapeRef parameter to 0. Otherwise, we simply increment the shapeRef parameter by 1 as shown in the snippet:

    if((plugin.shapeRef) === (plugin.options.shapes.length -1)){
    plugin.shapeRef = 0;
    }
    else{
    plugin.shapeRef = plugin.shapeRef+1;
    }

  13. Our final step is to add the new shape class to the shape element; this can be achieved by finding the shape element and using the jQuery.addClass function to add the shape from the shapes array. This is very similar to our removeClass command that we used earlier with addClass replacing removeClass.

    $(plugin.element).find(plugin.options.shapeClass).addClass(plugin.
    options.shapes[plugin.shapeRef]);

  14. At this point we should now have a working plugin; so if we fire up the browser and navigate to the index.html file, we should get a square with a button beneath it. Clicking on the button should show the next shape. If your code is working correctly, the shapes should be shown in the order: square, circle, triangle, oval, and then loop back to square.
  15. As a final test to show that each plugin instance is tied to one element, we will add a second element to the page. This is as simple as duplicating the original shape_wrapper and creating a second one as shown:

    <div class="shape_wrapper">
    <div class="shape">
    </div>
    </div>

  16. If everything is working correctly when loading the index.html page, we will have 2 squares each with a button underneath them, and on clicking the button only the shape above will change.

Summary

This article explained how to create your first jQuery plugin that manipulates the shape of a div element. We achieved this by writing some HTML to declare a shape and a button, declaring each shape in the CSS, and then using the JavaScript to toggle which shape is shown by toggling the CSS class appended to it.

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Instant jQuery Boilerplate for Plugins [Instant] Get started with jQuery plugin development with this rapid-paced guide with this book and ebook
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About the Author :


Jonathan Fielding

Jonathan Fielding attended the University of Hull where he studied Internet computing. Since completing his degree, he has worked for a variety of companies across banking and marketing fields, developing both frontend and backend systems.

Jonathan currently works for McCormack & Morrison, a digital agency based in London, UK, developing responsive websites for clients that include Virgin Active, Nyetimber,
and Intent Media.

As a regular contributor to open source, he has launched several of his own open source projects, including several jQuery plugins, and regularly publishes tutorials on his blog with the aim of sharing knowledge.

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