JQuery

Javascript is a complicated language and the environment of the browser is also complicated and can be hard to understand. Because of this, there have been many tool-kits and libraries developed to simplify one or other part of the task of writing applications in Javascript. Some of these took-kits take a very opinionated view of how you should write applications and provide a rich framework for the developer to work in. They can be quite powerful but they also have a steep learning curve.

This text and the course it supports is intended to teach you about how the technology of the web works and how things fit together to make web applications. From this perspective, we want to see how Javascript interacts with the DOM within the browser and how it responds to events to build an interactive application interface. Using a high level tool-kit hides a lot of the detail from the developer (for good reason) so doesn't serve our purpose very well. However, using just plain Javascript makes some things very complicated, in particular some of the things we want to be able to do such as navigating through elements in the page, adding event handlers and modifying the contents of the DOM. So, there is some value in using a toolkit that helps us with some of this but still allows us to see what is going on at the Javascript level. For this reason we'll make use of the JQuery toolkit and this chapter gives an introduction to it's capabilities.

Using JQuery

JQuery is a Javascript library but the language doesn't provide any way for your script to import a library in the same way that we can with Python or Java. Hence the way to use JQuery is to make sure that it is loaded into the browser before any scripts that make use of it. We mentioned before that the best practice is to load Javascript files at the end of the body of the html page, so if we are using JQuery we would load this first and then load any script files of our own.

There are two options for loading JQuery. You can download a version of the library and include it with your other static assets as part of your project. You would then refer to it as something like:

<script src="/static/js/jquery-3.3.1.min.js"></script>

Here 3.3.1 is the version of the JQuery library and .min refers to a minified version of the Javascript code. This is a version with all of the comments and whitespace removed and with all variables replaced by one or two character variable names. This is a way of compiling Javascript for delivery to the browser that makes it smaller. Tools such as Webpack do this to your Javascript code as well as many other things to help manage a web project.

The other alternative is to load JQuery from a Content Delivery Network (CDN). This is a hosting network that is used to host very commonly used files such as the JQuery library. The CDN will serve the file from a server as close to the client as possible from a collection of servers around the world. For JQuery the official CDN is at https://code.jquery.com/ so we can include a version of the library like this:

<script src="https://code.jquery.com/jquery-3.3.1.min.js"
        integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
        crossorigin="anonymous"></script>

Note the extra attributes here. integrity is a new standard called Subresource Integrity that includes a base64-encoded sha384 hash of the file in the integrity attribute. This allows the browser to check that the file it downloads is the one that was intended and guards against an attack that replaces a common library with compromised code. This is important when you are trusting a third party (the CDN) with hosting your code since they could easily replace your file with one containing malicious code. This standard is just starting to be supported by browsers.

Having loaded the JQuery library we can start to make use of it.

Using JQuery as $

The JQuery library defines one new object called jQuery and all functions can be called as methods on this object. However, since this will be used very frequently in our code it also defines the object to $ as well. $ is just a variable and any module could define it but it is most commonly used by JQuery. Most of our examples here will use $ as the main JQuery function. You should be aware that it is just a function name, not any new syntax for the Javascript language.

Selecting things with JQuery

The first thing to look at with JQuery is how to select parts of the HTML DOM to work with. While plain Javascript would use document.getElementById etc. JQuery makes it easy to select one or more elements in a similar way to the way we do in CSS stylesheets. So to find all of the anchor (<a>) tags in the document we use the expression:

$("a")

To find the element with a given id we would use the CSS # prefix:

$("#content")

JQuery can use the full range of CSS selectors plus a few other things to locate parts of the HTML DOM. The result is either a single element or a collection of elements.

Chaining Operations

Once we have identified some elements we can operate on them by chaining an operation using a dot after the selector. For example, to set the color of all second level headings to red:

$("h2").css("color", "red")

Another example would be changing the content of these elements. This expression would change them all to the same text:

$("h2").text("New Heading")

Chaining can keep going adding operations one after another, so we could achieve both of these changes like this:

$("h2").css("color", "red").text("New Heading")

(What is happening here is that each operation returns a JQuery object representing the node or nodes and so the next dotted method just applies to the same group after the change is made).

Operations can do various things in JQuery but we'll look at two main categories: changing the content of the DOM and adding event handlers. Changing the CSS style is an example of changing the DOM.

Adding Event Listeners

Javascript is an event driven language and the browser environment has many events that can be triggered by the user or by network events. We can set up listeners to handle these events using plain Javascript as described in the Events chapter. Using JQuery this process is simplified and because we can select multiple elements with one expression, it is easier to add event listeners to many elements at once.

A simple example would be to change the color of an element when the user clicks the mouse on the element. To do this we add a click handler as follows:

$("h3").click(function() {
    $(this).css("color", "red")
})

Here we define an anonymous function as the click handler for all <h3> elements. The function uses the selector $(this) which refers to the element that has been clicked on and changes the CSS for that element to red. To achieve the same thing with plain Javascript would require a few more lines of code and we would need to apply the click event handler to each element separately.

As a side note, it would be possible to define a named function and apply it as the click handler as follows:

function h3click() {
    $(this).css("color", "red")
}
$("h3").click(h3click)

this would have the same effect but is a little bit more verbose and not really any clearer than the version with the anonymous function. Common practice in JQuery coding is to use anonymous functions as in the first example; you should get used to this style of coding.

The Document Ready Event

One very common requirement is to run some code once the page has finished loading. As the browser loads and interprets the HTML of the page it builds the DOM for the page and loads any related resources (scripts, css, images). Until this has completed, we don't want to run any scripts that will modify the page. A common way to achieve this is to bind to the window.onload event which is triggered by the browser when the page has loaded. JQuery provides an interface to this which actually tries to run the code a little bit earlier - when the DOM has been built even if images and other resources are still being downloaded. This is the $(document).ready handler:

$(document).ready(funciton(){
    /* code to run when DOM is ready */
})

Your scripts can contain more than one call to $(document).ready() to have multiple code fragments run when the DOM is ready.

Note that if your scripts depend on looking at images, you will want to wait a little longer to run your scripts and bind to window.onload instead. In jQuery you can do this with:

$(window).load(function() {
    /* code that manipulates images goes here */
})

JQuery Examples

Form Handling

In this example, we will intercept the submission of a form so that instead of the data being sent in a request back to the server, we run some Javascript code to update the page instead.

We start with an HTML page with the following structure:

<body>
  <h1>What do you like?</h1>
  <form id=likeform>
    <input name=thing placeholder="a thing you like" size=30>
    <input type=submit>
  </form>

  <ul id=likes></ul>

  <!-- Scripts -->
  <script src="https://code.jquery.com/jquery-3.3.1.min.js"
        integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
        crossorigin="anonymous"></script>
  <script src="scripts/index.js"></script>
</body>

The page contains a form with a single input asking for a thing that we like. The goal of the application is to capture these things as the form is submitted and add them to the unordered list element below the form on the page. Note that the page loads the jQuery library from the CDN and then our local Javascript file.

The key to implementing this application is to add a handler for the submit event on the target form. Here is the framework for this in index.js:

(function(){
  $(document).ready(function() {
      $("#likeform").submit(function(event) {
        /* form handling code here */
        /* prevent the submission of the form */
        event.preventDefault()
      })
    })
})()

This code block uses the immediately called anonymous function pattern to wrap the code and then sets up the document ready handler to have some code run when the document has finished loading. The code we run is to select the form $('#likeform') and then add a submit handler for the form. The submit handler is an anonymous function with one argument event which will be a representation of the event that triggered the function call. We'll look at the content of this function below but the last thing that this function does is to call event.preventDefault() which blocks the default interpretation of the event which would be to to send an HTTP request back to the server.

The first step in handling the form submission is to get the input that has been entered into the form by the user. We can use jQuery to locate the input field and get the value like this:

        var input = $(this).children("input[name='thing']")
        var thing = $(input).val()

Here we are using $(this) to refer to the form being submitted. The children method searches among the child elements of the form and we search for the input element with a name attribute with the value thing. The next line then uses the val method to get the value entered into the form.

Now that we have this value, we can insert it into the page by appending some HTML to the target element:

        $("#likes").append("<li>" + thing + "</li>")

Our application is almost complete. If you try out this code up to this point you can enter text into the form, click on submit, and the text will be added as a list item in the page. However, after inserting the text, the entry is left behind in the form and we need to delete it to add something new. We can fix this by resetting the value in the form at the end of the function:

    $(input).val("")

This completes our little application. The data from the form is added into the page without any reference back to the server. Note that this is clearly not a useful application since as soon as the page is refreshed, the data disappears. However, it shows how this kind of in-page interaction can be implemented.

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