Home

Using Javascript with HTML DOM

Rohan Juneja / August 2023 (3304 Words, 19 Minutes)

Following along

Remember to “git pull” on teacher repository to update to lates.

Run this notebook in VSCode
Activate Help-Toogel Developer Tools to add console outputs to runtime experience

Referencing HTML elements using javascript

To get an HTML element, use document.getElementById("idTag")
You will use the ID that you set in your HTML
if you console.log the resulting variable you will get some information about the element

%%html
<!-- the ID must be specified within the element -->
<h1 id="domTitleID">My Title</h1>

<!-- javascript goes here -->
<script>
var titleElement = document.getElementById("domTitleID")
<!-- outputs h1 tag -->
console.log("Example #1, show element in DOM")
console.log(titleElement)
</script>

Getting the data within the HTML element

The variable titleElement stores the “object”
Basically think of this as the group of data enclosed in HTML tag
To access a certain type of data from an “object” we use “.” notation
- .innerHTML gets data within center of tag

%%html
<!-- the ID must be specified within the element -->
<h1 id="domTitleIDget">My Title</h1>

<!-- javascript goes here -->
<script>
var titleElement = document.getElementById("domTitleIDget")
<!-- outputs h1 innerHTML from h1 tag -->
console.log("Example #2, show innerHTML")
console.log(titleElement.innerHTML)
</script>

Setting the data within the HTML Element

The innerHTML data in this “object” can be set like a variable
- Change the value of the innerHTML using the “=” (assignment) operator

%%html
<!-- the ID must be specified on the element -->
<h1 id="domTitleIDset">My Title</h1>

<!-- javascript goes here -->
<script>
var titleElement = document.getElementById("domTitleIDset")
titleElement.innerHTML = "Set and Update My Title"
<!-- outputs h1 innerHTML after h1 tag has been updated -->
console.log("Example #3, update innerHTML")
console.log(titleElement.innerHTML)
</script>

Creating elements

Create a new element with the document.createElement function -> takes in the type of element
Set properties in the element just like the “h1” example

%%html
<!-- the ID must be specified on the element -->
<div id="divContainerID">
    <h1 id="h1ElementID">My Title</h1>
</div>

<!-- javascript goes here -->
<script>
   // creates a new element
   var pElement = document.createElement("p")
   pElement.innerHTML = "Starting a paragraph of text."
   
   // outputs p tag after it has been created
   console.log("Example #4, create a p tag within JS")
   console.log(pElement)
</script>

Issue! How to Create element that appears in HTML?

Here is a visualization of what is happening => the “p” is not placed inside the HRML page!

Solution

Correct by placeing the element somewhere in the page
For example, we could add the element within the div
- For this, use the appendChild function on the div object (the parameter would be the p element we created)
- Remember, use the getELementById to get the object for something in the html (the div!)
Updated Diagram

%%html
<!-- the ID must be specified on the element -->
<div id="divContainerIDset">
    <h1 id="h1ElementIDset">My Title</h1>
</div>

<!-- javascript goes here -->
<script>
   // creates a new element
   var pElement = document.createElement("p")
   pElement.innerHTML = "Starting a paragraph of text."
   
   // outputs p tag after it has been created
   console.log("Example #5, add p tag to HTML")
   console.log(pElement)
   
   // place the p element inside the HTML page
   var div = document.getElementById("divContainerIDset")
   div.appendChild(pElement)
</script>

Functions in JavaScript, using with DOM

Functions allow you to “do something”
- ex. “eat food” in a Snake Game
Functions were used in previous examples
- console.log = “print something”
- document.getElementById = “find an element with id”
Functions take in parameters, what to do (inside the parenthesis)
- the parameter tells console.log what to print
- the parameter in document.getElementById tells the id of the element
Functions can be used with DOM as well, thes will be shown below

Creeating functions

document functions functions were used to create a lot of functionality, but how can a developer create their own?
function are useful to avoid writing the same code over and over again
function can contain parameters for input (they effectively become variables)
function can contain a return, the are the “output” of the function

%%html
<!-- the ID must be specified on the element -->
<div id="divContainerIDfunction">
    <h1 id="h1ElementIDfunction">My Title</h1>
</div>

<!-- javascript goew here -->
<script>
    // define a function => takes parameter text, returns a new p tab
    function createPTag(text) {
        // creates a new element
        var pElement = document.createElement("p")

        // using the parameter like a variable
        pElement.innerHTML = text
        
        // outputs p tag after it has been created
        console.log("Example #6, add p tag using a function")
        console.log(pElement)

        return pElement;
    }

    // using a function to create p tag
    var pTag = createPTag("Starting a paragraph with cooler text than before.")

    // place the p element in the webpage
    var div = document.getElementById("divContainerIDfunction")
    div.appendChild(pTag)
</script>

OnClick Event

Run a function when an event occurs
- In this case, the p tag is created when the button is clicked

%%html
<!-- the ID must be specified on the elements -->
<button id="buttonID">Click here!</button>

<div id="divContainerIDbutton">
    <h1 id="h1ElementIDbutton">My Title</h1>
</div>

<!-- our javascript goe here -->
<script>
    // define a function => takes parameter text, returns a new p tab
    function createPTag(text) {
        // creates a new element
        var pElement = document.createElement("p")

        // using the parameter like a variable
        pElement.innerHTML = text
        
        // outputs p tag after it has been created
        console.log("Example #7.1, add p tag using a function")
        console.log(pElement)

        return pElement;
    }

    // create a function that sets specific text and adds to div
    function addPTagOnButton() {
        // using our new function
        var pTag = createPTag("Starting a paragraph with text created on button press.")

        // place the p element in the webpage
        var div = document.getElementById("divContainerIDbutton")

        // add p tag to the div
        div.appendChild(pTag)
        
        // outputs p tag after it has been created
        console.log("Example #7.2, update container adding a 'p' tag")
        console.log(div)
    }

    // add the P tag when our button is clicked
    var myButton = document.getElementById("buttonID")
    myButton.onclick = addPTagOnButton
    
</script>

Hacks

Copy your HTML code from the HTML hacks. Write a Javascript snippet to switch the links of the two a tags when a button is pressed. Once they are switched, change the inner HTML of the top p tag to the word “switched!”

Hack solution number 1

%%html

<style>
    .Text p{
        align-items: center;
    }
    button {
        padding: 10px 20px;
        background-color: #007bff;
        color: #fff;
        border: none;
        border-radius: 5px;
        cursor: pointer;
        transition: background-color 0.3s;
        margin-left: 40%;
      }
      button:hover {
        background-color: #0056b3;
      }
</style>
<div class="Text">
    <p> Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quae, quod reiciendis neque odit ipsam similique, adipisci rem iusto dignissimos est facere? Nemo repellendus perferendis corporis neque eius eveniet eligendi sequi!</p>
    <br>
    <button id="testbutton">Test Button</button>
</div>

<br>
<br>

<div class="Links">
    <a target="_blank" href="https://poway.instructure.com/">Canvas</a>
    <br>
    <br>
    <a target="_blank" href="https://github.com/">Github</a>
    <br>
    <p>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Provident maiores, dolorum officiis qui iure, delectus eum enim minus facere eius quisquam! Veniam nostrum tempora alias natus praesentium in accusantium laborum!</p>
</div>

<script>
    var originalText = document.querySelector('.Text p').innerHTML;
    var originalLinks = {
        firstLink: document.querySelectorAll('.Links a')[0].getAttribute('href'),
        firstLinkText: document.querySelectorAll('.Links a')[0].innerHTML,
        secondLink: document.querySelectorAll('.Links a')[1].getAttribute('href'),
        secondLinkText: document.querySelectorAll('.Links a')[1].innerHTML
    };

    var isReverted = false;

    document.getElementById('testbutton').addEventListener('click', function() {
        if (isReverted) {
            // Revert links and text to original values
            document.querySelectorAll('.Links a')[0].setAttribute('href', originalLinks.firstLink);
            document.querySelectorAll('.Links a')[0].innerHTML = originalLinks.firstLinkText;
            document.querySelectorAll('.Links a')[1].setAttribute('href', originalLinks.secondLink);
            document.querySelectorAll('.Links a')[1].innerHTML = originalLinks.secondLinkText;
            document.querySelector('.Text p').innerHTML = originalText;
        } else {
            // Change links and text
            var tempLink = document.querySelectorAll('.Links a')[0].getAttribute('href');
            var tempText = document.querySelectorAll('.Links a')[0].innerHTML;
            document.querySelectorAll('.Links a')[0].setAttribute('href', document.querySelectorAll('.Links a')[1].getAttribute('href'));
            document.querySelectorAll('.Links a')[0].innerHTML = document.querySelectorAll('.Links a')[1].innerHTML;
            document.querySelectorAll('.Links a')[1].setAttribute('href', tempLink);
            document.querySelectorAll('.Links a')[1].innerHTML = tempText;
            document.querySelector('.Text p').innerHTML = "Switched!";
        }

        isReverted = !isReverted; // Toggle the state for the next click
    });
</script>

Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quae, quod reiciendis neque odit ipsam similique, adipisci rem iusto dignissimos est facere? Nemo repellendus perferendis corporis neque eius eveniet eligendi sequi!

Canvas

Github

Lorem ipsum dolor sit amet consectetur, adipisicing elit. Provident maiores, dolorum officiis qui iure, delectus eum enim minus facere eius quisquam! Veniam nostrum tempora alias natus praesentium in accusantium laborum!

Hack solution number 2

%%html

<head>
<style>
    .Text p{
        align-items: center;
    }
    button {
        padding: 10px 20px;
        background-color: #007bff;
        color: #fff;
        border: none;
        border-radius: 5px;
        cursor: pointer;
        transition: background-color 0.3s;
        margin-left: 40%;
      }
      button:hover {
        background-color: #0056b3;
      }
</style>
</head>

<body>
    <h1 id="domTitleID">Adding paragraphs</h1>
    <div id="divContainerID">
        <h2>Dynamic Content:</h2>
    </div>
    <button id="addParagraphButton">Add Paragraph</button>
</body>

<script>
    // Function to create a new paragraph element with the given text
    function createParagraph(text) {
    var pElement = document.createElement("p");
    pElement.innerHTML = text;
    return pElement;
    }

    // Function to add a paragraph to the container
    function addParagraphToContainer() {
    var container = document.getElementById("divContainerID");
    var text = "Lorem ipsum dolor sit amet, consectetur adipisicing elit. Quae, quod reiciendis neque odit ipsam similique, adipisci rem iusto dignissimos est facere? Nemo repellendus perferendis corporis neque eius eveniet eligendi sequi!";
    var paragraph = createParagraph(text);
    container.appendChild(paragraph);
    }

    // Attach the click event handler to the button
    var addButton = document.getElementById("addParagraphButton");
    addButton.onclick = addParagraphToContainer;
</script>