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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>

%%html
<head>
<style> 
.title {
  width: 275px;
  height: 50px;
  background-color: red;
  position: relative;
  animation-name: backandforth;
  animation-duration: 4s;
  animation-iteration-count: infinite;
  text-align: center;
  border-radius: 8px;
}

@keyframes backandforth {
  0%   {background-color:red; left:0px; top:0px;}
  25%  {background-color:yellow; left:200px; top:0px;}
  50%  {background-color:orange; left:400px; top:0px;}
  75%  {background-color:yellow; left:200px; top:0px;}
  100% {background-color:red; left:0px; top:0px;}
}
</style>
</head>
<body>

<div class="title">
  <h1>City Guesser</h1>
</div>

<div style="font-family: 'Trebuchet MS', sans-serif;'">
  <p>Welcome to City Guesser, a game that helps you learn cities through their satellite images. Cities are displayed, and the goal is to guess them. Correct guesses give a point, while incorrect guesses lose all your points.</p>
  <button onclick="window.location.href = '../2023/09/07/JavaScript-Images.html';" style='width: 100%; background-color: orangered; color: white; padding: 14px 20px; margin: 8px 0; border: none;border-radius: 4px; cursor: pointer; font-size: 20px;'>Click Here to Play!</button>
</div>

<div style='font-family: "Lucida Console", "Courier New", monospace;'>
  <a id="linkToToggle" href='https://en.wikipedia.org/wiki/List_of_municipalities_in_California'>Valid Cities List (30K+)</a>
  <button style='width: 100%; background-color: #9acd32; color: white; padding: 14px 20px; margin: 8px 0; border: none;border-radius: 4px; cursor: pointer; font-size: 20px;' onclick="toggleLink()">Next Link</button>
  <br>
  <p>Some of the above links may be useful.</p>
</div>

<script>
var links = [
  { url: 'https://en.wikipedia.org/wiki/List_of_municipalities_in_California', text: 'Valid Cities List (30K+)' },
  { url: 'https://earth.google.com/web/', text: 'Google Earth - Image Source' },
  { url: 'https://discord.gg/usageo', text: 'USA GeoGuessr Discord' }
];
var currentIndex = 0;

function toggleLink() {
  var linkElement = document.getElementById('linkToToggle');
  currentIndex = (currentIndex + 1) % links.length;
  linkElement.href = links[currentIndex].url;
  linkElement.textContent = links[currentIndex].text;
}
</script>

</body>

City Guesser

Welcome to City Guesser, a game that helps you learn cities through their satellite images. Cities are displayed, and the goal is to guess them. Correct guesses give a point, while incorrect guesses lose all your points.

Valid Cities List (30K+)
Some of the above links may be useful.

Using Javascript with HTML DOM • 20 min read

Description