Lesson 28: JavaScript HTML DOM: Introduction to the DOM · DOM Methods · Finding Elements · Changing HTML · Changing CSS · Form Validation · Animations · The Document Object · Element Reference

Phase 1 of 13

1. What is the HTML DOM?

The Big Picture

When a browser loads an HTML page, it does not just display it · it reads every tag, builds a structured map of the entire page in memory, and hands that map to JavaScript. That map is called the DOM: the Document Object Model.

Think of a webpage like a family tree:

code

document
└── <html>
    ├── <head>
    │   └── <title>My Page</title>
    └── <body>
        ├── <h1>Hello World</h1>
        └── <p id="intro">Welcome!</p>

Every tag becomes a node in that tree. JavaScript can walk up and down the tree, read any node, change it, add new ones, or delete them · all without reloading the page.

Real-world analogy: A DOM is like a company org chart. The CEO is <html>, departments are <head> and <body>, and individual employees are your tags like <p>, <h1>, <button>. JavaScript is like a manager who can rename employees, add new ones, or fire them · instantly, without rebuilding the company.

Why Does the DOM Exist?

Before the DOM, if you wanted to update a webpage you had to reload the entire page from the server. The DOM was invented so that:

Pages could react to users (clicks, typing, hovering)
Content could update in real time (live scores, chat messages)
Forms could validate input before submitting
Elements could animate without Flash or plugins

Today, every interactive website you use · from Google Docs to YouTube · is built on DOM manipulation.

The DOM is a Standard

The DOM is not invented by a browser. It is a W3C standard · a set of rules every browser must follow. This means the same JavaScript DOM code works in Chrome, Firefox, Safari, and Edge.

The standard defines:

Standard	What it covers
Core DOM	All document types (XML, HTML)
XML DOM	XML-specific features
HTML DOM	HTML-specific features · this is what we learn

The DOM Standard Defines:

HTML elements as objects · each tag in your HTML becomes a JavaScript object
Properties · the data inside that object (e.g., innerHTML, id, style)
Methods · the actions you can call on it (e.g., getElementById(), appendChild())
Events · triggers that fire when something happens (e.g., onclick, onchange)

Micro-Demo 1 · The DOM in One Line

html

<p id="demo">I am original text.</p>

<script>
  document.getElementById("demo").innerHTML = "DOM changed me!";
</script>

What happens:

Browser builds the DOM from the HTML
document is the entry point to the entire tree
.getElementById("demo") finds the <p> node with id="demo"
.innerHTML = "DOM changed me!" replaces the text inside it

Expected output in browser: The paragraph now reads: DOM changed me!

💡 Thinking question: What would happen if you used an id that doesn't exist in the page? Try document.getElementById("ghost") · what would JavaScript return? Answer: It returns null. Trying to set .innerHTML on null will throw an error. Always check that your element exists!

✏️ Your Task

Practise what you just learned about 1. What is the HTML DOM?. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 2 of 13

2. DOM Methods · Your First Tools

What is a Method?

A method is an action you can perform. Just like a TV remote has buttons (change channel, adjust volume), DOM objects have methods (find element, change content, add child).

The two most foundational DOM methods are:

Method	What it does
`document.getElementById(id)`	Finds one element by its `id` attribute
`document.createElement(tag)`	Creates a brand new HTML element

`getElementById` · Your Most-Used Tool

html

<!DOCTYPE html>
<html>
<body>

  <p id="greet">Good morning!</p>
  <button onclick="changeGreeting()">Change Greeting</button>

  <script>
    function changeGreeting() {
      // Step 1: Find the element
      var element = document.getElementById("greet");

      // Step 2: Change its content
      element.innerHTML = "Good evening!";
    }
  </script>

</body>
</html>

Line-by-line breakdown:

Line	Meaning
`document.getElementById("greet")`	Searches the entire DOM for a tag with `id="greet"` and returns it as a JavaScript object
`element.innerHTML`	A property of the element object · it holds the HTML content inside the tags
`= "Good evening!"`	Assigns a new value, replacing the old content

Expected output: After clicking the button, the paragraph changes from "Good morning!" to "Good evening!" without any page reload.

`innerHTML` vs `innerText` · An Important Difference

These two properties look similar but behave differently:

html

<div id="box">Hello</div>

<script>
  var el = document.getElementById("box");

  // innerHTML treats the value as HTML markup
  el.innerHTML = "<strong>Bold text</strong>";
  // → Displays bold text in the browser

  // innerText treats the value as plain text
  el.innerText = "<strong>Bold text</strong>";
  // → Displays the literal string: <strong>Bold text</strong>
</script>

⚠️ Common beginner mistake: Using innerHTML to insert user-typed content directly. If a user types <script>alert("hacked")</script>, it will execute! For user input, always use innerText or sanitize the input first.

The `document` Object · Your Gateway

document is a special built-in JavaScript object that represents the entire loaded page. Every DOM method starts from document because it is the root of the tree.

code

window
  └── document        ← your starting point
        ├── head
        └── body
              ├── h1
              └── p

You do not create document · the browser creates it automatically when the page loads.

Defining HTML Elements · The id Attribute

getElementById works because HTML allows every tag to carry a unique identifier:

html

<p id="score">0</p>
<p id="status">Not started</p>

The id must be unique per page · just like a national ID number. If two elements share the same id, getElementById will only find the first one, and your code will behave unpredictably.

✏️ Your Task

Practise what you just learned about 2. DOM Methods · Your First Tools. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 3 of 13

3. Finding HTML Elements

Six Ways to Find Elements

Once you have the document, you need to locate the specific elements you want to work with. JavaScript gives you six main strategies:

Method	Returns	Use when...
`getElementById(id)`	Single element	You know the exact `id`
`getElementsByTagName(tag)`	HTMLCollection	You want all `<p>`, all `<li>`, etc.
`getElementsByClassName(cls)`	HTMLCollection	You want all elements sharing a CSS class
`querySelector(selector)`	Single element	You want the first match using CSS selector syntax
`querySelectorAll(selector)`	NodeList	You want all matches using CSS selector syntax
`getElementsByName(name)`	NodeList	Used with form inputs sharing a `name` attribute

Method 1: `getElementById`

html

<h2 id="title">JavaScript DOM</h2>

<script>
  var title = document.getElementById("title");
  console.log(title.innerHTML); // Output: JavaScript DOM
</script>

Returns one element (or null if not found)
Fastest method · browser optimises id lookups

Method 2: `getElementsByTagName`

Returns an HTMLCollection · like an array of all matching elements.

html

<p>First paragraph</p>
<p>Second paragraph</p>
<p>Third paragraph</p>

<script>
  var paragraphs = document.getElementsByTagName("p");

  console.log(paragraphs.length);     // Output: 3
  console.log(paragraphs[0].innerHTML); // Output: First paragraph
  console.log(paragraphs[2].innerHTML); // Output: Third paragraph
</script>

🔍 What is an HTMLCollection? It is a live list of DOM elements. "Live" means: if you add a new <p> to the page later, paragraphs automatically includes it · you don't need to call the method again.

Looping Through a Collection

html

<ul>
  <li>Apple</li>
  <li>Banana</li>
  <li>Cherry</li>
</ul>

<script>
  var items = document.getElementsByTagName("li");

  for (var i = 0; i < items.length; i++) {
    items[i].style.color = "green";  // Makes each item text green
  }
</script>

Expected output: All three list items turn green in the browser.

Method 3: `getElementsByClassName`

html

<p class="highlight">This is highlighted.</p>
<p>This is not.</p>
<p class="highlight">This is also highlighted.</p>

<script>
  var highlighted = document.getElementsByClassName("highlight");
  console.log(highlighted.length); // Output: 2
</script>

💡 Multiple classes: document.getElementsByClassName("box active") finds elements that have both classes.

Method 4: `querySelector` · CSS Selector Power

querySelector accepts any CSS selector string · giving you tremendous flexibility:

html

<div class="card">
  <p id="first">Hello</p>
  <p>World</p>
</div>

<script>
  // By id (like getElementById)
  var el1 = document.querySelector("#first");
  console.log(el1.innerHTML); // Output: Hello

  // By class
  var el2 = document.querySelector(".card");

  // By tag
  var el3 = document.querySelector("p");

  // By attribute
  var el4 = document.querySelector("input[type='text']");

  // By nesting — the first <p> inside .card
  var el5 = document.querySelector(".card p");
  console.log(el5.innerHTML); // Output: Hello
</script>

Method 5: `querySelectorAll`

Like querySelector but returns all matches as a NodeList.

html

<p class="item">One</p>
<p class="item">Two</p>
<p class="item">Three</p>

<script>
  var items = document.querySelectorAll(".item");

  items.forEach(function(item) {
    item.style.fontWeight = "bold";
  });
</script>

Expected output: All three paragraphs become bold.

🔑 Difference from HTMLCollection: NodeList returned by querySelectorAll is static · it does not update if you add new matching elements later.

Searching Within an Element (Not Just the Document)

You can call these methods on any element, not just document. This lets you narrow your search:

html

<div id="section-a">
  <p>Paragraph in A</p>
</div>
<div id="section-b">
  <p>Paragraph in B</p>
</div>

<script>
  var sectionA = document.getElementById("section-a");
  var pInA = sectionA.getElementsByTagName("p");
  console.log(pInA[0].innerHTML); // Output: Paragraph in A
  // Does NOT find the paragraph in section-b
</script>

Navigating the DOM Tree with Relationships

Every element knows about its relatives:

Property	What it returns
`element.parentNode`	The direct parent element
`element.childNodes`	All child nodes (including text nodes)
`element.children`	Only child elements (no text nodes)
`element.firstChild`	First child node
`element.lastChild`	Last child node
`element.nextSibling`	Next node at the same level
`element.previousSibling`	Previous node at the same level
`element.firstElementChild`	First child that is an element
`element.lastElementChild`	Last child that is an element

html

<ul id="menu">
  <li>Home</li>
  <li>About</li>
  <li>Contact</li>
</ul>

<script>
  var menu = document.getElementById("menu");
  var first = menu.firstElementChild;
  var last = menu.lastElementChild;

  console.log(first.innerHTML); // Output: Home
  console.log(last.innerHTML);  // Output: Contact
  console.log(menu.children.length); // Output: 3
</script>

✏️ Your Task

Practise what you just learned about 3. Finding HTML Elements. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 4 of 13

4. Changing HTML Content

What Can You Change?

Once you have found an element, you can change almost anything about it:

What to change	Property / Method
Text/HTML inside a tag	`.innerHTML` or `.innerText`
An attribute value (src, href, alt…)	`.setAttribute()` or direct property
The entire element	`.outerHTML`
Append new child elements	`.appendChild()`, `.insertBefore()`
Remove elements	`.removeChild()`, `.remove()`
Write directly to the page (during load)	`document.write()`

Changing innerHTML

html

<h1 id="heading">Old Title</h1>

<script>
  document.getElementById("heading").innerHTML = "New Title";
</script>

Expected output: The page displays "New Title".

Changing an Attribute

HTML attributes (like src, href, value, alt) can be changed by:

Method A · Direct property access:

html

<img id="photo" src="cat.jpg" alt="A cat">

<script>
  var img = document.getElementById("photo");
  img.src = "dog.jpg";   // Changes the image source
  img.alt = "A dog";     // Changes the alt text
</script>

Method B · Using setAttribute:

html

<a id="link" href="https://google.com">Visit</a>

<script>
  document.getElementById("link").setAttribute("href", "https://bing.com");
</script>

Read an attribute with getAttribute:

html

<script>
  var href = document.getElementById("link").getAttribute("href");
  console.log(href); // Output: https://bing.com
</script>

`document.write()` · Use With Caution

document.write() adds content directly into the HTML stream as the page loads:

html

<script>
  document.write("<p>Written during page load</p>");
</script>

⚠️ Critical warning: If you call document.write() after the page has finished loading, it overwrites the entire page. This is almost never what you want. Use innerHTML instead for post-load changes.

Creating and Inserting New Elements

This is one of the most powerful DOM skills: building elements from scratch and inserting them into the page.

html

<ul id="list">
  <li>Item 1</li>
  <li>Item 2</li>
</ul>

<button onclick="addItem()">Add Item</button>

<script>
  function addItem() {
    // Step 1: Create the new element
    var newItem = document.createElement("li");

    // Step 2: Give it content
    newItem.innerHTML = "Item 3";

    // Step 3: Find where to put it
    var list = document.getElementById("list");

    // Step 4: Append it as the last child
    list.appendChild(newItem);
  }
</script>

Expected output: Each button click adds a new <li>Item 3</li> to the list.

Inserting Before a Specific Element

html

<ul id="list">
  <li id="second">Second</li>
</ul>

<script>
  var newItem = document.createElement("li");
  newItem.innerHTML = "First (inserted)";

  var list = document.getElementById("list");
  var secondItem = document.getElementById("second");

  // insertBefore(newNode, referenceNode)
  list.insertBefore(newItem, secondItem);
</script>

Expected output: The list now shows "First (inserted)" above "Second".

Removing Elements

Modern approach (on the element itself):

html

<p id="remove-me">Delete me!</p>

<script>
  document.getElementById("remove-me").remove();
</script>

Classic approach (via parent):

html

<script>
  var child = document.getElementById("remove-me");
  child.parentNode.removeChild(child);
</script>

Both achieve the same result. The .remove() approach is shorter but slightly less supported in very old browsers.

Replacing an Element

html

<p id="old">Old paragraph</p>

<script>
  var newPara = document.createElement("p");
  newPara.innerHTML = "New paragraph";

  var old = document.getElementById("old");
  old.parentNode.replaceChild(newPara, old);
</script>

`outerHTML` · Replace Including the Tag Itself

html

<p id="box">Hello</p>

<script>
  // innerHTML changes only the CONTENT inside the tag
  // outerHTML changes the ENTIRE element including the tag itself

  document.getElementById("box").outerHTML = "<h2>I replaced the paragraph!</h2>";
</script>

Expected output: The <p> tag is completely replaced by an <h2>.

💡 Thinking question: After the code above runs, document.getElementById("box") will return null. Why? Answer: The element with id="box" no longer exists · it was replaced. The new <h2> doesn't have that id.

✏️ Your Task

Practise what you just learned about 4. Changing HTML Content. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 5 of 13

5. Changing CSS Styles

The `style` Property

Every DOM element has a .style property that maps directly to its inline CSS. You can read and write any CSS property through it.

html

<p id="message">Style me!</p>

<script>
  var el = document.getElementById("message");

  el.style.color = "red";
  el.style.fontSize = "24px";
  el.style.fontWeight = "bold";
  el.style.backgroundColor = "yellow";
  el.style.padding = "10px";
</script>

Expected output: The paragraph appears large, bold, red text on a yellow background with padding.

CSS Property Name Conversion Rule

CSS uses hyphens: background-color, font-size, border-radius. JavaScript uses camelCase: backgroundColor, fontSize, borderRadius.

The rule: remove the hyphen and capitalise the next letter.

CSS Property	JavaScript Property
`background-color`	`backgroundColor`
`font-size`	`fontSize`
`border-top-width`	`borderTopWidth`
`z-index`	`zIndex`
`flex-direction`	`flexDirection`
`text-decoration`	`textDecoration`

Toggle Visibility

A classic pattern: show or hide an element based on its current state.

html

<div id="panel" style="display:block;">
  <p>I am visible!</p>
</div>

<button onclick="togglePanel()">Toggle</button>

<script>
  function togglePanel() {
    var panel = document.getElementById("panel");

    if (panel.style.display === "none") {
      panel.style.display = "block";
    } else {
      panel.style.display = "none";
    }
  }
</script>

Expected output: Each button click hides/shows the div.

Using CSS Classes Instead of Inline Styles

Inline styles are powerful but mix logic with presentation. A cleaner approach is to add and remove CSS classes using JavaScript.

html

<style>
  .highlighted {
    background-color: yellow;
    font-weight: bold;
    border: 2px solid orange;
  }
</style>

<p id="note">Click a button to style me.</p>

<button onclick="addStyle()">Add Style</button>
<button onclick="removeStyle()">Remove Style</button>
<button onclick="toggleStyle()">Toggle Style</button>

<script>
  var note = document.getElementById("note");

  function addStyle() {
    note.classList.add("highlighted");
  }

  function removeStyle() {
    note.classList.remove("highlighted");
  }

  function toggleStyle() {
    note.classList.toggle("highlighted");
  }
</script>

The `classList` API · Full Reference

Method / Property	What it does
`classList.add("cls")`	Adds a class
`classList.remove("cls")`	Removes a class
`classList.toggle("cls")`	Adds if missing, removes if present
`classList.contains("cls")`	Returns `true` if the class is present
`classList.replace("old", "new")`	Swaps one class for another
`classList.length`	Number of classes on the element

html

<div id="box" class="card active large"></div>

<script>
  var box = document.getElementById("box");

  console.log(box.classList.contains("active")); // true
  console.log(box.classList.length);             // 3

  box.classList.remove("large");
  console.log(box.classList.length);             // 2

  box.classList.replace("active", "disabled");
  // Now: class="card disabled"
</script>

Reading Computed Styles

.style only reads inline styles. If a style comes from a CSS file or <style> tag, you need getComputedStyle:

html

<style>
  #text { color: blue; font-size: 18px; }
</style>

<p id="text">Blue text</p>

<script>
  var el = document.getElementById("text");

  // This reads the COMPUTED (final applied) style
  var computed = window.getComputedStyle(el);
  console.log(computed.color);    // Output: rgb(0, 0, 255)
  console.log(computed.fontSize); // Output: 18px
</script>

✏️ Your Task

Practise what you just learned about 5. Changing CSS Styles. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 6 of 13

6. Form Validation with the DOM

Why Validate on the Client Side?

When a user fills out a form and clicks Submit, the data travels to a server. Sending invalid data (empty fields, wrong email format) wastes bandwidth and frustrates users with slow error feedback. Client-side validation catches errors instantly, before the data ever leaves the browser.

⚠️ Important: Client-side validation is for user experience. Always also validate on the server side, because users can bypass JavaScript.

A Basic Required-Field Check

html

<form name="myForm" onsubmit="return validateForm()">
  <label>Name:</label>
  <input type="text" id="name" placeholder="Enter your name">
  <br>
  <button type="submit">Submit</button>
</form>

<script>
  function validateForm() {
    var nameInput = document.getElementById("name").value;

    // .trim() removes leading/trailing spaces
    if (nameInput.trim() === "") {
      alert("Name is required!");
      return false; // Stops form submission
    }

    return true; // Allows form submission
  }
</script>

Key concept: return false from an onsubmit handler prevents the form from submitting. return true allows it.

Validating an Email Address

html

<input type="text" id="email" placeholder="Enter email">
<span id="emailError" style="color:red;"></span>

<script>
  function validateEmail() {
    var email = document.getElementById("email").value;
    var errorSpan = document.getElementById("emailError");

    // A simple email pattern check
    var pattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;

    if (!pattern.test(email)) {
      errorSpan.innerText = "Please enter a valid email.";
      return false;
    } else {
      errorSpan.innerText = "";
      return true;
    }
  }
</script>

Numeric Range Validation

html

<label>Age (18–65):</label>
<input type="number" id="age">
<p id="ageError" style="color:red;"></p>

<script>
  function validateAge() {
    var age = Number(document.getElementById("age").value);
    var error = document.getElementById("ageError");

    if (isNaN(age) || age < 18 || age > 65) {
      error.innerText = "Age must be between 18 and 65.";
      return false;
    }
    error.innerText = "";
    return true;
  }
</script>

Automatic HTML5 Validation vs JavaScript Validation

Modern HTML5 includes built-in validation attributes:

Attribute	Behaviour
`required`	Field must not be empty
`type="email"`	Must match email format
`type="number"`	Must be a number
`min` / `max`	Number must be in range
`minlength` / `maxlength`	Text length limits
`pattern`	Must match a regex pattern

html

<input type="email" required placeholder="your@email.com">
<input type="number" min="18" max="65" required>

HTML5 validation fires automatically on submit. JavaScript validation gives you more control over the error messages and timing.

Real-Time Validation with Events

Instead of waiting for form submission, validate as the user types:

html

<input type="text" id="username" oninput="checkUsername(this.value)">
<small id="usernameMsg"></small>

<script>
  function checkUsername(value) {
    var msg = document.getElementById("usernameMsg");

    if (value.length < 3) {
      msg.style.color = "red";
      msg.innerText = "Too short (minimum 3 characters)";
    } else if (value.length > 20) {
      msg.style.color = "red";
      msg.innerText = "Too long (maximum 20 characters)";
    } else {
      msg.style.color = "green";
      msg.innerText = "✓ Username looks good!";
    }
  }
</script>

Expected output: As the user types, the message updates instantly with colour-coded feedback.

Password Confirmation Check

html

<input type="password" id="pass1" placeholder="Password">
<input type="password" id="pass2" placeholder="Confirm Password">
<p id="matchMsg"></p>

<script>
  function checkPasswords() {
    var p1 = document.getElementById("pass1").value;
    var p2 = document.getElementById("pass2").value;
    var msg = document.getElementById("matchMsg");

    if (p2 === "") {
      msg.innerText = "";
    } else if (p1 === p2) {
      msg.style.color = "green";
      msg.innerText = "✓ Passwords match";
    } else {
      msg.style.color = "red";
      msg.innerText = "✗ Passwords do not match";
    }
  }

  document.getElementById("pass2").addEventListener("input", checkPasswords);
</script>

✏️ Your Task

Practise what you just learned about 6. Form Validation with the DOM. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 7 of 13

7. DOM Animations

How DOM Animations Work (The Concept)

CSS transitions and animations are great for simple effects. But JavaScript DOM animations give you precise control using:

setInterval(fn, ms) · runs a function repeatedly every ms milliseconds
clearInterval(id) · stops a running interval
requestAnimationFrame(fn) · the modern, performance-friendly alternative

The core idea: change a CSS property by a small amount, many times per second, to create the illusion of smooth movement.

The Timer Functions Explained

Function	What it does
`setTimeout(fn, ms)`	Calls `fn` once after `ms` milliseconds
`setInterval(fn, ms)`	Calls `fn` repeatedly every `ms` milliseconds
`clearInterval(id)`	Stops an interval using its returned id
`requestAnimationFrame(fn)`	Calls `fn` before the next screen repaint (~60fps)

Micro-Demo: Blinking Text

html

<p id="blink">Notice me!</p>

<script>
  var visible = true;

  setInterval(function() {
    var el = document.getElementById("blink");
    el.style.visibility = visible ? "hidden" : "visible";
    visible = !visible;
  }, 500); // Every 500ms (half a second)
</script>

Expected output: The text alternates between visible and hidden twice per second.

Building a Slide Animation Step by Step

This is the classic W3Schools sliding box animation · let's build it together and understand every line.

HTML structure:

html

<div id="container" style="
  width: 400px;
  height: 400px;
  position: relative;
  background: #ddd;
">
  <div id="box" style="
    width: 50px;
    height: 50px;
    background: red;
    position: absolute;
    top: 0px;
    left: 0px;
  "></div>
</div>

<button onclick="startMove()">Start</button>
<button onclick="stopMove()">Stop</button>

Why position: relative on the container? The box uses position: absolute which positions it relative to its nearest positioned ancestor. By setting relative on the container, the box moves within the container, not the entire page.

JavaScript:

javascript

var timer = null;   // Stores the interval ID so we can stop it later

function startMove() {
  // Prevent starting multiple intervals at once
  clearInterval(timer);

  timer = setInterval(function() {
    var box = document.getElementById("box");

    // Read current left position (removing "px" to get a number)
    var currentLeft = parseInt(box.style.left);

    // If box has reached the right edge of the container, stop
    if (currentLeft >= 350) {  // 400px container - 50px box = 350px max
      clearInterval(timer);
    } else {
      // Move box 1px to the right
      box.style.left = (currentLeft + 1) + "px";
    }
  }, 5); // Every 5ms ≈ 200 frames per second
}

function stopMove() {
  clearInterval(timer);
}

Line-by-line breakdown:

Code	Meaning
`var timer = null`	Declares a variable outside the function so it persists between calls
`clearInterval(timer)`	Ensures no duplicate intervals run at the same time
`parseInt(box.style.left)`	Converts `"0px"` (a string) into `0` (a number) for arithmetic
`(currentLeft + 1) + "px"`	Increases by 1, then converts back to `"1px"` string
`clearInterval(timer)` at the boundary	Stops the animation when the box reaches the edge

Using `requestAnimationFrame` · The Modern Way

setInterval has a problem: it fires on its own schedule, not in sync with the browser's painting cycle. This can cause jank (stuttering). requestAnimationFrame is smarter:

javascript

var pos = 0;
var animating = false;
var animId;

function animate() {
  if (!animating) return;

  var box = document.getElementById("box");
  pos++;

  if (pos >= 350) {
    pos = 350;
    animating = false;
  }

  box.style.left = pos + "px";

  // Request the NEXT frame
  animId = requestAnimationFrame(animate);
}

function startMove() {
  if (!animating) {
    animating = true;
    animate();
  }
}

function stopMove() {
  animating = false;
  cancelAnimationFrame(animId);
}

Why is this better?

Syncs with the display refresh rate (usually 60fps)
Pauses automatically when the tab is not visible (saves CPU/battery)
Smoother than setInterval

Animating Multiple Properties

html

<div id="bubble" style="
  width: 60px;
  height: 60px;
  border-radius: 50%;
  background: blue;
  position: absolute;
  top: 200px;
  left: 0px;
  opacity: 1;
"></div>

<script>
  var x = 0;
  var opacity = 1;

  function animate() {
    var el = document.getElementById("bubble");

    x += 2;
    opacity -= 0.005;

    el.style.left = x + "px";
    el.style.opacity = opacity;

    if (x < 400 && opacity > 0) {
      requestAnimationFrame(animate);
    }
  }

  requestAnimationFrame(animate);
</script>

Expected output: A blue circle slides to the right while gradually fading out.

✏️ Your Task

Practise what you just learned about 7. DOM Animations. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 8 of 13

8. The Document Object in Depth

What Is the `document` Object?

The document object is the root of the HTML DOM · it represents the loaded HTML page and provides access to every element, attribute, and text node in it.

You never create document · it's automatically available in any JavaScript running in a browser.

Key `document` Properties

Property	What it returns
`document.title`	The page title (from `<title>` tag)
`document.URL`	Full URL of the current page
`document.domain`	Domain name (e.g., `"example.com"`)
`document.referrer`	URL of the page that linked here
`document.lastModified`	Date/time the page was last modified
`document.readyState`	Loading state: `"loading"`, `"interactive"`, `"complete"`
`document.characterSet`	Character encoding (usually `"UTF-8"`)
`document.doctype`	The DOCTYPE declaration
`document.head`	The `<head>` element
`document.body`	The `<body>` element
`document.documentElement`	The `<html>` element
`document.forms`	HTMLCollection of all `<form>` elements
`document.images`	HTMLCollection of all `<img>` elements
`document.links`	HTMLCollection of all `<a>` elements with href
`document.scripts`	HTMLCollection of all `<script>` elements
`document.styleSheets`	List of all linked/embedded stylesheets
`document.cookie`	Get or set cookies

Practical Examples

Reading page information:

javascript

console.log(document.title);        // "My Page Title"
console.log(document.URL);          // "https://example.com/page.html"
console.log(document.domain);       // "example.com"
console.log(document.lastModified); // "03/17/2026 10:30:00"
console.log(document.readyState);   // "complete"

Accessing the <body> directly:

javascript

document.body.style.backgroundColor = "lightblue";
document.body.style.fontFamily = "Arial, sans-serif";

Working with forms:

javascript

console.log(document.forms.length);         // Number of forms on the page
console.log(document.forms[0].id);          // id of first form
console.log(document.forms["loginForm"]);   // Access by name attribute

Working with images:

javascript

var images = document.images;
for (var i = 0; i < images.length; i++) {
  console.log(images[i].src); // Prints URL of each image
}

Working with links:

javascript

var links = document.links;
for (var i = 0; i < links.length; i++) {
  links[i].style.color = "purple"; // Changes all link colours
}

Creating Elements · Full Node API

The document object is responsible for creating all new nodes:

Method	Creates
`document.createElement(tag)`	A new HTML element
`document.createTextNode(text)`	A raw text node
`document.createAttribute(name)`	A new attribute node
`document.createComment(text)`	An HTML comment node
`document.createDocumentFragment()`	A lightweight container for building DOM structures off-screen

Text node example:

javascript

var para = document.createElement("p");
var text = document.createTextNode("Hello from a text node!");
para.appendChild(text);
document.body.appendChild(para);
// Result: <p>Hello from a text node!</p> added to the page

DocumentFragment · Performance Trick

When adding many elements, inserting them one by one causes the browser to repaint the screen repeatedly. A DocumentFragment lets you build the structure in memory first, then insert everything at once:

javascript

var fragment = document.createDocumentFragment();

for (var i = 1; i <= 100; i++) {
  var li = document.createElement("li");
  li.innerText = "Item " + i;
  fragment.appendChild(li); // No repaint yet
}

// ONE repaint only
document.getElementById("myList").appendChild(fragment);

This is significantly faster than 100 individual appendChild calls on the live DOM.

Cookies with `document.cookie`

javascript

// Set a cookie
document.cookie = "username=Alice; expires=Thu, 31 Dec 2026 12:00:00 UTC; path=/";

// Read all cookies (returns one big string)
console.log(document.cookie);
// Output: "username=Alice; theme=dark"

// To read a specific cookie, you need to parse the string manually
function getCookie(name) {
  var cookies = document.cookie.split("; ");
  for (var i = 0; i < cookies.length; i++) {
    var parts = cookies[i].split("=");
    if (parts[0] === name) return parts[1];
  }
  return null;
}

console.log(getCookie("username")); // Output: Alice

💡 For modern projects, use localStorage or sessionStorage for client-side data storage. Cookies are mainly for data that needs to be sent to the server.

`document.open()`, `document.write()`, `document.close()`

These three work together to write an entirely new document:

javascript

document.open();
document.write("<html><body><h1>New document written by JS!</h1></body></html>");
document.close();

⚠️ This completely replaces the existing page. Used mainly in old-style popup windows. Avoid in modern projects.

✏️ Your Task

Practise what you just learned about 8. The Document Object in Depth. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 9 of 13

9. HTML Element Reference

The Element as an Object

Every HTML element in the DOM is represented as a JavaScript object. That object has:

Properties · data about the element (its id, class, style, content, etc.)
Methods · actions you can perform (click it, focus it, blur it, etc.)

This section catalogues the most important ones.

Universal Element Properties (Available on Every Element)

Property	Type	Description
`id`	String	The element's `id` attribute
`tagName`	String	Tag name in UPPERCASE (`"DIV"`, `"P"`, `"INPUT"`)
`nodeName`	String	Same as tagName for elements
`nodeType`	Number	1=Element, 3=Text, 8=Comment, 9=Document
`innerHTML`	String	HTML content inside the element
`innerText`	String	Visible text content (no tags)
`textContent`	String	All text content (includes hidden text)
`outerHTML`	String	The element including its own opening/closing tags
`className`	String	All CSS classes as a space-separated string
`classList`	DOMTokenList	Live list of classes (use `.add()`, `.remove()`, `.toggle()`)
`style`	CSSStyleDeclaration	Inline styles
`attributes`	NamedNodeMap	All attributes of the element
`dataset`	DOMStringMap	Custom `data-*` attributes
`hidden`	Boolean	Whether the element is hidden
`tabIndex`	Number	Keyboard tab order
`title`	String	Tooltip text
`lang`	String	Language code
`dir`	String	Text direction: `"ltr"` or `"rtl"`
`accessKey`	String	Keyboard shortcut key
`contentEditable`	String	`"true"` if user can edit the element's content
`isContentEditable`	Boolean	Read-only. Is the element editable?
`draggable`	Boolean	Can the element be dragged?

Position and Size Properties

Property	Description
`offsetWidth`	Total width including padding and border
`offsetHeight`	Total height including padding and border
`offsetLeft`	Distance from left edge of offset parent
`offsetTop`	Distance from top edge of offset parent
`offsetParent`	Nearest positioned ancestor element
`clientWidth`	Width including padding, excluding border and scrollbar
`clientHeight`	Height including padding, excluding border
`scrollWidth`	Full scrollable width
`scrollHeight`	Full scrollable height
`scrollLeft`	Pixels scrolled horizontally
`scrollTop`	Pixels scrolled vertically
`getBoundingClientRect()`	Returns size and position relative to viewport

javascript

var box = document.getElementById("box");

console.log(box.offsetWidth);   // e.g. 300 (px)
console.log(box.offsetTop);     // e.g. 120 (px from top of offset parent)

var rect = box.getBoundingClientRect();
console.log(rect.top);    // Distance from top of viewport
console.log(rect.left);   // Distance from left of viewport
console.log(rect.width);  // Element width
console.log(rect.height); // Element height

Tree Navigation Properties

Property	Returns
`parentNode`	Parent node (may be a text node or document)
`parentElement`	Parent element (always an element or null)
`childNodes`	NodeList of all child nodes
`children`	HTMLCollection of child elements only
`firstChild`	First child node
`lastChild`	Last child node
`firstElementChild`	First child element
`lastElementChild`	Last child element
`nextSibling`	Next sibling node
`previousSibling`	Previous sibling node
`nextElementSibling`	Next sibling element
`previousElementSibling`	Previous sibling element
`childElementCount`	Number of child elements

Universal Element Methods

Method	Description
`getAttribute(name)`	Get the value of an attribute
`setAttribute(name, value)`	Set an attribute value
`removeAttribute(name)`	Remove an attribute
`hasAttribute(name)`	Returns boolean: does the attribute exist?
`appendChild(node)`	Add a child node at the end
`insertBefore(new, ref)`	Insert before a reference child
`removeChild(child)`	Remove a child node
`replaceChild(new, old)`	Replace a child node
`cloneNode(deep)`	Clone the element; `deep=true` clones children too
`contains(node)`	Returns boolean: is the node a descendant?
`matches(selector)`	Returns boolean: does the element match a CSS selector?
`closest(selector)`	Walks up the tree, returns first ancestor matching selector
`querySelector(sel)`	Finds first matching descendant
`querySelectorAll(sel)`	Finds all matching descendants
`insertAdjacentHTML(pos, html)`	Inserts HTML at a specific position
`insertAdjacentElement(pos, el)`	Inserts an element at a specific position
`scrollIntoView()`	Scrolls the element into the visible viewport
`focus()`	Gives focus to the element
`blur()`	Removes focus
`click()`	Programmatically clicks the element
`remove()`	Removes the element from the DOM

`insertAdjacentHTML` Positions

html

<div id="target">TARGET</div>

<script>
  var el = document.getElementById("target");

  el.insertAdjacentHTML("beforebegin", "<p>Before the div</p>");
  // <p>Before the div</p>
  // <div id="target">TARGET</div>

  el.insertAdjacentHTML("afterbegin", "<p>First inside div</p>");
  // <div id="target"><p>First inside div</p>TARGET</div>

  el.insertAdjacentHTML("beforeend", "<p>Last inside div</p>");
  // <div id="target">TARGET<p>Last inside div</p></div>

  el.insertAdjacentHTML("afterend", "<p>After the div</p>");
  // <div id="target">TARGET</div>
  // <p>After the div</p>
</script>

`cloneNode` · Copying Elements

html

<div id="original">
  <p>Hello</p>
  <span>World</span>
</div>

<script>
  var original = document.getElementById("original");

  // Shallow clone — copies only the div, NOT its children
  var shallow = original.cloneNode(false);

  // Deep clone — copies the div AND all its children
  var deep = original.cloneNode(true);

  document.body.appendChild(deep);
  // Adds an identical copy of the div (with <p> and <span>) to the body
</script>

`matches` and `closest`

html

<nav>
  <ul class="menu">
    <li class="item active"><a href="#">Home</a></li>
    <li class="item"><a href="#">About</a></li>
  </ul>
</nav>

<script>
  var link = document.querySelector("a");

  // Does this element match a CSS selector?
  console.log(link.matches("a"));          // true
  console.log(link.matches(".item a"));    // true
  console.log(link.matches(".active a"));  // true

  // Walk UP the tree to find the nearest ancestor matching a selector
  var closestMenu = link.closest(".menu");
  console.log(closestMenu.tagName); // Output: UL

  var closestNav = link.closest("nav");
  console.log(closestNav.tagName); // Output: NAV
</script>

`data-*` Attributes and the `dataset` API

Custom data attributes let you embed extra information in HTML without affecting layout:

html

<div id="product"
  data-id="42"
  data-name="Blue Shirt"
  data-price="29.99"
  data-in-stock="true">
  Blue Shirt
</div>

<script>
  var el = document.getElementById("product");

  console.log(el.dataset.id);       // "42"
  console.log(el.dataset.name);     // "Blue Shirt"
  console.log(el.dataset.price);    // "29.99"
  console.log(el.dataset.inStock);  // "true" (note: camelCase conversion)

  // Modify a data attribute
  el.dataset.price = "24.99";

  // Add a new data attribute
  el.dataset.discount = "10";
  // Now the HTML has data-discount="10"
</script>

💡 The naming conversion: data-in-stock → dataset.inStock (hyphen removed, next letter capitalised · same rule as CSS properties).

Input-Specific Properties

<input>, <textarea>, and <select> have special properties:

Property	Type	Description
`value`	String	Current value of the input
`defaultValue`	String	Original value (before user changes)
`checked`	Boolean	Whether a checkbox/radio is checked
`defaultChecked`	Boolean	Original checked state
`disabled`	Boolean	Whether the input is disabled
`readOnly`	Boolean	Whether the input is read-only
`required`	Boolean	Whether the input is required
`placeholder`	String	Placeholder text
`type`	String	Input type (`"text"`, `"number"`, etc.)
`name`	String	Name attribute (used in form submission)
`form`	HTMLFormElement	The form this input belongs to
`validity`	ValidityState	Validation state object
`validationMessage`	String	Browser's validation error message

javascript

var input = document.getElementById("myInput");

input.value = "Hello";          // Set value
console.log(input.value);       // Read value
input.disabled = true;          // Disable it
input.readOnly = false;         // Make it editable
input.focus();                  // Move cursor to this field
input.select();                 // Select all text in it

# PHASE 2 · APPLIED EXERCISES

✏️ Your Task

Practise what you just learned about 9. HTML Element Reference. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 10 of 13

10. Applied Exercises

Exercise 1: Live Word Counter

Objective: Use DOM manipulation to count characters and words in a textarea in real time.

Real-world scenario: You are building a Twitter-like post composer. Users must stay under 280 characters. You need to show them their remaining character count as they type.

Warm-up mini-example:

html

<input type="text" oninput="showLength(this)">
<p id="len">0 characters</p>

<script>
  function showLength(input) {
    document.getElementById("len").innerText = input.value.length + " characters";
  }
</script>

Your exercise:

Build a textarea with:

A character counter (e.g., "245/280 characters used")
A word counter (e.g., "32 words")
A colour indicator: green when under 240, orange from 240 · 279, red at 280+
A submit button that is disabled until at least 10 characters are entered

Step-by-step instructions:

Create a <textarea id="post"> with maxlength="280"
Add <p id="charCount"> and <p id="wordCount"> display elements
Add a <button id="submitBtn" disabled>Post</button>
Write an oninput handler that:

- Reads textarea.value.length - Updates the character counter text - Splits by whitespace to count words: text.trim().split(/\s+/).filter(Boolean).length - Changes the counter colour using style.color based on length - Sets button.disabled = (length < 10)

Hints:

"".trim().split(/\s+/).filter(Boolean).length returns 0, not 1. The .filter(Boolean) removes empty strings from split on an empty input.
Access the textarea's value: document.getElementById("post").value

Self-check questions:

What does filter(Boolean) do when the textarea is empty?
Why use trim() before splitting?
What happens to disabled when the user clears the textarea?

Optional what-if: Change the colour of the textarea border (not just the counter) based on length. Also display "X characters remaining" instead of "X used".

Exercise 2: Dynamic Student Grade Table

Objective: Let the user add rows to a table using the DOM.

Real-world scenario: A school administrator app where teachers can add student grades without a page reload.

Warm-up mini-example:

html

<table id="t"><tr><th>Name</th><th>Grade</th></tr></table>
<button onclick="addRow()">Add Row</button>

<script>
  function addRow() {
    var row = document.createElement("tr");
    row.innerHTML = "<td>Student</td><td>A</td>";
    document.getElementById("t").appendChild(row);
  }
</script>

Your exercise:

Build a grade entry system with:

Input fields: student name, subject, grade (number 0 · 100)
An "Add Student" button
A <table> that shows each added student
A colour-coded grade: green (≥70), orange (50 · 69), red (<50)
A running class average displayed below the table
A Delete button on each row

Step-by-step instructions:

Create three <input> fields with ids "name", "subject", "grade"
Create an empty <table id="gradeTable"> with headers
Create <p id="average">Class Average: · </p>
In addStudent():

- Read input values - Validate: name not empty, grade is 0 · 100 - Create a <tr> with <td> cells, setting the grade td's background colour - Add a Delete button: set its onclick to this.parentElement.parentElement.remove() - After removing, recalculate the average

Grade colour logic:

javascript

var gradeNum = parseInt(grade);
var gradeCell = document.createElement("td");
gradeCell.innerText = gradeNum;
if (gradeNum >= 70) gradeCell.style.backgroundColor = "#c8f7c5";
else if (gradeNum >= 50) gradeCell.style.backgroundColor = "#fde8a0";
else gradeCell.style.backgroundColor = "#fdc5c5";

Self-check questions:

Why do you need parseInt() when reading a number input's .value?
After clicking Delete, how does the average recalculate without knowing which row was removed?
Why use this.parentElement.parentElement.remove() instead of an id in the Delete button?

Exercise 3: CSS Theme Switcher

Objective: Toggle between light and dark themes using classList.

Real-world scenario: Most modern web apps (GitHub, VS Code, Twitter) offer a dark mode. This is typically done by toggling a class on the <body>.

Step-by-step instructions:

In <style>, define two themes:

css

body.light {
  background-color: white;
  color: black;
}
body.dark {
  background-color: #1a1a2e;
  color: #eee;
}
.card {
  padding: 20px;
  border: 1px solid currentColor;
  margin: 10px;
}

Set <body class="light">
Create a toggle button
In the toggle handler:

- Use document.body.classList.toggle("dark") - Use document.body.classList.toggle("light") - Update the button text: "Switch to Dark" / "Switch to Light"

Remember theme with localStorage: localStorage.setItem("theme", "dark") and read it on page load

Self-check questions:

Why toggle both classes instead of just adding/removing "dark"?
What happens if a user opens the page in a new tab · will their theme preference persist?
How would you detect the user's operating system dark mode preference using window.matchMedia?

Exercise 4: Animated Progress Bar

Objective: Build an animated progress bar using DOM animations.

Real-world scenario: File upload progress indicators, quiz result reveals, loading screens.

Step-by-step instructions:

HTML:

html

<div style="width:400px; background:#ddd; height:30px; border-radius:15px;">
  <div id="bar" style="
    width: 0%;
    height: 100%;
    background: linear-gradient(to right, #4CAF50, #8BC34A);
    border-radius: 15px;
    transition: width 0.1s;
  "></div>
</div>
<p id="percent">0%</p>
<button onclick="startProgress()">Start</button>
<button onclick="resetProgress()">Reset</button>

JavaScript:

- Create a variable progress = 0 - Use setInterval every 50ms to increase progress by 1 - Update bar.style.width = progress + "%" - Update percent.innerText = progress + "%" - When progress reaches 100, call clearInterval

Enhancement: Change the bar colour as it fills:

- 0 · 40%: red - 41 · 70%: orange - 71 · 100%: green

Self-check questions:

Why is clearInterval essential · what happens without it?
How do you prevent the user from clicking "Start" multiple times and creating multiple intervals?
What does transition: width 0.1s in CSS do? How does this interact with the JS animation?

Exercise 5: Form Validation Suite

Objective: Build a complete registration form with comprehensive validation.

Fields to validate:

Field	Rules
Full Name	Required, letters and spaces only, 2 · 50 chars
Email	Required, valid email format
Phone	Required, exactly 10 digits
Password	Required, min 8 chars, at least 1 uppercase, 1 digit
Confirm Password	Must match password
Age	Required, number 16 · 120
Terms checkbox	Must be checked

Step-by-step instructions:

Create the form HTML with one <span class="error"> next to each field
Write a validation function for each field
Call all validators on form submit
Show error messages next to each invalid field (not alert boxes)
Highlight invalid inputs with a red border using classList.add("invalid") and a CSS rule .invalid { border: 2px solid red; }
Clear error messages and borders for valid fields

Hint for password strength check:

javascript

function validatePassword(pass) {
  var hasUpper = /[A-Z]/.test(pass);
  var hasDigit = /[0-9]/.test(pass);
  var hasMinLength = pass.length >= 8;
  return hasUpper && hasDigit && hasMinLength;
}

# PHASE 3 · PROJECT SIMULATION

✏️ Your Task

Practise what you just learned about 10. Applied Exercises. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 11 of 13

11. Project Simulation: Interactive Student Dashboard

Project Overview

You are a junior front-end developer at EduTrack, a school management software company. Your task is to build a client-side Student Performance Dashboard · a single HTML page that teachers can use to:

Add students and their test scores
View a dynamically-built grade table
Filter students by performance category
Animate a summary panel when scores are entered
Validate all inputs before accepting them

No frameworks. No libraries. Pure HTML + CSS + JavaScript DOM.

Stage 1: Setup & Core Logic

Goal: Build the data entry form and display students in a table.

Preview micro-example:

javascript

var students = []; // Our in-memory data store

function addStudent(name, score) {
  students.push({ name: name, score: Number(score) });
  renderTable();
}

function renderTable() {
  var tbody = document.getElementById("studentBody");
  tbody.innerHTML = ""; // Clear existing rows

  students.forEach(function(student, index) {
    var row = document.createElement("tr");
    row.innerHTML = `
      <td>${student.name}</td>
      <td>${student.score}</td>
      <td>${getGrade(student.score)}</td>
    `;
    tbody.appendChild(row);
  });
}

function getGrade(score) {
  if (score >= 90) return "A";
  if (score >= 80) return "B";
  if (score >= 70) return "C";
  if (score >= 60) return "D";
  return "F";
}

Expected output after Stage 1: A functional table that grows as students are added.

Milestones:

[ ] HTML form: name input, score input (0 · 100), subject dropdown, Add button
[ ] Input validation: name required, score is 0 · 100, duplicate names warned
[ ] Data array stores all entries
[ ] Table renders from array (re-rendered on every change)
[ ] Each row has a colour-coded grade cell
[ ] Delete button on each row removes from array and re-renders

Stage 2: Adding Features

Goal: Add filtering, sorting, and a statistics panel.

Preview micro-example · filtering:

javascript

function filterStudents(category) {
  var filtered;

  if (category === "all") {
    filtered = students;
  } else if (category === "passing") {
    filtered = students.filter(function(s) { return s.score >= 60; });
  } else if (category === "failing") {
    filtered = students.filter(function(s) { return s.score < 60; });
  } else if (category === "excellent") {
    filtered = students.filter(function(s) { return s.score >= 90; });
  }

  renderTable(filtered); // Pass the filtered list to the renderer
}

Expected output after Stage 2: Buttons that instantly filter the table. A stats panel showing count, average, highest, and lowest scores.

Milestones:

[ ] Filter buttons: All / Passing (≥60) / Failing (<60) / Excellent (≥90)
[ ] Sort by name (A · Z) or score (high · low, low · high)
[ ] Statistics panel showing:

- Total students - Class average (to 1 decimal place) - Highest scorer + name - Lowest scorer + name - Count per grade letter (A, B, C, D, F)

[ ] Statistics panel animates in (fade from opacity 0 to 1) when first student is added
[ ] Export button that generates a summary <pre> block with all data formatted as text

Stage 3: Displaying Results

Goal: Add a visual grade distribution bar chart using DOM animation.

Preview micro-example · drawing bars:

javascript

function drawChart() {
  var grades = { A: 0, B: 0, C: 0, D: 0, F: 0 };

  students.forEach(function(s) {
    grades[getGrade(s.score)]++;
  });

  var total = students.length;

  Object.keys(grades).forEach(function(letter) {
    var barEl = document.getElementById("bar-" + letter);
    var percent = total > 0 ? (grades[letter] / total) * 100 : 0;
    barEl.style.width = "0%"; // Reset

    // Animate to actual width
    setTimeout(function() {
      barEl.style.transition = "width 0.8s ease";
      barEl.style.width = percent + "%";
    }, 50);

    document.getElementById("label-" + letter).innerText =
      letter + ": " + grades[letter] + " students";
  });
}

Each bar structure:

html

<div style="margin-bottom: 8px;">
  <span id="label-A" style="display:inline-block; width:100px;">A: 0 students</span>
  <div style="display:inline-block; background:#ddd; width:300px; height:20px;">
    <div id="bar-A" style="background:#4CAF50; height:100%; width:0%;"></div>
  </div>
</div>

Expected output after Stage 3: When students are added or removed, the bar chart animates to reflect the new grade distribution.

Full Project Template

html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>EduTrack — Student Dashboard</title>
  <style>
    body { font-family: Arial, sans-serif; max-width: 900px; margin: 20px auto; padding: 0 20px; }
    h1 { color: #2c3e50; }
    .form-group { margin-bottom: 10px; }
    .form-group label { display: inline-block; width: 100px; }
    input, select { padding: 6px; margin: 4px; border: 1px solid #ccc; border-radius: 4px; }
    input.invalid { border-color: red; }
    .error { color: red; font-size: 0.85em; }
    button { padding: 8px 16px; cursor: pointer; border: none; border-radius: 4px; }
    .btn-primary { background: #3498db; color: white; }
    .btn-danger { background: #e74c3c; color: white; font-size: 0.8em; padding: 4px 8px; }
    .btn-filter { background: #ecf0f1; color: #2c3e50; margin: 2px; }
    .btn-filter.active { background: #3498db; color: white; }

    table { border-collapse: collapse; width: 100%; margin-top: 10px; }
    th, td { border: 1px solid #ddd; padding: 8px 12px; text-align: left; }
    th { background: #2c3e50; color: white; }
    tr:nth-child(even) { background: #f9f9f9; }

    .grade-A { background: #c8f7c5 !important; }
    .grade-B { background: #d5f5e3 !important; }
    .grade-C { background: #fef9e7 !important; }
    .grade-D { background: #fde8a0 !important; }
    .grade-F { background: #fdc5c5 !important; }

    #statsPanel { opacity: 0; transition: opacity 1s; margin-top: 20px; padding: 15px; background: #ecf0f1; border-radius: 8px; }
    #statsPanel.visible { opacity: 1; }

    .chart-bar-bg { display: inline-block; background: #ddd; width: 250px; height: 18px; vertical-align: middle; }
    .chart-bar { height: 100%; width: 0%; transition: width 0.8s ease; }
    .chart-label { display: inline-block; width: 120px; font-size: 0.9em; }
  </style>
</head>
<body>

  <h1>📊 EduTrack — Student Performance Dashboard</h1>

  <!-- SECTION 1: Entry Form -->
  <section>
    <h2>Add Student</h2>
    <div class="form-group">
      <label>Name:</label>
      <input type="text" id="studentName" placeholder="Full name">
      <span class="error" id="nameError"></span>
    </div>
    <div class="form-group">
      <label>Subject:</label>
      <select id="subject">
        <option value="Math">Math</option>
        <option value="English">English</option>
        <option value="Science">Science</option>
        <option value="History">History</option>
      </select>
    </div>
    <div class="form-group">
      <label>Score (0–100):</label>
      <input type="number" id="score" min="0" max="100" placeholder="0–100">
      <span class="error" id="scoreError"></span>
    </div>
    <button class="btn-primary" onclick="handleAdd()">+ Add Student</button>
  </section>

  <!-- SECTION 2: Filter Controls -->
  <section style="margin-top: 20px;">
    <strong>Filter:</strong>
    <button class="btn-filter active" onclick="setFilter('all', this)">All</button>
    <button class="btn-filter" onclick="setFilter('passing', this)">Passing (≥60)</button>
    <button class="btn-filter" onclick="setFilter('excellent', this)">Excellent (≥90)</button>
    <button class="btn-filter" onclick="setFilter('failing', this)">Failing (&lt;60)</button>

    <strong style="margin-left: 20px;">Sort:</strong>
    <button class="btn-filter" onclick="setSort('name')">Name A–Z</button>
    <button class="btn-filter" onclick="setSort('score-high')">Score ↓</button>
    <button class="btn-filter" onclick="setSort('score-low')">Score ↑</button>
  </section>

  <!-- SECTION 3: Grade Table -->
  <table id="gradeTable">
    <thead>
      <tr>
        <th>#</th>
        <th>Name</th>
        <th>Subject</th>
        <th>Score</th>
        <th>Grade</th>
        <th>Action</th>
      </tr>
    </thead>
    <tbody id="studentBody">
      <tr><td colspan="6" style="text-align:center; color:#aaa;">No students yet.</td></tr>
    </tbody>
  </table>

  <!-- SECTION 4: Statistics Panel (animates in) -->
  <div id="statsPanel">
    <h3>📈 Class Statistics</h3>
    <div id="statsContent"></div>
    <h4>Grade Distribution</h4>
    <div id="gradeChart"></div>
  </div>

  <script>
    var students = [];
    var currentFilter = "all";
    var currentSort = "none";
    var statsShown = false;

    // ─── Validation ───────────────────────────────────────
    function validateInputs(name, score) {
      var valid = true;

      document.getElementById("nameError").innerText = "";
      document.getElementById("scoreError").innerText = "";
      document.getElementById("studentName").classList.remove("invalid");
      document.getElementById("score").classList.remove("invalid");

      if (name.trim() === "") {
        document.getElementById("nameError").innerText = "Name is required.";
        document.getElementById("studentName").classList.add("invalid");
        valid = false;
      }

      var scoreNum = Number(score);
      if (score === "" || isNaN(scoreNum) || scoreNum < 0 || scoreNum > 100) {
        document.getElementById("scoreError").innerText = "Enter a score between 0 and 100.";
        document.getElementById("score").classList.add("invalid");
        valid = false;
      }

      return valid;
    }

    // ─── Add Student ──────────────────────────────────────
    function handleAdd() {
      var name = document.getElementById("studentName").value;
      var subject = document.getElementById("subject").value;
      var score = document.getElementById("score").value;

      if (!validateInputs(name, score)) return;

      students.push({
        id: Date.now(),
        name: name.trim(),
        subject: subject,
        score: Number(score)
      });

      // Clear inputs
      document.getElementById("studentName").value = "";
      document.getElementById("score").value = "";

      render();

      // Animate stats panel in on first student
      if (!statsShown) {
        statsShown = true;
        setTimeout(function() {
          document.getElementById("statsPanel").classList.add("visible");
        }, 100);
      }
    }

    // ─── Delete Student ───────────────────────────────────
    function deleteStudent(id) {
      students = students.filter(function(s) { return s.id !== id; });
      render();
    }

    // ─── Grading ──────────────────────────────────────────
    function getGrade(score) {
      if (score >= 90) return "A";
      if (score >= 80) return "B";
      if (score >= 70) return "C";
      if (score >= 60) return "D";
      return "F";
    }

    // ─── Filter & Sort ────────────────────────────────────
    function setFilter(filter, btn) {
      currentFilter = filter;
      document.querySelectorAll(".btn-filter").forEach(function(b) {
        b.classList.remove("active");
      });
      btn.classList.add("active");
      render();
    }

    function setSort(sort) {
      currentSort = sort;
      render();
    }

    function applyFilterSort(list) {
      // Filter
      var filtered = list.filter(function(s) {
        if (currentFilter === "passing") return s.score >= 60;
        if (currentFilter === "failing") return s.score < 60;
        if (currentFilter === "excellent") return s.score >= 90;
        return true;
      });

      // Sort
      if (currentSort === "name") {
        filtered.sort(function(a, b) { return a.name.localeCompare(b.name); });
      } else if (currentSort === "score-high") {
        filtered.sort(function(a, b) { return b.score - a.score; });
      } else if (currentSort === "score-low") {
        filtered.sort(function(a, b) { return a.score - b.score; });
      }

      return filtered;
    }

    // ─── Render Table ─────────────────────────────────────
    function renderTable(list) {
      var tbody = document.getElementById("studentBody");
      tbody.innerHTML = "";

      if (list.length === 0) {
        tbody.innerHTML = "<tr><td colspan='6' style='text-align:center; color:#aaa;'>No students match the filter.</td></tr>";
        return;
      }

      list.forEach(function(student, index) {
        var grade = getGrade(student.score);
        var row = document.createElement("tr");
        row.className = "grade-" + grade;

        row.innerHTML =
          "<td>" + (index + 1) + "</td>" +
          "<td>" + student.name + "</td>" +
          "<td>" + student.subject + "</td>" +
          "<td>" + student.score + "</td>" +
          "<td><strong>" + grade + "</strong></td>" +
          "<td><button class='btn-danger' data-id='" + student.id + "'>Delete</button></td>";

        row.querySelector(".btn-danger").addEventListener("click", function() {
          deleteStudent(Number(this.getAttribute("data-id")));
        });

        tbody.appendChild(row);
      });
    }

    // ─── Render Stats ─────────────────────────────────────
    function renderStats() {
      if (students.length === 0) {
        document.getElementById("statsContent").innerHTML = "<p>Add students to see statistics.</p>";
        document.getElementById("gradeChart").innerHTML = "";
        return;
      }

      var scores = students.map(function(s) { return s.score; });
      var avg = (scores.reduce(function(a, b) { return a + b; }, 0) / scores.length).toFixed(1);
      var highest = Math.max.apply(null, scores);
      var lowest = Math.min.apply(null, scores);
      var topStudent = students.find(function(s) { return s.score === highest; });
      var bottomStudent = students.find(function(s) { return s.score === lowest; });

      var gradeCounts = { A: 0, B: 0, C: 0, D: 0, F: 0 };
      students.forEach(function(s) { gradeCounts[getGrade(s.score)]++; });

      document.getElementById("statsContent").innerHTML =
        "<p>👥 <strong>Total Students:</strong> " + students.length + "</p>" +
        "<p>📊 <strong>Class Average:</strong> " + avg + "</p>" +
        "<p>🏆 <strong>Top Scorer:</strong> " + topStudent.name + " (" + highest + ")</p>" +
        "<p>📉 <strong>Lowest Scorer:</strong> " + bottomStudent.name + " (" + lowest + ")</p>";

      // Build animated grade chart
      var chartHTML = "";
      var colors = { A: "#4CAF50", B: "#8BC34A", C: "#FFC107", D: "#FF9800", F: "#F44336" };

      Object.keys(gradeCounts).forEach(function(letter) {
        var pct = students.length > 0 ? (gradeCounts[letter] / students.length * 100) : 0;
        chartHTML +=
          "<div style='margin-bottom:8px;'>" +
          "<span class='chart-label'>" + letter + ": " + gradeCounts[letter] + " student(s)</span>" +
          "<div class='chart-bar-bg'><div id='bar-" + letter + "' class='chart-bar' style='background:" + colors[letter] + ";'></div></div>" +
          "</div>";
      });

      document.getElementById("gradeChart").innerHTML = chartHTML;

      // Animate bars
      Object.keys(gradeCounts).forEach(function(letter) {
        var pct = students.length > 0 ? (gradeCounts[letter] / students.length * 100) : 0;
        setTimeout(function() {
          var bar = document.getElementById("bar-" + letter);
          if (bar) bar.style.width = pct + "%";
        }, 50);
      });
    }

    // ─── Master Render ────────────────────────────────────
    function render() {
      var displayList = applyFilterSort(students);
      renderTable(displayList);
      renderStats();
    }
  </script>

</body>
</html>

Reflection Questions

Why do we re-render the entire table on every change instead of just modifying one row? What are the pros and cons of this "full re-render" approach vs a "targeted update" approach?

The statistics panel fades in on the first student added. How would you make it fade back out if all students are deleted?

The filter buttons only filter the displayed table, not the underlying data array. Why is this the right approach? What would go wrong if you filtered the array itself?

How would this project work in a real company? What would need to change if student data had to be saved to a database and persist across page reloads?

The Delete button uses data-id instead of the student's array index. Why is this safer than using the index directly?

Optional Advanced Features

[ ] Edit in place: Clicking a student's name makes it editable inline using contenteditable
[ ] Export to CSV: A button that builds a CSV string and triggers a download using URL.createObjectURL
[ ] LocalStorage persistence: Save the students array to localStorage on every change, restore it on page load
[ ] Search filter: A text input that filters students by name in real time using querySelectorAll and classList
[ ] Drag-to-reorder: Implement drag-and-drop row reordering using draggable and the drag events

# 12. Completion Checklist & Summary

✏️ Your Task

- Add students and their test scores - View a dynamically-built grade table - Filter students by performance category - Animate a summary panel when scores are entered - Validate all inputs before accepting them No frameworks. No libraries. Pure HTML + CSS + JavaScript DOM. ·

Phase 12 of 13

Common Beginner Errors · Quick Reference

Mistake	Why it happens	Fix
`getElementById` returns `null`	Script runs before the element exists in the DOM	Place `<script>` at the end of `<body>`, or use `DOMContentLoaded`
`style.left` returns `""` not `"0px"`	Initial style is not set inline	Set `style.left = "0px"` in HTML, or check for `""` and default to 0
`parseInt` returns `NaN`	Style value is empty string	Always provide a fallback: `parseInt(el.style.left) \	\	0`
Multiple intervals stacking	`startMove()` called multiple times	Always call `clearInterval(timer)` before `setInterval`
`innerHTML` XSS risk	User input inserted as HTML	Use `innerText` or sanitise HTML
`getElementsByTagName` not updating	Expected live, got stale index	HTMLCollections are live, but indices can shift; loop in reverse when removing
`querySelectorAll` result has no `.forEach` in IE	NodeList in old browsers	Use `Array.from(list).forEach(...)` for compatibility
`document.write` destroying the page	Called after load	Use `innerHTML` instead for post-load content insertion
Forgetting `return false` in onsubmit	Form submits even on validation failure	Always `return false` (or `event.preventDefault()`) when validation fails
Using `id` that contains spaces	Browsers allow it but JS selectors break	Never put spaces in id values

✏️ Your Task

Practise what you just learned about Common Beginner Errors · Quick Reference. Open your editor, type the examples above by hand, modify them, and observe what changes.

Phase 13 of 13

One-Sentence Summary

The HTML DOM is JavaScript's live map of your webpage · master its methods for finding, changing, creating, and animating elements, and you gain complete programmatic control over everything users see and interact with.

Tutorial generated by AI_TUTORIAL_GENERATOR · Source: W3Schools JavaScript HTML DOM series (9 pages) · Format: Markdown

✏️ Your Task

Practise what you just learned about One-Sentence Summary. Open your editor, type the examples above by hand, modify them, and observe what changes.

🏗️ Build It — Mini Project

Lesson 28 Project

<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>My Project</title> </head> <body> <!-- Your code h

starter.html

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>My Project</title>
</head>
<body>
<!-- Your code here -->
</body>
</html>

Lesson 28 complete! 🎉

You covered:

✅ 1. What is the HTML DOM?
✅ 2. DOM Methods · Your First Tools
✅ 3. Finding HTML Elements
✅ 4. Changing HTML Content
✅ 5. Changing CSS Styles
✅ 6. Form Validation with the DOM
✅ 7. DOM Animations
✅ 8. The Document Object in Depth

← Lesson 27 Lesson 29 →

JavaScript HTML DOM: Introduction to the DOM · DOM Methods · Finding Elements · Changing HTML · Changing CSS · Form Validation · Animations · The Document Object · Element Reference

👋 Welcome to Lesson 28

The Big Picture

Why Does the DOM Exist?

The DOM is a Standard

The DOM Standard Defines:

Micro-Demo 1 · The DOM in One Line

What is a Method?

getElementById · Your Most-Used Tool

innerHTML vs innerText · An Important Difference

The document Object · Your Gateway

Defining HTML Elements · The id Attribute

Six Ways to Find Elements

Method 1: getElementById

Method 2: getElementsByTagName

Looping Through a Collection

Method 3: getElementsByClassName

Method 4: querySelector · CSS Selector Power

Method 5: querySelectorAll

Searching Within an Element (Not Just the Document)

Navigating the DOM Tree with Relationships

What Can You Change?

Changing innerHTML

Changing an Attribute

document.write() · Use With Caution

Creating and Inserting New Elements

Inserting Before a Specific Element

Removing Elements

Replacing an Element

outerHTML · Replace Including the Tag Itself

The style Property

CSS Property Name Conversion Rule

Toggle Visibility

Using CSS Classes Instead of Inline Styles

The classList API · Full Reference

Reading Computed Styles

Why Validate on the Client Side?

A Basic Required-Field Check

Validating an Email Address

Numeric Range Validation

Automatic HTML5 Validation vs JavaScript Validation

Real-Time Validation with Events

Password Confirmation Check

How DOM Animations Work (The Concept)

The Timer Functions Explained

Micro-Demo: Blinking Text

Building a Slide Animation Step by Step

Using requestAnimationFrame · The Modern Way

Animating Multiple Properties

What Is the document Object?

Key document Properties

Practical Examples

Creating Elements · Full Node API

DocumentFragment · Performance Trick

Cookies with document.cookie

document.open(), document.write(), document.close()

The Element as an Object

Universal Element Properties (Available on Every Element)

Position and Size Properties

Tree Navigation Properties

Universal Element Methods

insertAdjacentHTML Positions

cloneNode · Copying Elements

matches and closest

data-* Attributes and the dataset API

Input-Specific Properties

Exercise 1: Live Word Counter

Exercise 2: Dynamic Student Grade Table

Exercise 3: CSS Theme Switcher

Exercise 4: Animated Progress Bar

Exercise 5: Form Validation Suite

Project Overview

Stage 1: Setup & Core Logic

Stage 2: Adding Features

Stage 3: Displaying Results

Full Project Template

Reflection Questions

Optional Advanced Features

Lesson 28 complete! 🎉

`getElementById` · Your Most-Used Tool

`innerHTML` vs `innerText` · An Important Difference

The `document` Object · Your Gateway

Method 1: `getElementById`

Method 2: `getElementsByTagName`

Method 3: `getElementsByClassName`

Method 4: `querySelector` · CSS Selector Power

Method 5: `querySelectorAll`

`document.write()` · Use With Caution

`outerHTML` · Replace Including the Tag Itself

The `style` Property

The `classList` API · Full Reference

Using `requestAnimationFrame` · The Modern Way

What Is the `document` Object?

Key `document` Properties

Cookies with `document.cookie`

`document.open()`, `document.write()`, `document.close()`

`insertAdjacentHTML` Positions

`cloneNode` · Copying Elements

`matches` and `closest`

`data-*` Attributes and the `dataset` API