⚡ Lesson 15: Events & Event Listeners

What Are Events?

An event is something that happens in the browser — a user clicks a button, types in a text field, hovers over an image, scrolls the page, or submits a form. JavaScript lets you listen for these events and respond with code.

Think of it like a doorbell: the event is someone pressing the button, and the event listener is the code that runs when it rings — maybe you open the door, maybe you check a camera, maybe you ignore it. You decide what happens.

addEventListener()

The modern way to listen for events is addEventListener. It takes two required arguments: the event type (a string) and a callback function (what to do when it happens).

const button = document.querySelector("#my-button");

// Basic pattern: element.addEventListener(eventType, callback)
button.addEventListener("click", function () {
    console.log("Button was clicked!");
});
                

Using Arrow Functions

Arrow functions are a clean, common choice for event callbacks.

button.addEventListener("click", () => {
    console.log("Clicked with an arrow function!");
});
                

Using Named Functions

Named functions are useful when you want to reuse the same handler or remove it later.

function handleClick() {
    console.log("Button clicked!");
}

button.addEventListener("click", handleClick);

// ❌ Common mistake — this CALLS the function immediately!
// button.addEventListener("click", handleClick());
// Pass the function reference, don't call it
                

<!-- ❌ Old-school inline handler — avoid this -->
<button onclick="alert('clicked!')">Click Me</button>

<!-- ✅ Clean HTML + addEventListener in your script -->
<button id="my-btn">Click Me</button>
                    

Common Event Types

Mouse Events

const box = document.querySelector(".box");

box.addEventListener("click", () => {
    console.log("Clicked!");
});

box.addEventListener("dblclick", () => {
    console.log("Double-clicked!");
});

box.addEventListener("mouseenter", () => {
    box.style.backgroundColor = "#eff6ff";
});

box.addEventListener("mouseleave", () => {
    box.style.backgroundColor = "";
});
                

Keyboard Events

document.addEventListener("keydown", (event) => {
    console.log(`Key pressed: ${event.key}`);
});

document.addEventListener("keyup", (event) => {
    console.log(`Key released: ${event.key}`);
});

// Listen on a specific input
const searchInput = document.querySelector("#search");
searchInput.addEventListener("keydown", (event) => {
    if (event.key === "Enter") {
        console.log("Search submitted:", searchInput.value);
    }
});
                

Form & Input Events

const input = document.querySelector("#username");

// Fires on every keystroke / change
input.addEventListener("input", (event) => {
    console.log("Current value:", event.target.value);
});

// Fires when the field loses focus (after editing)
input.addEventListener("change", (event) => {
    console.log("Final value:", event.target.value);
});

// Fires when the input gains focus
input.addEventListener("focus", () => {
    input.style.outline = "2px solid #3b82f6";
});

// Fires when the input loses focus
input.addEventListener("blur", () => {
    input.style.outline = "";
});
                

Page & Window Events

// When the entire page has loaded
window.addEventListener("load", () => {
    console.log("Page fully loaded (including images)");
});

// When the DOM is ready (before images finish loading)
document.addEventListener("DOMContentLoaded", () => {
    console.log("DOM is ready!");
});

// When the user scrolls
window.addEventListener("scroll", () => {
    console.log("Scroll position:", window.scrollY);
});
                

The Event Object

Every event handler receives an event object as its first argument. This object contains details about what happened — which element was clicked, which key was pressed, where the mouse was, and more.

button.addEventListener("click", (event) => {
    // The element that was clicked
    console.log(event.target);         // <button id="my-button">...</button>

    // The element the listener is attached to
    console.log(event.currentTarget);  // Same as above (usually)

    // The type of event
    console.log(event.type);           // "click"

    // Mouse position relative to the viewport
    console.log(event.clientX, event.clientY);

    // Timestamp
    console.log(event.timeStamp);
});
                

event.target vs. event.currentTarget

This distinction matters when an event bubbles up from a child element.

// HTML: <div id="card"><button>Click</button></div>

const card = document.querySelector("#card");

card.addEventListener("click", (event) => {
    // If the BUTTON inside the card is clicked:
    console.log(event.target);         // <button> (what was actually clicked)
    console.log(event.currentTarget);  // <div id="card"> (where the listener lives)
});
                

Keyboard Event Properties

document.addEventListener("keydown", (event) => {
    console.log(event.key);       // "a", "Enter", "ArrowUp", "Shift"
    console.log(event.code);      // "KeyA", "Enter", "ArrowUp", "ShiftLeft"
    console.log(event.shiftKey);  // true if Shift was held
    console.log(event.ctrlKey);   // true if Ctrl was held
    console.log(event.altKey);    // true if Alt was held
    console.log(event.metaKey);   // true if Cmd (Mac) / Win key was held

    // Check for keyboard shortcuts
    if (event.ctrlKey && event.key === "s") {
        event.preventDefault();   // Stop the browser from saving
        console.log("Custom save!");
    }
});
                

Preventing Default Behavior

Many events have a default action — clicking a link navigates to a URL, submitting a form reloads the page, pressing space scrolls down. preventDefault() cancels that default behavior so you can handle it yourself.

Forms — The Most Common Use Case

const form = document.querySelector("#signup-form");

form.addEventListener("submit", (event) => {
    // Stop the page from reloading
    event.preventDefault();

    // Now handle the form data with JavaScript
    const name = document.querySelector("#name").value;
    const email = document.querySelector("#email").value;
    console.log("Submitted:", { name, email });
});
                

Links

const link = document.querySelector("a.internal");

link.addEventListener("click", (event) => {
    event.preventDefault();  // Don't navigate
    console.log("Link clicked but not followed:", link.href);
    // Handle navigation with JavaScript instead
});
                

Other Defaults

// Prevent right-click context menu
document.addEventListener("contextmenu", (event) => {
    event.preventDefault();
    console.log("Custom right-click menu would go here");
});

// Prevent spacebar from scrolling when a game is active
document.addEventListener("keydown", (event) => {
    if (event.key === " " && gameIsActive) {
        event.preventDefault();  // Stop scrolling
        jump();                  // Do game action instead
    }
});
                

Event Propagation

When you click a button inside a <div> inside the <body>, the click event doesn't just fire on the button — it travels through the DOM. This is called propagation, and it has three phases.

Bubbling (Default Behavior)

By default, event listeners fire during the bubbling phase — the event starts at the target and bubbles up through each ancestor.

// HTML: <div id="outer"><div id="inner"><button>Click</button></div></div>

document.querySelector("#outer").addEventListener("click", () => {
    console.log("Outer div");
});

document.querySelector("#inner").addEventListener("click", () => {
    console.log("Inner div");
});

document.querySelector("button").addEventListener("click", () => {
    console.log("Button");
});

// Clicking the button logs:
// "Button"     ← target
// "Inner div"  ← bubbles up
// "Outer div"  ← bubbles up more
                

stopPropagation()

If you want to stop the event from bubbling up to parent elements, use stopPropagation().

document.querySelector("button").addEventListener("click", (event) => {
    event.stopPropagation();  // Stop here — don't bubble up
    console.log("Only the button handler runs");
});
                

Capturing Phase

You can listen during the capturing phase (top-down) by passing a third argument. This is rarely needed, but good to know.

// Third argument: { capture: true } or just true
document.querySelector("#outer").addEventListener("click", () => {
    console.log("Outer (capture phase — fires FIRST)");
}, true);

document.querySelector("button").addEventListener("click", () => {
    console.log("Button (normal bubbling phase)");
});

// Clicking the button:
// "Outer (capture phase — fires FIRST)"
// "Button (normal bubbling phase)"
                

Event Delegation

Event delegation is a powerful pattern that takes advantage of bubbling. Instead of attaching a listener to every child element, you attach one listener to the parent and use event.target to figure out which child was clicked.

The Problem

// ❌ Adding a listener to every button — doesn't work for new buttons!
const buttons = document.querySelectorAll(".item-btn");
buttons.forEach(btn => {
    btn.addEventListener("click", () => {
        console.log("Item clicked:", btn.textContent);
    });
});

// If you add a new button later, it won't have a listener
// You'd have to manually attach one every time
                

The Solution: Delegation

// ✅ One listener on the parent — works for current AND future children
const list = document.querySelector("#task-list");

list.addEventListener("click", (event) => {
    // Check if the clicked element is a button
    if (event.target.matches(".item-btn")) {
        console.log("Item clicked:", event.target.textContent);
    }
});

// New buttons added later automatically work!
const newBtn = document.createElement("button");
newBtn.classList.add("item-btn");
newBtn.textContent = "New Task";
list.appendChild(newBtn);
// Clicking this new button triggers the same handler
                

Using closest() for Nested Targets

Sometimes the click target is a child of the element you care about (like an icon inside a button). Use closest() to find the nearest ancestor matching a selector.

// HTML: <button class="delete-btn"><span class="icon">🗑️</span> Delete</button>

list.addEventListener("click", (event) => {
    // event.target might be the <span> icon, not the button
    const deleteBtn = event.target.closest(".delete-btn");

    if (deleteBtn) {
        const taskId = deleteBtn.dataset.taskId;
        console.log("Delete task:", taskId);
    }
});
                

Removing Event Listeners

Sometimes you need to stop listening — maybe a button should only work once, or you're cleaning up when a component is removed. Use removeEventListener with a reference to the same function.

function handleClick() {
    console.log("This only fires once!");
    button.removeEventListener("click", handleClick);
}

button.addEventListener("click", handleClick);
                

The { once: true } Shortcut

If you want a listener to fire only once, pass { once: true } as the third argument. The browser removes it automatically after it fires.

button.addEventListener("click", () => {
    console.log("This only runs once, then auto-removes!");
}, { once: true });
                

// ❌ Can't remove — no reference to the function
button.addEventListener("click", () => {
    console.log("You can't remove me!");
});
button.removeEventListener("click", ???);  // What goes here?

// ✅ Named function — can be removed
function handleClick() {
    console.log("You CAN remove me!");
}
button.addEventListener("click", handleClick);
button.removeEventListener("click", handleClick);  // Works!
                    

AbortController — Remove Multiple Listeners at Once

For more complex cleanup, use an AbortController. When you abort it, all listeners associated with its signal are removed at once.

const controller = new AbortController();

button.addEventListener("click", handleClick, { signal: controller.signal });
input.addEventListener("input", handleInput, { signal: controller.signal });
window.addEventListener("scroll", handleScroll, { signal: controller.signal });

// Later: remove ALL three listeners at once
controller.abort();
                

Hands-on Exercise

<!-- Save this as tasks.html and open in a browser -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Task List</title>
    <style>
        body { font-family: sans-serif; padding: 2rem; background: #f1f5f9; }
        .task-app { max-width: 500px; margin: 0 auto; }
        .add-form { display: flex; gap: 0.5rem; margin-bottom: 1rem; }
        .add-form input {
            flex: 1; padding: 0.5rem; border: 2px solid #cbd5e1;
            border-radius: 6px; font-size: 1rem;
        }
        .add-form button {
            padding: 0.5rem 1rem; background: #3b82f6; color: white;
            border: none; border-radius: 6px; cursor: pointer; font-size: 1rem;
        }
        #task-list { list-style: none; padding: 0; }
        .task-item {
            display: flex; align-items: center; gap: 0.5rem;
            padding: 0.75rem; background: white; border-radius: 8px;
            margin-bottom: 0.5rem; box-shadow: 0 1px 3px rgba(0,0,0,0.1);
        }
        .task-item.done .task-text { text-decoration: line-through; color: #94a3b8; }
        .task-text { flex: 1; }
        .complete-btn { background: #22c55e; color: white; border: none;
            border-radius: 4px; padding: 0.25rem 0.5rem; cursor: pointer; }
        .delete-btn { background: #ef4444; color: white; border: none;
            border-radius: 4px; padding: 0.25rem 0.5rem; cursor: pointer; }
        .counter { text-align: center; color: #64748b; margin-top: 1rem; }
    </style>
</head>
<body>
    <div class="task-app">
        <h1>📋 Task List</h1>
        <form class="add-form" id="add-form">
            <input type="text" id="task-input" placeholder="Add a new task..."
                   aria-label="New task" required>
            <button type="submit">Add</button>
        </form>
        <ul id="task-list"></ul>
        <p class="counter" id="counter">0 tasks</p>
    </div>

    <script>
    // TODO: Complete each task

    // 1. Listen for the form "submit" event
    //    - Prevent the default form submission
    //    - Get the input value (trim whitespace)
    //    - If empty, return early
    //    - Create a new <li class="task-item"> with this HTML inside:
    //      <span class="task-text">{task}</span>
    //      <button class="complete-btn">✅</button>
    //      <button class="delete-btn">🗑️</button>
    //    - Append it to #task-list
    //    - Clear the input and refocus it
    //    - Update the counter

    // 2. Use event delegation on #task-list to handle clicks:
    //    - If a .complete-btn is clicked: toggle the "done" class
    //      on the parent .task-item
    //    - If a .delete-btn is clicked: remove the parent .task-item
    //    - Update the counter after each action

    // 3. Write an updateCounter() function that counts the <li>
    //    elements in #task-list and updates #counter text

    // 4. Bonus: Add keyboard support
    //    - Press Escape in the input to clear it
    //    - Listen on the document for "d" key to toggle
    //      a "dark-mode" class on the body
    </script>
</body>
</html>
                    

const form = document.querySelector("#add-form");
const taskInput = document.querySelector("#task-input");
const taskList = document.querySelector("#task-list");
const counter = document.querySelector("#counter");

// Helper: update the task counter
function updateCounter() {
    const total = taskList.querySelectorAll("li").length;
    const done = taskList.querySelectorAll("li.done").length;
    counter.textContent = `${total} task${total !== 1 ? "s" : ""} (${done} done)`;
}

// 1. Handle form submission
form.addEventListener("submit", (event) => {
    event.preventDefault();

    const text = taskInput.value.trim();
    if (!text) return;

    const li = document.createElement("li");
    li.classList.add("task-item");
    li.innerHTML = `
        <span class="task-text">${text}</span>
        <button class="complete-btn">✅</button>
        <button class="delete-btn">🗑️</button>
    `;

    taskList.appendChild(li);
    taskInput.value = "";
    taskInput.focus();
    updateCounter();
});

// 2. Event delegation for complete and delete
taskList.addEventListener("click", (event) => {
    const taskItem = event.target.closest(".task-item");
    if (!taskItem) return;

    if (event.target.matches(".complete-btn")) {
        taskItem.classList.toggle("done");
    }

    if (event.target.matches(".delete-btn")) {
        taskItem.remove();
    }

    updateCounter();
});

// 4. Bonus: Keyboard support
taskInput.addEventListener("keydown", (event) => {
    if (event.key === "Escape") {
        taskInput.value = "";
    }
});

document.addEventListener("keydown", (event) => {
    // Only toggle dark mode if not typing in the input
    if (event.key === "d" && document.activeElement !== taskInput) {
        document.body.classList.toggle("dark-mode");
    }
});
                        

Summary

📚 Additional Resources

• MDN — addEventListener()
• MDN — Introduction to Events
• MDN — Event Interface
• javascript.info — Browser Events
• javascript.info — Event Delegation

🚀 What's Next?

You can now respond to anything a user does on the page. But what about adding entirely new elements — building lists, creating cards, or removing items dynamically? The next lesson covers creating and removing elements, completing your DOM toolkit.