Browser

Table of Content

Event Handling

Events are actions or occurrences that happen in the browser, such as clicks, key presses, or form submissions. JavaScript provides various ways to handle these events to create interactive web applications.

This Privacy Policy describes how Alevio ("we,"

"our," or "us"), founded and led by Felix Danyluk (CEO and Founder), collects, uses, and discloses your information when you use our application, website, and services (collectively, the "Service").

Wichtig: Alevio ist als privacy-first Anwendung konzipiert. Alle deine persönlichen Daten werden lokal auf deinem Gerät gespeichert und nicht an unsere Server übertragen, es sei denn, du aktivierst explizit Cloud-Funktionen.

1. Information We Collect

Alevio operates primarily as a local application.

The information stored includes:

• Personal information: name and preferences (stored locally only).

• Usage data: daily responses and insights (stored locally only).

• Device information: app preferences and settings (stored locally only).

• Al Chat data: when using Al features, messages are sent to OpenAl for processing.

2. How We Use Your Information

We use the information we collect to:

• Provide, maintain, and improve our Service.

• Personalize your experience.

• Communicate with you about our Service.

• Monitor and analyze trends, usage, and activities.

3. Sharing Your Information

We may share your information with:

• Service providers who perform services on our behalf.

• Business partners with whom we jointly offer products or services.

• Law enforcement or other third parties when required by law.

4. Data Security

We implement appropriate technical and organizational measures to protect the security of your personal information. However, please note that no method of transmission over the note that no method of transmission over the Internet or method of electronic storage is 100% secure.

5. Your Choices

You can access, update, or delete your account information at any time by logging into your account settings. You may also contact us directly to request access to, correction of, or deletion of any personal information that you have provided to us.

6. Children's Privacy

Our Service is not intended for children under the age of 13. We do not knowingly collect personal information from children under 13. If you are a parent or guardian and you are aware that your child has provided us with personal information, please contact us.

7. Changes to This Privacy Policy

We may update our Privacy Policy from time to time. We will notify you of any changes by posting the new Privacy Policy on this page.

8. Company Information

Alevio is founded and led by Felix Danyluk, CEO and Founder. Our commitment to privacy and data protection is a core value of our leadership team.

9. Contact Us

If you have any questions about this Privacy Policy, please contact us at privacy.alevio@gmail.com

Adding Event Listeners

The addEventListener() method attaches an event handler to an element.

let button = document.getElementById("myButton");
button.addEventListener("click", () => {
  console.log("Button clicked!");
});

This method provides several advantages:

Multiple event listeners can be attached to a single element.
It allows removing event listeners later.
It provides better separation of JavaScript from HTML.

Common Event Types

Events can be triggered by user actions or system interactions.

Mouse Events

click – Triggered when an element is clicked.
dblclick – Triggered on a double-click.
mouseover – Triggered when the mouse enters an element.
mouseout – Triggered when the mouse leaves an element.
mousedown / mouseup – Triggered when a mouse button is pressed or released.

Example:

document.getElementById("box").addEventListener("mouseover", () => {
  console.log("Mouse entered the box!");
});

Keyboard Events

keydown – Triggered when a key is pressed down.
keyup – Triggered when a key is released.
keypress – (Deprecated) Used for detecting character key presses.

Example:

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

Form Events

submit – Triggered when a form is submitted.
change – Triggered when the value of an input field changes.
focus / blur – Triggered when an input field gains or loses focus.

Example:

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault(); // Prevents form submission
  console.log("Form submitted!");
});

Window Events

load – Triggered when the page finishes loading.
resize – Triggered when the window is resized.
scroll – Triggered when the page is scrolled.

Example:

window.addEventListener("resize", () => {
  console.log("Window resized!");
});

Event Object (`event`)

When an event occurs, an event object is automatically passed to the event handler, containing useful information about the event.

Example:

document.addEventListener("click", event => {
  console.log("Event Type:", event.type);
  console.log("Target Element:", event.target);
  console.log("Mouse Coordinates:", event.clientX, event.clientY);
});

Event Propagation (Bubbling and Capturing)

JavaScript events propagate through the DOM in two phases:

Capturing Phase – The event moves from the root element down to the target.
Bubbling Phase – The event moves from the target back up to the root.

By default, events bubble up.

Example of event bubbling:

document.getElementById("parent").addEventListener("click", () => {
  console.log("Parent clicked!");
});
  
document.getElementById("child").addEventListener("click", () => {
  console.log("Child clicked!");
});

Clicking the child element will log:

To stop event propagation:

document.getElementById("child").addEventListener("click", event => {
  event.stopPropagation();
  console.log("Child clicked, but event won't bubble up.");
});

To use the capturing phase, pass true as the third argument in addEventListener():

document.getElementById("parent").addEventListener("click", () => {
  console.log("Parent clicked during capture phase.");
}, true);

Removing Event Listeners

Use removeEventListener() to detach an event handler.

function showMessage() {
  console.log("Button clicked!");
}
  
let button = document.getElementById("myButton");
button.addEventListener("click", showMessage);
  
// Remove the event listener
button.removeEventListener("click", showMessage);

Important: The function reference must match exactly for removeEventListener() to work.

Event Delegation

Instead of adding event listeners to multiple elements, event delegation attaches a single listener to a parent element and checks which child triggered the event.

document.getElementById("list").addEventListener("click", event => {
  if (event.target.tagName === "LI") {
    console.log("List item clicked:", event.target.textContent);
  }
});

Benefits of event delegation:

Improves performance by reducing the number of event listeners.
Useful for dynamically added elements.

Preventing Default Behavior

Some events have default browser behavior (e.g., form submissions, link clicks). The event.preventDefault() method prevents this.

document.getElementById("myLink").addEventListener("click", event => {
  event.preventDefault();
  console.log("Link click prevented!");
});

Conclusion

Event handling is essential for interactive web applications. This section covered event listeners, event propagation, event delegation, and preventing default behaviors. The next section will focus on working with forms and user input in JavaScript.