Event Handler

Definition of Event Handler

An event handler is a function or method in software programming that reacts to a specific event, such as a user input or system occurrence. It listens for a pre-defined trigger, such as a mouse click or keyboard input, and then executes the corresponding code. This process allows for a more interactive and responsive user experience within applications or websites.


The phonetic pronunciation of the keyword “Event Handler” is:/ɪˈvɛnt ˈhændlər/ee-vent hand-lur

Key Takeaways

  1. An event handler is a function or method in JavaScript that is used to manage user interactions with HTML elements, like onclick or onmouseover.
  2. Event handlers can be added to HTML elements using inline attributes, like onClick=”myFunction()”, or through JavaScript using the addEventListener() method.
  3. Multiple event handlers can be added to the same element and event type, giving you more flexibility and control over the desired user interactivity.

Importance of Event Handler

The technology term “Event Handler” is important because it plays a crucial role in enabling smooth interaction between users and software applications.

Event handlers are essentially functions or methods in programming that respond to specific events or triggers in a software system, such as a button click, a mouse movement, or a keyboard input.

By defining and executing event handlers, developers can create dynamic and responsive applications that react to user actions in real-time.

This functionality helps in enhancing the overall user experience, making software applications more intuitive, engaging, and user-friendly.

Without event handlers, user interactions with the application would be static, limited, and far less interactive, significantly undermining the software’s usability and appeal.


Event handlers play a crucial role in the realm of software development, particularly in creating interactive and responsive applications. Their primary purpose is to monitor and respond to specific occurrences, known as events, which may take various forms such as user inputs, system triggers, or external messages. A typical scenario involves the user interacting with the application by clicking buttons, entering data, or performing gestures, which are detected by the event handlers.

These actions enable the application to deliver a seamless, dynamic, and user-oriented experience by executing appropriate functions or updating the interface in response to those events. Event handlers work by establishing a clear path of communication between the application’s user interface and its underlying logic. Developers assign specific event handler functions to different elements of the interface, which listen for predefined event occurrences and respond accordingly.

This mechanism ensures that the applications not only become more interactive but also provide the necessary feedback to the user, making the experience feel intuitive, natural, and engaging. Handling events is a vital aspect of software development across numerous platforms and frameworks, irrespective of the programming language used. From building immersive website interfaces to designing delightful mobile applications, event handlers effectively set the stage for a rich exchange between users and the digital environment.

Examples of Event Handler

Web-based e-commerce platforms: Event handlers are used extensively in web-based e-commerce platforms like Amazon and Shopify. In these systems, event handlers respond to user interactions such as clicking on a product, adding an item to the cart, or starting the checkout process. For instance, when a user clicks the “Add to Cart” button on Amazon, an event handler detects the action and updates the cart with the selected item.

Social media applications: Social media platforms like Facebook, Twitter, and Instagram all use event handlers to ensure seamless user interaction with their applications. Event handlers are responsible for features such as liking a post, commenting, sharing, and sending direct messages. When a user clicks the “Like” button on a Facebook post, an event handler records the action, updates the user interface, and communicates with the server to save the new information in the database.

Online gaming: Online games, such as Fortnite and League of Legends, rely on event handlers for their gameplay mechanics. These games have built-in event handlers that react to user inputs like mouse clicks, keyboard presses, or touchscreen interactions. For example, when a player initiates a command to move their character in the game, an event handler processes the input, communicates with the game engine, and updates the character’s position accordingly, ensuring a real-time interactive gaming experience.

Event Handler FAQ

What is an event handler in JavaScript?

An event handler in JavaScript is a function that handles the execution of a specific action or process when an event, such as a user action (click, keypress, etc.) or a browser event (page load, resize, etc.), occurs on an HTML element. Event handlers allow JavaScript code to be more interactive and dynamic.

How to create an event handler in JavaScript?

To create an event handler, first, create a JavaScript function that will serve as the handler. Then, assign that function to the corresponding event attribute of the HTML element you want the event to be associated with. You can do this inline with the HTML, or set the property in JavaScript using an HTMLElement reference, or by using .addEventListener() method for more advanced handling.

What are some common event handler types?

Some common event handler types include: onload (page load), onclick (when an element is clicked), onmouseover (when the mouse pointer is over an element), onmouseout (when the mouse pointer leaves an element), onkeydown (when a key is pressed), and onsubmit (when a form is submitted).

Can an element have multiple event handlers?

Yes, an element can have multiple event handlers. One way to achieve this is using the .addEventListener() method. You can call this method multiple times on the same element for a single event type, with a different handler function each time, and each handler will be called when the event fires.

How to remove an event handler?

To remove an event handler, you can use the .removeEventListener() method. This method allows you to remove a specific event handler function from an element. Note that in order to remove the event listener, you need to have access to the same function reference as the one passed to .addEventListener(), typically requiring the handler to be a named function or an existing variable reference.

Related Technology Terms

  • Callback function
  • Event listener
  • Event-driven programming
  • Asynchronous execution
  • DOM (Document Object Model) events

Sources for More Information


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