How to ensure an event listener is only fired once in JavaScript

Key takeaways:

• Use the once option in addEventListener() for a simple, modern solution to limit event firing.

• Alternatively, manually remove the listener using removeEventListener() to gain more control.

• Use closures to create a custom one-time listener function if more complex logic is needed.

Sometimes, we only need to listen once for an event, such as when we want to initiate a process or animation without repeating it on subsequent clicks. In this Answer, we’ll learn different methods to ensure an event listener is only fired once, allowing us to create smoother and more efficient user experiences.

Understanding event listeners

An event listener in JavaScript allows us to execute a function whenever a specific event occurs, such as a click, mouse movement, or key press. By default, event listeners will execute their associated functions every time the event occurs. However, there are several ways to limit this behavior so that the listener fires only once.

This can be handled in the following ways:

• Using the once option

• Removing the event listener manually

• Using a closure

Let’s learn each method below:

Using the once option

We can pass an object as an argument to the addEventListener() method and specify that the event is only handled once. This is achieved by passing the property once to the object. If we set once to true, the event will only be fired once.

In the above code:

• Line 6: We create a button element with the ID named myBtn.

• Line 10–12: We use the document.getElementById() to select the myBtn and call the addEventListener() on it, specifying that we are listening for a click event. This button will trigger an event when clicked. When the button is clicked, the function inside addEventListener() executes. Inside this function:

  • Line 11: We use console.log("Button clicked!") to log a message to the browser’s console indicating that the button was clicked.

  • Line 12: We use the { once: true } option to ensure that this listener will be removed automatically after its first invocation.

Removing the event listener manually

Another approach to ensure an event listener only fires once is to remove the listener manually within the event handler itself. We can do this using the removeEventListener() method.

In the above code:

• Line 11: We use the querySelector() method to select the button element from the DOM.

• Lines 14–17: We define the handleClick() function that logs a message and removes itself as a listener when executed.

• Line 20: We attach the handleClick() function as an event listener.

Using a closure

We can also use a closure to create a function that fires only once. This method involves creating a function that returns another function that handles the event.

In the above code:

• Lines 14–20: We define the oneTimeListener() function that takes an element, event type, and handler. Inside this function, we create a wrappedHandler that calls the original handler and removes itself.

• Lines 23–25: We invoke the oneTimeListener(), passing the button, event type, and handler.

Knowledge test

Let’s attempt a short quiz to assess your understanding.

Conclusion

We’ve learned various methods to ensure an event listener fires only once in JavaScript. Whether we choose to use the modern once option, manually remove the listener, or implement a closure, these techniques enhance our ability to manage events in our web applications efficiently. By applying these methods, we can create a more responsive and polished user experience.