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This post is part of my Today I learned series in which I share all my web development learnings.

To build interactive web interfaces, you can attach event listeners to DOM events. How does this work?

You query an element, define an event type, call addEventListener and pair it with a callback function — and violà, you are ready to react to clicks, keypresses, scrolls and many other events.

Use the following code to react to button clicks:

document.querySelector('button')
  .addEventListener('click', () => {
    console.log('element clicked');
  });

According to MDN, target.addEventListener defines the following parameters:

target.addEventListener(type, listener [, options]);
target.addEventListener(type, listener [, useCapture]);
target.addEventListener(type, listener [, useCapture, wantsUntrusted  ]); // Gecko/Mozilla only

addEventListener accepts the event type, a listener callback function and an options or useCapture parameter.

(To learn more about possible options or useCapture, head over to the MDN addEventListener documentation.)

What if I told you that the listener parameter can be a function but an object, too?

addEventListener and the EventListener interface

It turns out that MDN documents the listener parameter as follows:

[listener] must be an object implementing the EventListener interface, or a JavaScript function.

The early DOM events specification (we're talking pre-HTML5 here) defined an EventListener interface. Objects defining a handleEvent method were valid in addEventListener.

// a class implementing
// the `EventListener` interface
class EventHandler {
  constructor() {
    this.eventCount = 0;
  }

  handleEvent() {
    this.eventCount++;
    console.log(`Event triggered ${this.eventCount} time(s)`);
  }
}

document.querySelector('button')
  .addEventListener('click', new EventHandler());

The code above defines a JavaScript class EventHandler. An initialized event handler object can then be passed to addEventListener to react to specific events. And because it's an object, it can hold state.

The event handler object keeps track of the number of times a specific event occurred (check it on CodePen) by implementing an eventCount field. All information is stored in the object itself, and the code works without outer scope variables.

I quite like this pattern, and I can see it coming in handy when dealing with sequential events.

According to MDN, the EventListener interface is supported by all major browsers, and you can safely pass objects that implement it to addEventListener.

When would you pass EventListener objects to addEventListener? I'd love to learn about more examples!

Edit: Someone shared the following snippet on Reddit.

class MyComponent {
  constructor (el) {
    this.el = el
    this.el.addEventListener('click', this)
  }
  handleEvent (event) {
    console.log('my component element was clicked')
  }
  destroy () {
    this.el.removeEventListener('click', this)
  }
}

const component = new MyComponent(
  document.querySelector('button')
);

The MyComponent class accepts a DOM element and automatically attaches/detaches event listeners. It also implements a handleEvent method so you can pass this to addEventListener. Fancy!

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