Creating and triggering events

This article demonstrates how to create and dispatch DOM events. Such events are commonly called synthetic events, as opposed to the events fired by the browser itself.

Creating custom events

Events can be created with the Event constructor as follows:

js
const event = new Event("build");

// Listen for the event.
elem.addEventListener(
  "build",
  (e) => {
    /* … */
  },
  false,
);

// Dispatch the event.
elem.dispatchEvent(event);

The above code example uses the EventTarget.dispatchEvent() method.

This constructor is supported in most modern browsers. For a more verbose approach, see the old-fashioned way below.

Adding custom data – CustomEvent()

To add more data to the event object, the CustomEvent interface exists and the detail property can be used to pass custom data. For example, the event could be created as follows:

js
const event = new CustomEvent("build", { detail: elem.dataset.time });

This will then allow you to access the additional data in the event listener:

js
function eventHandler(e) {
  console.log(`The time is: ${e.detail}`);
}

The old-fashioned way

The older approach to creating events uses APIs inspired by Java. The following shows an example with document.createEvent():

js
// Create the event.
const event = document.createEvent("Event");

// Define that the event name is 'build'.
event.initEvent("build");

// Listen for the event.
elem.addEventListener(
  "build",
  (e) => {
    // e.target matches elem
  },
  false,
);

// target can be any Element or other EventTarget.
elem.dispatchEvent(event);

Event bubbling

It is often desirable to trigger an event from a child element, and have an ancestor catch it; optionally, with data:

html
<form>
  <textarea></textarea>
</form>
js
const form = document.querySelector("form");
const textarea = document.querySelector("textarea");

// Create a new event, allow bubbling, and provide any data you want to pass to the "detail" property
const eventAwesome = new CustomEvent("awesome", {
  bubbles: true,
  detail: { text: () => textarea.value },
});

// The form element listens for the custom "awesome" event and then consoles the output of the passed text() method
form.addEventListener("awesome", (e) => console.log(e.detail.text()));

// As the user types, the textarea inside the form dispatches/triggers the event to fire, and uses itself as the starting point
textarea.addEventListener("input", (e) => e.target.dispatchEvent(eventAwesome));

Creating and dispatching events dynamically

Elements can listen for events that haven't been created yet:

html
<form>
  <textarea></textarea>
</form>
js
const form = document.querySelector("form");
const textarea = document.querySelector("textarea");

form.addEventListener("awesome", (e) => console.log(e.detail.text()));

textarea.addEventListener("input", function () {
  // Create and dispatch/trigger an event on the fly
  // Note: Optionally, we've also leveraged the "function expression" (instead of the "arrow function expression") so "this" will represent the element
  this.dispatchEvent(
    new CustomEvent("awesome", {
      bubbles: true,
      detail: { text: () => textarea.value },
    }),
  );
});

Triggering built-in events

This example demonstrates simulating a click (that is programmatically generating a click event) on a checkbox using DOM methods. View the example in action.

js
function simulateClick() {
  const event = new MouseEvent("click", {
    view: window,
    bubbles: true,
    cancelable: true,
  });
  const cb = document.getElementById("checkbox");
  const cancelled = !cb.dispatchEvent(event);

  if (cancelled) {
    // A handler called preventDefault.
    alert("cancelled");
  } else {
    // None of the handlers called preventDefault.
    alert("not cancelled");
  }
}

See also