Simulated DOM events for automated testing


Simulated DOM events for automated testing

Sometimes you need to create fake DOM events so that you can test the parts of your app or library that depend on user input. But doing so is a royal pain in the arse:

try {
  // DOM Level 3 
  event = new MouseEvent( 'mousemove', {
    bubbles: true,
    cancelable: true,
    relatedTarget: previousNode
  node.dispatchEvent( event );
catch ( err ) {
  if ( document.createEvent ) {
    // DOM Level 2 
    event = document.createEvent( 'MouseEvents' );
    event.initMouseEvent( 'mousemove', true, true, window, null, 0, 0, 0, 0, '', false, false, false, false, 0, previousNode );
    node.dispatchEvent( event );
  else {
    // IE8 and below 
    event = document.createEventObject();
    event.relatedTarget = previousNode;
    node.fireEvent( 'onmousemove', event );
// WITH SIMULANT.JS node, 'mousemove', { relatedTarget: previousNode });

Simulant was created to make automated testing of Ractive.js across different browsers easier.

In some cases you can. But events created with $(element).trigger('click'), for example, won't trigger handlers bound using element.addEventListener('click', handler) in many situations, such as when you're doing automated tests with PhantomJS.

Simulant uses native DOM events, not fake events, so its behaviour is more predictable.

npm install simulant
// Create a simulated event 
event = simulant( 'click' );
// Create a simulated event with parameters, e.g. a middle-click 
event = simulant( 'click', { button: 1, which: 2 });
// Fire a previously created event 
target = document.getElementById( 'target' ); target, event );
// Create an event and fire it immediately target, 'click' ); target, 'click', { button: 1, which: 2 });
// Polyfill addEventListener in old browsers 
if ( !window.addEventListener ) {

Normally, events have side-effects - a click in a text input will focus it, a mouseover of an element will trigger its hover state, and so on. When creating events programmatically, these side-effects don't happen - so your tests shouldn't expect them to. For example you shouldn't fire simulated keypress events at an input element and expect its value to change accordingly.

There are exceptions - a click event on a checkbox input will cause a secondary change event to be fired, for example.

Copyright (c) 2013-14 Rich Harris (@rich_harris). Released under an MIT license.