simulant.js

Simulated DOM events for automated testing

What's this for?

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:

// WITHOUT SIMULANT.JS
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
simulant.fire( node, 'mousemove', { relatedTarget: previousNode });

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

Why not just use jQuery?

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.

Installation

npm install simulant

Usage

// 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' );
simulant.fire( target, event );

// Create an event and fire it immediately
simulant.fire( target, 'click' );
simulant.fire( target, 'click', { button: 1, which: 2 });

// Polyfill addEventListener in old browsers
if ( !window.addEventListener ) {
  simulant.polyfill();
}

Limitations

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.

Building and testing

Simulant uses jsdom for testing, which requires io.js rather than node.js.

To build the library, do npm run build.

To test the library using jsdom, do npm test.

To test the library in browsers, do npm start. This will build the library (and watch the source files for changes) and serve a simple test page to localhost:4567.

Rich-Harris/simulant

Rich-Harris

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