# Events

We dispatch a number of events to make it easy for you to react to changes in the presentation. Reveal.on() is used to add event listeners, and Reveal.off() is used to remove them.

Reveal.on( 'eventname', callbackFunction );

The ready event is fired when reveal.js has loaded all non-async dependencies and is ready to accept API calls. To check if reveal.js is already 'ready' you can call Reveal.isReady().

Reveal.on( 'ready', event => {  // event.currentSlide, event.indexh, event.indexv} );

We also add a .ready class to the .reveal element so that you can hook into this with CSS.

The Reveal.initialize method also returns a Promise which resolves when the presentation is ready. The following is synonymous to adding a listener for the ready event:

Reveal.initialize().then( () => {  // reveal.js is ready} )

## Slide Changed

The slidechanged event is fired each time the slide changes. The event object holds the index values of the current slide as well as a reference to the previous and current slide HTML elements.

Some libraries, like MathJax (see #226), get confused by the transforms and display states of slides. Often times, this can be fixed by calling their update or render function from this callback.

Reveal.on( 'slidechanged', event => {  // event.previousSlide, event.currentSlide, event.indexh, event.indexv} );

## Slide Transition End

The slidechanged event fires instantly as soon as the slide changes. If you'd rather invoke your event listener when the slide has finished transitioning and is fully visible, you can use the slidetransitionend event. The slidetransitionend event includes the same event data as slidechanged.

Reveal.on( 'slidetransitionend', event => {	console.log( event.currentSlide );});

## Resize

The resize event is fired when reveal.js changes the scale of the presentation.

Reveal.on( 'resize', event => {  // event.scale, event.oldScale, event.size} );