The Document interface's onfullscreenchange property is an event handler for the fullscreenchange event that is fired immediately before a document transitions into or out of full-screen mode. 
targetDocument.onfullscreenchange = fullscreenChangeHandler;
 An event handler which is invoked whenever the document receives a fullscreenchange event, indicating that the document is transitioning into or out of full-screen mode. 
 The fullscreenchange event does not directly specify whether the transition is into or out of full-screen mode, so your event handler should look at the value of Document.fullscreenElement. If it's null, the event indicates a transition out of full-screen mode. Otherwise, the specified element is about to take over the screen. 
document.onfullscreenchange = function ( event ) { console.log("FULL SCREEN CHANGE") }; document.documentElement.onclick = function () { // requestFullscreen() must be in an event handler or it will fail document.documentElement.requestFullscreen(); }
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onfullscreenchange | 
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15 
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12-14 
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11  | 
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5.1  | 
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14 
12.1-14 
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 12 
Only available on iPad, not on iPhone. 
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
    https://developer.mozilla.org/en-US/docs/Web/API/Document/onfullscreenchange