<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<button id="sum"><img width="20" height="20" src="Без названия.png" alt=""></button>
<script>
let sum = document.querySelector('#sum');
sum.addEventListener('click', function(){
if (document.fullscreenElement) {
document.exitFullscreen();
}
else {
document.documentElement.requestFullscreen();
}
});
</script>
</body>
</html>