<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
div{
font-size: 2em;
}
</style>
<script>
addEventListener('DOMContentLoaded', function() {
let str = 'Всем привет ',
elem = document.querySelector('div'),
timer = () => {
let [...a] = str;
elem.textContent = str = a.pop() + a.join('');
setTimeout(timer, 100)
};
timer()
});
</script>
</head>
<body>
<div></div>
</body>
</html>