<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script type="text/javascript">
window.onload=function(){
var btn=document.getElementById("btn");
var num=document.getElementById("num");
var a=0;
btn.onclick=function(){
function addNumber(){
num.value=a;
a++;
setTimeout(addNumber,1000);
}
addNumber();
}
}
</script>
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<input type="text" id="num">
</body>
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