Нашел вариант с вычитанием. На сложение можно переделать?
<!DOCTYPE html>
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<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<meta name="robots" content="noindex, nofollow">
<meta name="googlebot" content="noindex, nofollow">
<style type="text/css">
</style>
<title></title>
<script type='text/javascript'>
//<![CDATA[
window.onload=function(){
var first = document.getElementById("first");
var second = document.getElementById("second");
var result = document.getElementById("result");
var calcButton = document.getElementById("calc");
calcButton.onclick = calc;
function calc(){
var base = 10;
var op1 = str2bigInt(first.value, base);
var op2 = str2bigInt(second.value, base);
result.value = bigInt2str(sub(op1, op2), base);
}
}//]]>
</script>
</head>
<body>
<script src="http://www.leemon.com/crypto/BigInt.js"></script>
<input id="first" type="text"/>
<br/>
-
<br/>
<input id="second" type="text"/>
<br/>
=
<br/>
<input id="result" type="text" value="0" disabled/>
<br/>
<input id="calc" type="button" value="calc"/>
</body>
</html>