const BigInteger = require('./path/to/biginteger').BigInteger; var a = BigInteger('123456789'); var b = BigInteger('10000000000'); var c = BigInteger('987654321'); console.log(a.multiply(b).add(c).toString()); // '1234567890987654321' 

<script src="path/to/biginteger.js"></script> <script> var a = BigInteger('123456789'); var b = BigInteger('10000000000'); var c = BigInteger('987654321'); console.log(a.multiply(b).add(c).toString()); // '1234567890987654321' </script>