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>