I felt compelled to put this on github and publish to npm. I haven't tested every other big integer library out there, but the few that I have tested in comparison to this one have not even come close in performance. I am aware of the bi module on npm, however it has been modified and I wanted to publish the original without modifications. This is jsbn and jsbn2 from Tom Wu's original website above, with the module pattern applied to prevent global leaks and to allow for use with node.js on the server side.
var BigInteger = require('jsbn').BigInteger;
var bi = new BigInteger('91823918239182398123');
console.log(bi.bitLength()); // 67bi.toString([radix]) => stringReturns a string representing the BigInteger bi.
radix - Optional. An integer between 2 and 36 specifying the base to use for representing numeric values.If the radix is not specified, the preferred radix is assumed to be 10.
bi.negate() => BigIntegerReturns a new BigInteger equal to the negation of bi.
bi.abs() => BigIntegerReturns a BigInteger equal to the absolute value of bi.
note: if the bi is a positive value, it will be returned as is, otherwise a new instance of BigInteger is returned.
bi.compareTo(other) => numberCompare to BigIntegers. The return value will be a negative JavaScript number if bi is less than other, a positive number if bi is greater than other, and 0 if bi and other represents the same integer.
bi.bitLength() => numberReturns the number of bits used to store bi as a JavaScript number.
bi.mod(m) => BigIntegerReturns a BigInteger with the value of (bi mod m).
bi.modPow(exponent, m) => BitIntegerReturns a BigInteger with the value of (biexponent mod m).