It looks like this has been solved before:
If you only need integers you can use BigInteger.js.
While looking for an big integer library for an ElGamal crypto implementation I tested several libraries with the following results:
I recommend this one: Tom Wu's jsbn.js (http://www-cs-students.stanford.edu/~tjw/jsbn/)
Leemon Baird's big integer library (http://www.leemon.com/crypto/BigInt.js)
I haven't tested this by myself: BigNumber (http://jsfromhell.com/classes/bignumber)
(Note that I haven't used this myself. Try it and see what you think.)
There is also Silent Matt's library for Big Integers. It does not handle decimals.
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