it is a string "115792089237316195423570985008687907853269984665640564039457584007908834671663" or something else..?
It's the value of the moduli. It is the biggest number in the curve secp256k1 that bitcoin uses.
=2^256-2^32-2^9-2^8-2^7-2^6-2^4-1
If your generated private key is bigger than that, then you have to subtract that number from your generated key as many times as necessary, until it becomes smaller than that number.
The security of bitcoin depends on big numbers. That is the reason why the moduli has to be so big.
https://en.bitcoin.it/wiki/Secp256k1for example :
Here is my private key ="9c63e8e2f6574c197c0626bad843eb47104adf3f01f2901aad1258936feb007e"
var newprimarykey = (9c63e8e2f6574c197c0626bad843eb47104adf3f01f2901aad1258936feb007e * 36453278) mod 115792089237316195423570985008687907853269984665640564039457584007908834671663 );
console.log('newkey = ',newprimarykey);
If i am executing this one then it'll give error :
var t1 = (9c63e8e2f6574c197c0626bad843eb47104adf3f01f2901aad1258936feb007e * 36453278);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SyntaxError: Unexpected identifier
at exports.runInThisContext (vm.js:53:16)
at Module._compile (module.js:373:25)
at Object.Module._extensions..js (module.js:416:10)
at Module.load (module.js:343:32)
at Function.Module._load (module.js:300:12)
at Function.Module.runMain (module.js:441:10)
at startup (node.js:140:18)
at node.js:1043:3