Why does parsing a large number, of string form, give a larger number?

Question

See snippet:

var num = "9223372036854775808";
document.write(num);
document.write("<br>");
document.write(parseInt(num, 10));

On running the code snippet, the first write yields:

9223372036854775808

and the third write yields:

9223372036854776000

But, I am only parsing the number in string form to number. Why does it give a still larger number?

I thought it might have to do something with limits of storage capacity, but then, why would it yield a larger number if it could not store the smaller?

I read this question: Why parsing a very large number to integer return 1 and the parseInt doc but they did not help much.

So, Why does parsing a large number, of string form, return a larger number?

Answer 1

The first result is fine as it is treated as a string.

In the second one, it crosses the value of int which is +/- 9007199254740992 ie, the maximum value which parseint can parse is 9007199254740992 and since your value 9223372036854775808 is larger than the maximum value. it is giving some garbage value.

As commented correctly by blunderboy it seems that that the reason could be this

The numbers in Javascript are 64 bit "double" precision which follow the IEE754 floating point. Sothe largest positive whole number that can therefore be accurately represented is 2^53 and the rest of the remaining bits are reserved for the exponent.

If you check the ECMA specs this has been explained.