JSON transfer of bigint: 12000000000002539 is converted to 12000000000002540?

Question

The value is actually not exceeding the maximum numeric value in JavaScript (which is "only" 1.7³⁰⁸ or so).

However, the value is exceeding the range of "integral precision". It is not that the wrong number is sent: rather, it is that the literal 12000000000002539 can only be represented as precisely as 12000000000002540, and thus there was never the correct numeric value in JavaScript. (The range of integrals is about +/- 2⁵³.)

This is an interesting phenomena of using a double relative-precision (binary64 in IEEE-754 speak) type to store all numeric values, including integers:

12000000000002539 === 12000000000002540 // true

The maximum significant number of decimal digits that be precisely stored as a numeric value is 15 (15.95, really). In the above, there are 17 significant digits, so some of the least-significant information is silently lost. In this case, as the JavaScript parser/engine reads in the literal value.

The only safe way to handle integral numbers of this magnitude in JavaScript is to use a string literal or to break it down in another fashion (e.g. a custom numeric type or a "bigint library"). However, I recommend just using a string, as it is human readable, relatively compact (only two extra characters in JSON), and doesn't require special serialization. Since the value is just an "id" in this case, I hope that math does not need to be performed upon it :)

Happy coding.