JSON.parse broken

[lastmjs]

In Chrome, Node, and Firefox this works just fine:

JSON.parse('19686109595169230000')
// 19686109595169230000

In QuickJS in wasm32-wasi (using Javy) and on my Ubuntu laptop x86 this is broken:

JSON.parse('19686109595169230000')
// 19686109595169227000

[ulan]

JSON.parse() uses parseFloat() under the hood, so all integers in JSON are treated as floating point numbers and precision errors are expected.

That said, the spec requires that parseFloat() is consistent with parsing of literals in the program:

> let x = 19686109595169230000;
> let y = parseFloat("19686109595169230000");
> x == y

This should return true, but currently it returns false and it appears to be a bug indeed.

I debugged the issue to this part of js_strtod: https://github.com/bellard/quickjs/blob/4bb8c35da7c83b5eb67541772b857e822da855da/quickjs.c#L9976
Precision errors are introduced when a large n is assigned to double d.

One way to fix this issue would be to restrict the fast-path parsing only to integers up to 2^53 and fall back to the slower strtod for larger integers:
ulan/javy@149ff4c

[bellard]

fixed