In a detailed exploration of JSON parsing challenges between Postgres and JavaScript, the author introduces a new JavaScript library, json-custom-numbers, designed to handle the differences in number representation between the two systems. As JavaScript only supports IEEE 754 float64 numbers, while Postgres supports a wider range of number types, parsing issues can arise, especially with large numbers like bigint. Existing solutions, json-bigint and lossless-json, offer partial solutions but lack full conformance to native JSON.parse() behavior and vary significantly in performance. The json-custom-numbers library is presented as a more effective alternative, offering conformance to JSON.parse(), improved performance, and the flexibility to customize number parsing, addressing the specific issue of handling big numbers. The article delves into the technical aspects of developing the library, including the use of sticky regular expressions for performance gains and non-recursive implementations to avoid call stack limitations. The author also shares insights gained during the development process, highlighting the importance of experimentation and the influence of JavaScript engine optimizations on performance benchmarking.