Help me where I can find documentation for the TO_NUMBER function and what does the %(percentage sign) mean when placed before MathExpRT (which also can't find any docs about it)
https://github.com/v8/v8/blob/b0e4dce6091a8777bda80d962df76525dc6c5ea9/src/js/math.js#L62-L64
TO_NUMBER is a V8-internal macro defined here:
https://github.com/v8/v8/blob/ac886b0c1c173d6ceff762952e1d6cf78e8bd172/src/js/macros.py#L96
Not that this will help you much: it simply calls into an intrinsic, which is implemented as generated code, and has many different implementations depending on compiler (V8 has four), hardware platform (V8 supports about 10), and available type information. One generic code generator can be seen here:
If you just want to know the semantics, you better look at the ECMAScript 262 standard.