Any half serious library will be able to handle 42. If I leave the signed int32 range as a designer of the API I should be aware of the problem and either clear

Any half serious library will be able to handle 42. If I leave the signed int32 range as a designer of the API I should be aware of the problem and either clearly document what I do or find some alternative. (Like making it a string with quotes) Random side note: ECMAScript has no integer type, but only Number, which is a float, thus when dealing with such numbers in a JavaScript frontend you are in Problem Land anyways ... which again shows that boundaries have to be thought of, independently from the specification of the data exchange layer.