Documents for human readable documentation and runtime validation

The described technology is directed towards having document files (e.g., a JSON object) associated with objects of a computer program. The document files may include validation information that is used during runtime to validate one or more parts of the object via validation code such as a validation function. For example, a function of an object may have one validation function that during runtime validates whether the input parameter(s) are valid (their types and/or values are correct), and another validation function that during runtime validates whether the return values from the function are valid. Also described is processing the document files into more easily readable documentation, e.g., without any source code.