Synchronous errors from onSubmit are ignored in v3

[radekmie]

As found it while working on #812, there's a problem with errors from onSubmit. These errors are treated as results and not actual problems. The problematic code is here.

onSubmit={model => {
  const errorOrNull = synchronousValidation(model);

   // Does not work but should.
  return errorOrNull;

  // Works as expected. 
  return Promise.reject(errorOrNull);
}}

[radekmie]

Some update: while working on that, I've come to the conclusion that synchronous errors from onSubmit are problematic. Returning them may be error-prone, as people may expect it to work in the same way as returning a value from asynchronous submission. Allowing throwing synchronous errors in onSubmit is problematic as well, as it may hide actual problems. Another option would be to make it work as follows:

onSubmit={model => { return value; }} // [A] This value is accessible in `form.submit()` Promise. [B] This value is ignored.
onSubmit={model => { throw error; }} // This is an error, `onSubmit` cannot throw an error.
onSubmit={model => { return Promise.resolve(value); }} // This value is accessible in `form.submit()` Promise.
onSubmit={model => { return Promise.reject(error); }} // This error is treated as a validation error, being accessible in the form.

I think that is the best, as most onSubmit functions are asynchronous. Another, maybe easier to understand option ([B]), would be to force onSubmit: (model: Model) => void | Promise<any>, so ignore any synchronous result.

[Monteth]

I would opt for the 'B' as it's simpler and more consistent. I believe wrapping errors to Promise in sync scenarios won't be harmful.