onSubmit handler typing issue

Hey there 👋

I’ve been using formik for quite some time but recently I’ve been using it with Typescript for the first time and I’ve came across some issue (I think) regarding the onSubmit handler typing.

https://github.com/jaredpalmer/formik/blob/b50e2ce19dcbe38bffcd12bfe8d5407be98e3022/src/formik.tsx#L138-L141

Because of value: object, I’m not able to know what’s going to be the shape of my value object in TS. I’ve tried abstracting a type such as:

type FormikSubmitHandler<V> = (value: V, actions: FormikActions<V>) => void;

Although, once done that, I get a signature mismatch when I pass the function to formik. I’m not sure what’s the right fix atm but to get TS (and autocomplete) not complaining anymore I had a workaround by doing a type assertion. Here’s a complete example that illustrates the issue:

type FormikSubmitHandler<V> = (value: object, actions: FormikActions<V>) => void;

interface FormValues {
  foo: string;
  bar: number
}

// At this point, I don't know what's in `value` since it's type is `object`.
const myHandler: FormikSubmitHandler<FormValues> = (value, actions) => {
  const typedValue = value as FormValues;
  value.foo; // string
  value.bar; // number
}

render() {
  return <Formik onSubmit={myHandler} />
}

I’d be happy to provide a PR but I’m not sure what’s to do to get it fixed (if needs be).

Thanks.

[jaredpalmer]

I didn’t bother updating to TS 2.6.1, but this generic ^^ should do the trick.

[Magellol]

Cool, that should do it 😃 — Thanks!