allow typescript compiler to call setState on only one react state property

I think that the best way to do it is to use Partial

Declare your component in the following way

class Main extends React.Component<MainProps, Partial<MainState>> {
}

Partial automatically changes all of the keys to optional.

Edit

The definitions for react were updated and the signature for setState are now:

setState<K extends keyof S>(state: Pick<S, K>, callback?: () => any): void;

Where Pick<S, K> is a built-in type which was added in Typescript 2.1:

type Pick<T, K extends keyof T> = {
    [P in K]: T[P];
}

See Mapped Types for more info.
If you still have this error then you might want to consider updating your react definitions.

Original answer:

I'm facing the same thing.

The two ways I manage to get around this annoying issue are:

(1) casting/assertion:

this.setState({
    editorState: editorState
} as MainState);

(2) declaring the interface fields as optional:

interface MainState {
    todos?: Todo[];
    hungry?: Boolean;
    editorState?: EditorState;
}

If anyone has a better solution I'd be happy to know!

---

Edit

While this is still an issue, there are two discussions on new features that will solve this problem:
Partial Types (Optionalized Properties for Existing Types)
and
More accurate typing of Object.assign and React component setState()

We can tell setState which field it should expect, by parametrising it with <'editorState'>:

this.setState<'editorState'>({
  editorState: editorState
});

If you are updating multiple fields, you can specify them like this:

this.setState<'editorState' | 'hungry'>({
  editorState: editorState,
  hungry: true,
});

although it will actually let you get away with specifying only one of them!

Anyway with the latest TS and @types/react both of the above are optional, but they lead us to...

---

If you want to use a dynamic key, then we can tell setState to expect no fields in particular:

this.setState<never>({
  [key]: value,
});

and as mentioned above, it doesn't complain if we pass an extra field.

(GitHub issue)

Update state explicitly, example with the counter

this.setState((current) => ({ ...current, counter: current.counter + 1 }))