is it possible to React.useState(() => {}) in React?

It is possible to set a function in state using hooks, but because state can be initialized and updated with a function that returns the initial state or the updated state, you need to supply a function that in turn returns the function you want to put in state.

const { useState } = React;

function App() {
  const [ooops, setOoops] = useState(() => () => console.log("default ooops"));

  return (
    <div>
      <button onClick={ooops}>Show Ooops</button>

      <button
        onClick={() => {
          setOoops(() => () => console.log("other ooops"));
        }}
      >
        change oops
      </button>
    </div>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>

TL;DR

Yes, it is possible to use a function as the React Component's state. In order to do this, you must use a function returning your function in React.useState:

const [ooops, setOoops] = React.useState<OoopsFunction>(
    () => () => console.log('default ooops')
);

// or

const yourFunction = () => console.log('default ooops');
const [ooops, setOoops] = React.useState<OoopsFunction>(
    () => yourFunction
);

To update your function you also must use a function returning your function:

setOoops(() => () => console.log("other ooops"));

// or

const otherFunction = () => console.log("other ooops");
setOoops(() => otherFunction);

Detailed Answer

Notes about React.useState

The signature of useState in React with types is

function useState<S>(initialState: S | (() => S)): [S, Dispatch<SetStateAction<S>>];

It shows, that there are two ways to set the initial value in your state:

1. Provide the initial value as is (React.useState(0) - initial value 0),
2. Provide a function, which returns the initial value to be set (React.useState(() => 0) - initial value also 0).

Important to note: If you provide a function in React.useState, then this function is executed, when React.useState is executed and the returned value is stored as the initial state.

How to actually store functions

The problem here is if you want to store a function as state you can not provide it as initial state as is, because this results in the function being executed and its return value stored as state instead of the function itself. Therefore when you write

const [ooops, setOoops] = React.useState<OoopsFunction>(
    () => console.log('default ooops')
);

'default ooops' is logged immediately when React.useState is called and the return value (in this case undefined) is stored.

This can be avoided by providing a higher order function returning your function you want to store:

const [ooops, setOoops] = React.useState<OoopsFunction>(
    () => () => console.log('default ooops')
);

This way the outer function will be definitely executed when first running React.useState and its return value will be stored. Since this return value is now your required function, this function will be stored.

Notes about the state setter function

The state setter function's (here setOoops) signature is given as

Dispatch<SetStateAction<S>>

with

type Dispatch<A> = (value: A) => void;
type SetStateAction<S> = S | ((prevState: S) => S);

Like in React.useState there is also the possibility to update state with a value or a function returning the value. So in order to update state the higher order function from above has to be used as well.