function Example() {
   const f = n=>{console.log('firing '+n); return n;};
   const [a, setA] = React.useState(f(0));           // calling a function
   const [b, setB] = React.useState(()=>f(1));    // passing a raw function
   return (<div><p>{a} {b}</p>
                <button onClick={() => {setA(a+1);}}>firing f(0) on every render</button>
                                <button onClick={() => {setB(b+1);}}>firing f(1) on first render only</button>
                                   </div>);}ReactDOM.render(<Example/>,document.querySelector("div"));