var f = x => x;
f(x => x + (y=2, z=undefined, ...rest) => y);
() => null;
const FC = props => <p>functional component</p>;

const good = () => 0;
const good = (x) => 0;
const bad = (a => [...a, b]);
const bad = (_ => doSomething());
const bad = (() => 0);
const bad = ((a, b) => [...a, b]);
const array = [1, 2, 3].reduce((acc, next) => [...acc, next], []);
sides.every((length,width=(3+2+(4/5))) => length > 0 );