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 );