js arrow function syntax

example.js

// good
items.map(item => {
  return <div />
})

// good - implicit return, doesn't require curly braces {} or return keyword
items.map(item => <div />)

// good - using parens when on multiple lines is a common convention
items.map(item => (
  <div />
))

// bad - when using curly braces, a return is needed
// here, the div just gets ignored and you map to an array full of `undefined`s
items.map(item => {
  <div />
})