Discussion on: Arrow functions: a walkthrough and gotchas

and you need {} as soon as you have more than 1 line of function body

Almost but not completely correct. You need {} when the function body is not just one expression, that is, when you need additional statements before the return value. You can easily write something like

const sum = (array) => array
   .reduce( (a,b) => a+b);

but you can't do

const sumPlusOne = (a, b) => a+=1; a+b