[chrismorgan]

> last variable return of arrow functions. (not using widely due to compatibility)

It’s not last variable return such as you get in expression-oriented languages like Ruby or Rust; rather, it’s that if what follows the fat arrow is not a curly brace, it’s parsed as an expression, which is made to be the value returned. So `a => { a }` is the void function, but `a => { return a }` and `a => a` are identity functions. Expressed as an approximate grammar, it’s

  arrow-function = arrow-function-parameters "=>" ( "{" *statement "}" | expression-minus-object-literals )

All forms of arrow functions were introduced at the same time, in ES6 (roughly meaning “everything since but excluding IE”). `a => { return a; }` has identical browser support to `a => a`, so if you use arrow functions you might as well switch to the expression form.