JavaScript hoisting explanation

return
function a() {}

is same as

return;
function a() {}

after automatic semicolon insertion. In second case, function a is moved to the top of its scope. The code is same as

var a = 0;

function b() {
  function a() {};
  a = 10;
  return;
}
b();
console.log(a);

As a inside b() is inner function and then overriden, it is not accessible from outside of b().

Here is the demo to understand how the hoisting works.

var a = 0;

function b() {
  console.log(a); // function
  a = 10;
  console.log(a); // 10
  return

  function a() {};
}
console.log(b()); // undefined
console.log(a);

return function a(){};

Here function ... is an expression. A named function expression to be precise. The a here doesn't matter much, it just gives the anonymous function a .name, but it's still just a function expression that you're returning.

return 
function a(){};

This here is equivalent to:

return;
function a(){};

Here function a is a declaration, not an expression. It is hoisted, creating a local name a in the scope, shadowing the outer a. I.e. it is equivalent to:

function b(){
    var a = function () {};
    a = 10;
    return;
}