JavaScript hoisting explanation

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;
}