Execution Of Function Bar - Returns Undefined

Suppose we have following javascript codes: Case 1.
var foo = function (){
var x = "hello";
var bar = function () {
alert(x);
} return bar;
} var bar_ref= foo();
document.write(bar_ref()); // it pops up "hello" and print-outs "undefined".

If we modified above code slightly, shown as follow: Case 2.
var foo = function (){
var x = "hello";
var bar = function () {
alert(x);
} return bar();
} var bar_ref= foo();
document.write(bar_ref()); // it only pops up "hello".

As you can see, Case 2 modified the return value from "return bar" to "return bar()," which won't cause the "undefined" output. To me, it looks like when the JS interpreter executes the line "bar_ref();" it triggers the execution of function "foo", besides both "return bar" and "return bar()" do the same job which is to execute function body of "bar".

The only difference is that after the execution of function bar, its function body does not exist anymore, so when the interpreter executes the line "return bar;" it follows the function identifier "bar" and ends up with "undefined". This is why the Case 1 gives us "undefined", but I am not quite clear about why the Case 2 can trace down to the function body of "bar". Do you have any ideas about such difference outputs?