Confused about a function returning a function in java script

Question

function a(){
   function b(){
      alert("hi");
   }
   return b();
}
function c(){
   var d = a();
}

c();

When I execute the above, I get the alert "hi". but if I do as below

function a(){
    function b(){
       alert("hi");
    }
    return b();
}
function c(){
    var d = a();
    d();
}

c();

I am expecting to see two alerts for assignment and function call statements in c();, but i get only one alert. What am I doing wrong?

Answer 1

Because method a is not returning anything since a is calling b in the return statement but b is not returning anything so d is undefined whcih will cause d() to throw an error

Demo: Fiddle

function c() {
    var d = a();
    alert(d)
    d();
}

If you see the fiddle, the second alert shows undefined as the value of d

Answer 2

You are calling an undefined function.

function a(){
    function b(){
       alert("hi");
       // no return => return undefined
    }

    // the result of b() === undefined
    return b();
}
function c(){
    // assign the result of a to variable d
    // (which is itself the result of b, which is undefined, because b has no return)
    var d = a();

    // call undefined function, this will throw an error
    d();
}

c();

Answer 3

The confusion stems from the difference between invoking/calling a function and having a reference to that function. When you add the () to a function reference you are invoking that function which returns the result of executing that function (and not the function itself).

So in this line:

function c(){
    var d = a();
    d();
}

You are assigning d to the result of the function invocation rather than the function reference. As the returned value is not a function itself, the call to d() will fail.

The same also applies in the top function where you have (this seems to be the more likely area of mistake):

return b();

The present code will execute the function and return the result, if the desired behavior was to return a reference to the function itself it should not have the ().

Answer 4

you need to return the function reference, not call it:

return b;

Answer 5

In the second code, you are basically returning the result of the alert, which should be undefined.

var d <= a() <= c() <= b() <= alert('hi') // alert returns undefined

Calling d will throw an error.

Answer 6

If you execute c() in the second example the following will occur:

• executing function a
• in a: executing function b
• in b: executing alert (no return, so undefined is returned)
• in a: returning undefined value from b
• var d now contains undefined
• executing d should throw exception: TypeError: undefined is not a function

Use some tools like chrome's js console to run and debug js.