closure in JavaScript

Question

I want the variable i to be a counter, but it is being initialized to 100 each time.

How do I call myFunction().f() directly?

function myFunction() {
 var i=100;
 function f() {
 i=i+1;
 return i;
 }
 return f(); // with parenthesis
};
var X = myFunction();
console.log(X);
X = myFunction();
console.log(X);

• Phillip Senn: The purpose of closure is to keep variables scoped within the function, and yet they maintain their state after the function returns. Oops. This was in reply to someone else's comment about "What's the purpose of closure?"

Answer 1

You can't call f directly. It is wrapped in a closure, the point of which is to close over all the local variable. You have to expose it to the outside of myFunction.

First:

return f; //(); // withOUT parenthesis

Then just call X, as you'll have assigned a function to it.

var X = myFunction();
X();

• Phillip Senn: Thanks Quentin! When I console.log(X()) a second time, it still returns 101.
• Phillip Senn: OK, I got it! Thanks! (I was assigning X = myFunction() a second time as well, which was wrong).

Answer 2

This example would return 101 and 102: Be sure to try it.

function myFunction() {
 var i=100;
 function f() {
 i=i+1;
 return i;
 }
 return f; // without any parenthesis
};
var X = myFunction();
// X is a function here
console.log(X());
// when you call it, variable i gets incremented
console.log(X());
// now you can only access i by means of calling X()
// variable i is protected by the closure

If you need to call myFunction().f() that will be a pointless kind of closure:

function myFunction() {
 var i=100;
 function f() {
 i=i+1;
 return i;
 }
 return {x : f}
};
var X = myFunction().x();
// X now contains 101
var X = myFunction().x();
// X still contains 101
// pointless, isn't it?

• Phillip Senn: This returns 101 two times. I would like it to return 101, 102.
• Phillip Senn: Well, I'm trying to understand the correct way to use closure. I don't want to learn something that would be a pointless kind of closure.
• Phillip Senn: Oh, I see... You were showing me both the right way AND the wrong way to do it...