Closures

The rest of the chapter is about closures (what better way to close a chapter?). Closures can be a little hard to grasp initially, so don't feel discouraged if you don't "get it" during the first read. You should go through the rest of the chapter and experiment with the examples on you own, but if you feel you don't fully understand the concept, you can come back to it later when the topics discussed previously in this chapter have had a chance to sink in.

Before moving on to closures, let's first review and expand on the concept of scope in JavaScript.

Scope chain

As you know, in JavaScript, there is no curly braces scope, but there is function scope. A variable defined in a function is not visible outside the function, but a variable defined in a code block (for example an if or a for loop) is visible outside the block:

> var a = 1; 
> function f() {
    var b = 1; 
    return a;
  }
> f();
1
> b;
ReferenceError: b is not defined

The variable...