Closures

A closure is bascially an inner function made accessible from outside of its parent function with in-scope variables and their values incorporated from the environment in which it was created.

In that environment, it has access to its own local variables, to its outer function's variables and the global variables (declared outside its outer function).

To begin with, we declare a function called f() containing a variable pi, and an inner function called getPi(), which returns the value of the variable pi. The function f() returns the function getPi() without executing it (opening and closing parentheses are not there).

				
				function f() {
					var pi = 3.141;
					function getPi() {
						return pi;
					}
					return getPi;
				}
				
			

The variable pi and the inner function getPi() are both local to the function f(), and neither is accessible outside of f(). Trying to access them directly will result in ReferenceError being thrown into the browser's console.

getPi() however can access variables declared in its parent function f(), and hence can also access pi.

Now we can call the function f() and assign it to a variable, say, p

				
				var p = f();
				
				

f() returns its inner function getPi() and stores it in p, without executing it. p now has access to the local variables and functions of f(), which are supposed to be private to f(). On calling p()

				
				p();
				
			

f()'s inner function getPi() gets called, and returns the assigned value of pi, which is f()'s local variable. p() now has become a closure.