Closures

【Closures】

　　A closure is a special kind of object that combines two things: a function, and the environment in which that function was created. Just like block in Objective-C.But Objective-C Block is more weird, cause Block contains a distinct terminology between copy & retain.

　　Here's a slightly more interesting example — a makeAdder function:

function makeAdder(x) {
  return function(y) {
    return x + y;
  };
}

var add5 = makeAdder(5);
var add10 = makeAdder(10);

print(add5(2));  // 7
print(add10(2)); // 12

　　In this example, we have defined a function makeAdder(x) which takes a single argument x and returns a new function. The function it returns takes a single argument y, and returns the sum of x and y.

　　In essence, makeAdder is a function factory — it creates functions which can add a specific value to their argument. In the above example we use our function factory to create two new functions — one that adds 5 to its argument, and one that adds 10.

　　add5 and add10 are both closures. They share the same function body definition, but store different environments. In add5's environment, x is 5. As far as add10 is concerned, x is 10.

 

　　More information will be found here:https://developer.mozilla.org/en/JavaScript/Guide/Closures