Closures: Line by Line explanation of “[removed] Good Parts” example?

Question

I\'m reading \"Javascript: The Good Parts\" and am totally baffled by what\'s really going on here. A more detailed and/or simplified explanation would be greatly appreciate

Answer 1

I think this is a very common source of confusion for newcomers to JavaScript. First I would suggest checking out the following Mozilla Dev article for brief introduction on the topic of closures and lexical scoping:

• Mozilla Dev Center: Working with Closures

Let's start with the bad one:

var add_the_handlers = function (nodes) {
// Variable i is declared in the local scope of the add_the_handlers() 
// function. 
   var i;

// Nothing special here. A normal for loop.
   for (i = 0; i < nodes.length; i += 1) {

// Now we are going to assign an anonymous function to the onclick property.
       nodes[i].onclick = function (e) {

// The problem here is that this anonymous function has become a closure. It 
// will be sharing the same local variable environment as the add_the_handlers()
// function. Therefore when the callback is called, the i variable will contain 
// the last value it had when add_the_handlers() last returned. 
           alert(i);
       }
   }

// The for loop ends, and i === nodes.length. The add_the_handlers() maintains
// the value of i even after it returns. This is why when the callback
// function is invoked, it will always alert the value of nodes.length.
};

We can tackle this problem with more closures, as Crockford suggested in the "good example". A closure is a special kind of object that combines two things: a function, and the environment in which that function was created. In JavaScript, the environment of the closure consists of any local variables that were in-scope at the time that the closure was created:

 // Now we are creating an anonymous closure that creates its own local 
 // environment. I renamed the parameter variable x to make it more clear.
 nodes[i].onclick = function (x) {

     // Variable x will be initialized when this function is called.

     // Return the event callback function.
     return function (e) {
         // We use the local variable from the closure environment, and not the 
         // one held in the scope of the outer function add_the_handlers().
         alert(x);
     };
 }(i); // We invoke the function immediately to initialize its internal 
       // environment that will be captured in the closure, and to receive
       // the callback function which we need to assign to the onclick.

Rather than having the callbacks all sharing a single environment, the closure function creates a new environment for each one. We could also have used a function factory to create a closure, as in the following example:

function makeOnClickCallback (x) {
   return function (e) {
      alert(x);
   };
}

for (i = 0; i < nodes.length; i += 1) {
   nodes[i].onclick = makeOnClickCallback(i);
}