How does a function in a loop (which returns another function) work?

Question

I've been trying to assign a function to onclick event of a dynamically created "a" tag in JavaScript. All of the tags are created in a loop as follows:

for ( var i = 0; i < 4; i++ )
{
  var a = document.createElement( "a" );
  a.onclick = function( ) { alert( i ) };
  document.getElementById( "foo" ).appendChild( a );
}

The alerted value for all four links is always "4". Pretty obvious. When googling I came across a post that shows the following code snippet:

a.onclick = (function(p, d) {
return function(){ show_photo(p, d) }
})(path, description);

I managed to tweak it for my needs and got the alert( i ) thing to work correctly but I'll appreciate if someone could explain exactly what the above code does.

Answer 1

When you assign the function to the click handler, a closure is created.

Basically a closure is formed when you nest functions, inner functions can refer to the variables present in their outer enclosing functions even after their parent functions have already executed.

At the time that the click event is executed, the handler refers to the last value that the i variable had, because that variable is stored on the closure.

As you noticed, by wrapping the click handler function in order to accept the i variable as an argument, and returning another function (basically create another closure) it works as you expect:

for ( var i = 0; i < 4; i++ ) {
  var a = document.createElement( "a" );
  a.onclick = (function(j) { // a closure is created
    return function () {
      alert(j); 
    }
  }(i));
  document.getElementById( "foo" ).appendChild( a );
}

When you iterate, actually create 4 functions, each function store a reference to i at the time it was created (by passing i), this value is stored on the outer closure and the inner function is executed when the click event fires.

I use the following snippet to explain closures (and a very basic concept of curry), I think that a simple example can make easier to get the concept:

// a function that generates functions to add two numbers
function addGenerator (x) { // closure that stores the first number
  return function (y){ // make the addition
    return x + y;
  };
}

var plusOne = addGenerator(1), // create two number adding functions
    addFive = addGenerator(5);

alert(addFive(10)); // 15
alert(plusOne(10)); // 11