Please explain the use of JavaScript closures in loops [duplicate]

Answer 1

A closure is a construct in which you reference a variable outside the scope in which it's defined. You usually talk about closures in the context of a function.

var helloFunction;
var finished = false;

while (!finished) {
 var message = 'Hello, World!';
 helloFunction = function() {
   alert(message);
 }
 finished = true;
}

helloFunction();

Here, I define the variable message, and define a function that references message. When I define the function to use message, I am creating a closure. This means helloFunction holds a reference to message, so that I can continue to use message, even outside of the scope (the loop body) where message is defined.

Addendum

The (i) in parenthesis is a function call. What's happening is:

1. You define some function(num) {}. This is called an anonymous function, because it's defined inline and doesn't have a name.
2. function(num) takes an integer argument, and returns a reference to another function, which is defined as alert(num)
3. The outer anonymous function is immediately called, with the argument i. So num=i. The result of this call is a function which will do alert(i).
4. The end result is more or less equivalent to: link.onclick = function() { alert(i); };