What are the benefits of a closure, and when are they typically used?

I'm a web developer, but lots of folks are looking for slightly more advanced skills and understanding closures seems to be at the forefront of this.

I get the whole "execution context creating a reference to a variable that doesnt ever get destroyed" thing, but really, is this some sort of private 开发者_如何学编程or static variable implementation in JavaScript?

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They can be good for lots of things, for example, visibility (like private members in traditional OO).

var count = function(num) {

   return function(add) {
       add = add || 1;
       num += add;
       return num;
   }

}

See it.

My count() can be seeded with a number. When I assign a variable to the return, I can call it with an optional number to add to the internal num (an argument originally, but still part of the scope of the returned function).

This is a pretty good overview.

See also on Stack Overflow

• What is a practical use for a closure in JavaScript?
• How does a javascript closure work ?
• When actually is a closure created?
• more...

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A closure is a code block with bound variables - it "catches" its variables from their outer context - but it is independent from that same context.

A practical use in javascript is when defining events or callbacks - you declare a closure to be executed on the click of a button, for instance, but this closure can reference variables declared on the caller scope.

jQuery uses closures a lot - btw this is a good link for understanding closures for jQuery: A Graphical Explanation Of Javascript Closures In A jQuery Context.

Hope it helps.

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There are many JS books available, but you really should grab a copy of David Flanagan's JavaScript: The Definitive Guide if you really want to learn this (still the best JS reference IMHO). Read the chapter on closures and you'll get a lot more in-depth knowledge than anyone can give you in a reply here.