From the wiki article, a closure (computer science) is a function or reference to a function together with a referencing environment-- a table storing a reference to each of the non-local variables of that function.
Closure-like constructs include callbacks and as such, are important in asynchronous programming. Here is a simple example in PHP that uses a closure as a callback to compute the total price of a shopping cart by defining a reference table for the callback function and including variables tax and total:
The concept of closures in javascript is important to understand because you might not even know you're using it. If you write in coffee-script classes or do classical inheritance patterns in vanilla javascript or even write callbacks in general for asynchronous programming, you are probably using closures. The following example is a starting point for classical inheritance in javascript. This shows how to hide private variables. It doesn't use "new" but the pattern is very similar.
According to Effective Javascript: 68 Specific Ways To Harness The Power of Javascript, there are three essential facts regarding closures:
- JavaScript allows you to refer to variables that were defined outside of the current function
- Functions can refer to variables defined in outer functions even after those outer functions have returned
- Closures can update values of outer variables
Knowing this, we can do some fun stuff in Node.JS with asynchronous programming. With closures, we can pull a document collection from a NoSQL database, manipulate the results, and push it to an array stored via closure in the parent scope.
Hopefully you will use closures to your advantage, especially when developing in javascript, be it server side or client side or even in the database (Postgres with v8).
