In javascript, functions are closures. This means that the function "closes" over the environment in which it was defined. In the above example, the function ha

In javascript, functions are closures. This means that the function "closes" over the environment in which it was defined. In the above example, the function has access to the sum variable because it was available at the time the function was defined. It is then free to add to that variable when the function is called at a later time.