William Jiang

JavaScript,PHP,Node,Perl,LAMP Web Developer – http://williamjxj.com; https://github.com/williamjxj?tab=repositories

JavaScript: closure

JavaScript Closure

This is from http://stackoverflow.com/questions/111102/how-do-javascript-closures-work, talking about Closure, which normally use in setTimeout() etc:

To clear up several (possible) misunderstandings about closure:

  • A closure is not only created when you return an inner function. In fact, the enclosing function does not need to return at all in order for its closure to be created. You might instead assign your inner function to a variable in an outer scope, or pass it as an argument to another function where it could be called immediately or any time later. Therefore, the closure of the enclosing function is probably created as soon as the enclosing function is called since any inner function has access to that closure whenever the inner function is called, before or after the enclosing function returns.
  • A closure does not reference a copy of the old values of variables in its scope. The variables themselves are part of the closure, and so the value seen when accessing one of those variables is the latest value at the time it is accessed. This is why inner functions created inside of loops can be tricky, since each one has access to the same outer variables rather than grabbing a copy of the variables at the time the function is created or called.
  • The "variables" in a closure include any named functions declared within the function. They also include arguments of the function. A closure also has access to its containing closure’s variables, all the way up to the global scope.
  • Closures use memory, but they don’t cause memory leaks since JavaScript by itself cleans up its own circular structures that are not referenced. IE memory leaks involving closures are created when it fails to disconnect DOM attribute values that reference closures, thus maintaining references to possibly circular structures.

Summary:

  • Whenever you use function inside another function, a closure is used.
  • Whenever you use eval() inside a function, a closure is used. The text you eval can reference local variables of the function, and within eval you can even create new local variables by using eval(‘var foo = …
  • When you use Function() inside a function, it does not create a closure. (The new function cannot reference the local variables of the function calling Function()).
  • A closure in JavaScript is like keeping a copy of the all the local variables, just as they were when a function exited.
  • It is probably best to think that a closure is always created just on entry to a function, and the local variables are added to that closure.
  • A new set of local variables is kept every time a function with a closure is called (Given that the function contains a function declaration inside it, and a reference to that inside function is either returned or an external reference is kept for it in some way).
  • Two functions might look like they have the same source text, but have completely different behaviour because of their ‘hidden’ closure. I don’t think JavaScript code can actually find out if a function reference has a closure or not.
  • If you are trying to do any dynamic source code modifications ( for example: myFunction = Function(myFunction.toString().replace(/Hello/,’Hola’)); ), it won’t work if myFunction is a closure (Of course, you would never even think of doing source code string substitution at runtime, but…).
  • It is possible to get function declarations within function declarations within functions – and you can get closures at more than one level.
  • I think normally a closure is the term for both the function along with the variables that are captured. Note that I do not use that definition in this article!
  • I suspect that closures in JavaScript differ from those normally found in functional languages.
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