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Summary of JavaScript closures

If everything seems completely unclear then the best thing to do is to play with the examples. Reading an explanation is much harder than understanding examples. My explanations of closures and stack-frames etc are not technically correct - they are gross simplifications intended to help understanding. Once the basic idea is grokked, you can pick up the details later.


Final points:

  • 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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