How do JavaScript closures work?
Like the old Albert Einstein said:
Well, I tried to explain JavaScript closures to a 27-year-old friend and completely failed. How would you explain it to someone with a knowledge of the concepts which make up closures (for example, functions, variables and the like), but does not understand closures themselves? I have seen the Scheme example given on Stack Overflow, and it did not help.
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JavaScript Closures for Dummies
Closures Are Not MagicThis page explains closures so that a programmer can understand them — using working JavaScript code. It is not for gurus or functional programmers. Closures are not hard to understand once the core concept is grokked. However, they are impossible to understand by reading any academic papers or academically oriented information about them! This article is intended for programmers with some programming experience in a mainstream language, and who can read the following JavaScript function:
An Example of a ClosureTwo one sentence summaries:
The following code returns a reference to a function:
Most JavaScript programmers will understand how a reference to a function is returned to a variable ( There is a critical difference between a C pointer to a function and a JavaScript reference to a function. In JavaScript, you can think of a function reference variable as having both a pointer to a function as well as a hidden pointer to a closure. The above code has a closure because the anonymous function In C, and most other common languages after a function returns, all the local variables are no longer accessible because the stack-frame is destroyed. In JavaScript, if you declare a function within another function, then the local variables can remain accessible after returning from the function you called. This is demonstrated above, because we call the function
Looking at the output of The magic is that in JavaScript a function reference also has a secret reference to the closure it was created in — similar to how delegates are a method pointer plus a secret reference to an object. More examplesFor some reason, closures seem really hard to understand when you read about them, but when you see some examples you can click to how they work (it took me a while). I recommend working through the examples carefully until you understand how they work. If you start using closures without fully understanding how they work, you would soon create some very weird bugs! Example 3This example shows that the local variables are not copied — they are kept by reference. It is kind of like keeping a stack-frame in memory when the outer function exits!
Example 4All three global functions have a common reference to the same closure because they are all declared within a single call to
The three functions have shared access to the same closure — the local variables of Note that in the above example, if you call Example 5This one is a real gotcha for many people, so you need to understand it. Be very careful if you are defining a function within a loop: the local variables from the closure do not act as you might first think.
The line
Note that when you run the example, Example 6This example shows that the closure contains any local variables that were declared inside the outer function before it exited. Note that the variable
Tricky: note also that the Example 7This final example shows that each call creates a separate closure for the local variables. There is nota single closure per function declaration. There is a closure for each call to a function.
SummaryIf 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:
Links
ThanksIf you have just learnt closures (here or elsewhere!), then I am interested in any feedback from you about any changes you might suggest that could make this article clearer. Send an email to morrisjohns.com (morris_closure @). Please note that I am not a guru on JavaScript — nor on closures. |
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