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JavaScript Closures 101- they're not magic

Credits: This tutorial is written by Morris Johns. Morris maintains a personal blog called Developing Thoughts where he can be contacted via.

This tutorial explains closures so that a regular programmer can understand them - using working JavaScript code. It is not for gurus nor 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 main-stream language, and who can read the following JavaScript function:

function sayHello(name) {
  var text = 'Hello ' + name;
  var sayAlert = function() { alert(text); }
  sayAlert();
}

An Example of a Closure

Two one sentence summaries:

  • a closure is the local variables for a function - kept alive after the function has returned, or
  • a closure is a stack-frame which is not deallocated when the function returns. (as if a 'stack-frame' were malloc'ed instead of being on the stack!)

The following code returns a reference to a function:

function sayHello2(name) {
  var text = 'Hello ' + name; // local variable
  var sayAlert = function() { alert(text); }
  return sayAlert;
}

Most JavaScript programmers will understand how a reference to a function is returned to a variable in the above code. If you don't, then you need to before you can learn closures. A C programmer would think of the function as returning a pointer to a function, and that the variables sayAlert and say2 were each a pointer to a function.

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 function() { alert(text); } is declared inside another function, sayHello2() in this example. In JavaScript, if you use the function keyword inside another function, you are creating a closure.

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 say2(); after we have returned from sayHello2(). Notice that the code that we call references the variable text, which was a local variable of the function sayHello2().

function() { alert(text); }

Click the button above to get JavaScript to print out the code for the anonymous function. You can see that the code refers to the variable text. The anonymous function can reference text which holds the value 'Jane' because the local variables of sayHello2() are kept in a closure.

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.