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JavaScript: The Definitive Guide

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11.7 Garbage Collection

In any programming language in which you can dynamically create new objects (such as with the new operator in JavaScript) there must be some form of "garbage collection"--a way of reclaiming the memory occupied by objects that are no longer in use. In C and C++, garbage collection is manual--the programmer explicitly decides when to free memory for reuse. In Java, on the other hand, garbage collection is handled automatically--the system can detect when objects are no longer in use and free them appropriately.

JavaScript also supports automatic garbage collection. In Internet Explorer 3.0, garbage collection is implemented in a technically sound way and you don't have to understand any of its details--it is enough to know that when your objects are no longer in use, the memory they occupy will automatically be reclaimed by the system. Navigator 4.0 will also have a perfectly transparent garbage collection scheme like this. Unfortunately, garbage collection in earlier versions of Navigator is less than perfect. In Navigator 3.0, it is pretty good, but requires you to be aware of a couple of issues. In Navigator 2.0, garbage collection is seriously flawed, and you must take a number of steps to avoid crashing the browser! The following subsections provide the details.

Reference Counting in Navigator 3.0

In Navigator 3.0, garbage collection is performed by reference counting. This means that every object (whether a user object created by JavaScript code, or a built-in HTML object created by the browser) keeps track of the number of references there are to it. Recall that objects are assigned by reference in JavaScript, rather than having their complete value copied.

When an object is created and a reference to it is stored in a variable, the object's reference count is 1. When the reference to the object is copied and stored in another variable, the reference count is incremented to 2. When one of the two variables that holds these references is overwritten with some new value, the object's reference count is decremented back to 1. If the reference count reaches zero, then there are no more references to the object, and since there are no references to copy, there can never again be a reference to the object in the program. Therefore, JavaScript knows that it is safe to destroy the object and "garbage collect" the memory associated with it.

This reference-counting scheme has some important implications. (These implications are also true of the Internet Explorer garbage collector, but, as we'll see, they are not true of the garbage collection scheme in Navigator 2.0.) If JavaScript code running in a window creates an object, and a reference to that object is stored in a variable of another window, then that object will continue to exist even after the window that created it is closed, or loads in a different page. The original reference to the object is lost, but since a reference still exists from another window, the object will not be garbage collected.

Perhaps a more surprising implication is that a top-level browser window may be closed by the user or by JavaScript code, but the Window object associated with it may continue to exist. This occurs when a variable in one window contains a reference to the window that is closed. Since there is still a reference to the Window object, that object cannot be garbage collected. Note, however, that many of the methods and properties of a Window object that is closed cannot be meaningfully used. In Navigator 3.0, you should be sure to check the closed property (a Boolean value) of any Window object before using its properties or methods, if there is any chance that it could have been closed.

Shortcomings of Garbage Collection by Reference Counting

As you may already be aware, there are some shortcomings to using reference counting as a garbage collection scheme. In fact, some people don't even consider reference counting to be true garbage collection, and reserve that term for algorithms such as "mark-and-sweep" garbage collection. The computer science literature on garbage collection is large and technical, and we won't get into it here. For our purposes it is enough to know that reference counting is a very simple form of garbage collection to implement, and it works fine in many situations. There are situations, however, in which reference counting cannot correctly detect and collect all "garbage", and you need to be aware of these.

The basic flaw with reference counting has to do with cyclical references. If object A contains a reference to object B and object B contains a reference to object A, then a cycle of references exists. A cycle would also exist, for example, if A referred to B, B referred to C, and C referred back to A. In cycles such as these, there is always a reference from within the cycle to every element in the cycle. Thus, even if none of the elements of the cycle has any remaining references, their reference count will never drop below one, and they can never be garbage collected. The entire cycle may be garbage, because there is no way to refer to any of these objects from a program, but because they all refer to each other, a reference-counting garbage collector will not be able to detect and free this unused memory.

This problem with cycles is the price that must be paid for a simple, lightweight, portable garbage collection scheme. The only way to prevent this problem is by manual intervention. If you create code in which A refers to B, B refers to C, and C refers to A, then you must be able to recognize that you've created a cycle, and take steps to force the cycle to be garbage collected when it is no longer needed.

When you know that the objects in your cycle are no longer in use, you can force them to be garbage collected by breaking the cycle. You can do this by picking one of the objects in the cycle and setting the property of it that refers to the next object to null. For example, suppose that A, B, and C are objects that each have a next property, and the value of this property is set so that these objects refer to each other and form a cycle. When these objects are no longer in use, you can break the cycle by setting A.next to null. This means that object B no longer has a reference from A, so its reference count can drop to zero and it can be garbage collected. Once it has been garbage collected, then it will no longer refer to C, so its reference count can drop to zero and it can be garbage collected. Once C is garbage collected, A can be garbage collected.

Note, of course, that none of this can happen if A, B, and C are stored in global variables in a window that is still open, because those variables A, B, and C still refer to the objects. If these were local variables in a function, and you broke their cycle before the function returned, then they could be garbage collected. But if they are stored in global variables, they will remain referenced until the window that contains them closes. In this case, if you want to force them to be garbage collected you must break the cycle and set the variables to null:

A.next = null;      // break the cycle
A = B = C = null;   // remove the last remaining external references

Per-Page Memory Management in Navigator 2.0

The garbage collection scheme in Navigator 2.0 is much simpler than that in Navigator 3.0, and, unfortunately, it is inadequate for the needs of JavaScript programs that use multiple windows and frames. In Navigator 2.0, all objects created by JavaScript code running in any particular window allocate memory from a pool of memory owned by the window. Then, when the window is destroyed, or when the document (containing the JavaScript program) displayed in the window is unloaded, the entire pool of memory is freed at once. No memory is freed until then.

With this garbage collection scheme, all memory allocated by the JavaScript running in a window can be freed in a single stroke. It is a simple and efficient scheme to implement. Unfortunately, it suffers from two major drawbacks.

First, if an object is created in one window, and then a reference to that object is stored in a variable in a second window, that object will be destroyed when the first window moves on to a new page, despite the fact that there is still an active reference to it from the other window. If this other window attempts to use this reference to the destroyed object, an error will result, possibly crashing the browser! This is an especially pernicious problem, because doing something as simple as assigning a string can cause this problem. Consider the following code:

newwin = window.open("", "temp_window");
newwin.defaultStatus = "temporary browser window".

The defaultStatus property is set to a string "owned" by the original window. If that window is closed, the string will be destroyed and the next reference to defaultStatus will go looking for a non-existing string.

The second problem with this scheme is that if a window never unloads, the memory associated with it will never be freed. For a page that runs some JavaScript once and then is static, this is not a problem. But consider a page that performs a status-bar animation, for example. If it updates the status bar several times a second for a long time, the memory consumed by that page will grow and grow. Another example occurs with the use of frames. One frame might serve as a navigation window, with controls that allow a user to easily browse a large site in other frames or other windows. These other frames and windows may load and unload pages frequently, freeing memory. But the navigation frame itself remains the same, and the memory associated with it is not freed. Depending on how the event handlers are written, there is a good chance that each time the user interacts with the navigation controls some new string or object will be created, and no memory will ever be freed. Eventually, the browser will run out of memory, and may well crash.

Workarounds for Navigator 2.0

It is possible to compensate, somewhat, for these memory management problems in Navigator 2.0. For the problem of memory not being released until the page is unloaded, the solution is simply to be careful about how much memory your scripts consume. If your page loops a lot or does a repetitive animation, look very carefully at the code that is executed over and over, and minimize the number of objects created on each iteration. Similarly, if you write a script that the user may use frequently without ever unloading, be sure to keep careful tabs on your memory usage.

Note that string manipulation is a big memory sink--each time you call a method on a string object, a new string object is generally created for the result. The same is true for string concatenation with the + operator.

For the problem of dangling references from one window to destroyed objects that were owned by another, one solution is to avoid programs that rely on inter-window references. Another solution is to be sure to make copies of all strings and other objects that are passed from one window to another. Suppose that in window 1, you want to set the defaultStatus property of window 2, as we saw earlier. If you do this directly with code in window 1, then window 2 will contain a reference to an object owned by window 1. But, if you call a function in window 2 to do the assignment, and make sure that the function makes a copy of the object, then the object assigned in window 2 will be owned by window 2. You could, for example, ensure that window 2 contains a definition of the following function:

function set_string_property(name, value)
{
    // Assign a property to this window, using associative array notation.
    // We add the empty string to the value to force JavaScript to make
    // a copy. If this function is called from another window, we won't
    // own the value string, but by making a copy, we do own the result.
    self[name] = value + "";
}

With this function defined, you could then set the property from window 1 with a line like the following:

window2.set_string_property("defaultStatus", "temporary browser window");


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