javascript

Chapter 4: Variables, Scope, and Memory
There are several other patterns that may cause circular references, which they will
be covered throughout this book.
Performance
The garbage collector runs periodically and can potentially be an expensive process if there is a large
number of variable allocations in memory, so the timing of the garbage - collection process is important.
IE was infamous for its performance issues related to how often the garbage collector ran — it ran based
on the number of allocations, specifically 256 variable allocations, 4,096 object/array literals and array
slots, or 64kb of strings. If any of these thresholds were reached, the garbage collector would run. The
problem with this implementation is that a script with so many variables will probably continue to have
that many variables throughout its lifetime, meaning the garbage collector will run quite frequently. This
issue caused serious performance problems that led to changes in the garbage - collection routine in IE 7.
With the release of IE 7, the JavaScript engine ’ s garbage - collection routine was tuned to dynamically
change the allocation threshold of variables, literals, and/or array slots that triggered garbage collection.
The IE 7 thresholds start out equal to those in IE 6. If the garbage - collection routine reclaims less than
15% of the allocations, the threshold for variables, literals, and/or array slots doubles. If the routine ever
reclaims 85% of the allocations, then the threshold is reset to the default. This simple change greatly
improved the performance of the browser on JavaScript - heavy web pages.
It ’ s possible, though not recommended, to trigger the garbage - collection process
in some browsers. In IE, the window.CollectGarbage() method causes garbage
collection to occur immediately. In Opera 7 and higher, calling window.opera
.collect() initiates the garbage - collection process.
Managing Memory
In a garbage - collected programming environment, developers typically don ’ t have to worry about
memory management. However, JavaScript runs in an environment where memory management and
garbage collection operate uniquely. The amount of memory available for use in web browsers is
typically much less than is available for desktop applications. This is more of a security feature than
anything else, ensuring that a web page running JavaScript can ’ t crash the operating system by using up
all the system memory. The memory limits affect not only variable allocation but also the call stack and
the number of statements that can be executed in a single thread.
Keeping the amount of used memory to a minimum leads to better page performance. The best way to
optimize memory usage is to ensure that you ’ re keeping around only data that is necessary for the
execution of your code. When data is no longer necessary, it ’ s best to set the value to null , freeing up the
reference — this is called dereferencing the value. This advice applies mostly to global values and
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