Why ActionScript and JavaScript are Slow

Slow is a relative term. Compared to the Commodore 64 that I use to program, today's computers are incredibly fast. Still, a lot of complaints about JavaScript and ActionScript (the scripting language Flash uses) is how slow they are. While both ActionScript and JavaScript use JIT compilers to greatly improve the speed of the code, there is overhead in compiling. Some proponents of "real" programming languages will say that this is still not as fast as a proper compiled language would produce. As someone who has programmed in a variety of Machine Languages, I should point out that compiled languages do not produce as good of code as hand-coded assembly language will produce. Some compiler makers claim that their compiled code is better than what the average assembly language programmer would produce. This is possibly true if that assembly language programmer was converting the higher level language line by line into assembly language like a compiler does, but if he or she is writing the routine from scratch the results would likely favour the programmer.

While some of the speed difference can be attributed to the nature of JIT compiled code, I think the bigger problem is that both ActionScript and JavaScript were designed to be easy to write code for. Remember that although these languages are now being used for more serious projects, they were intended to be for adding more flexibility to web pages or animations. In theory, these languages are suppose to be accessible to anybody, though if you look at more complex project, it is clear that real programming is still at the heart of these languages.

Concepts like dynamic objects definitely make these languages far more flexible than older languages like C/C++ but flexibility has a lot of overhead. Instead of just calling a function directly, the language has to go to the object in memory and find the pointer to the function (as it may have changed from the default one) before making the function call. It may not be much of an overhead, but it does add up. Then there is the issue of the libraries included with these languages. Using Strings for a lot of things that other libraries would have used integers for has a drastic effect on performance. If you used an integer, you would have one quick comparison to do. When you use a string, each letter of the string has to be compared. Using strings certainly makes it easier to program but there is a big cost in speed.

I could probably go on for pages, but ultimately, what it comes down to is that these languages are easier for humans to write code at a cost of efficiency, but as JIT compilers get better, the difference is not as great as purists would like to believe and the much greater productivity that results probably outweighs the speed costs.