Thursday, September 18, 2008

JavaScript runs at C++ speed, if you let it

The common perception (ignorance of the crowd) is that JavaScript is slow. What I'm constantly finding, however, is that people will hand-craft a JavaScript loop to do, say, string parsing, when they could and should be using the language's built-in String methods (which always run fast).

Example: You need a "trim" function to remove leading and trailing whitespaces from user-entered text in a form. If you go out on the web and look at what people are doing in their scripts, you see a lot of things like:

function trim10 (str) {
var whitespace = ' \n\r\t\f\x0b\xa0\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u200b\u2028\u2029\u3000';
for (var i = 0; i < str.length; i++) {
if (whitespace.indexOf(str.charAt(i)) === -1) {
str = str.substring(i);
for (i = str.length - 1; i >= 0; i--) {
if (whitespace.indexOf(str.charAt(i)) === -1) {
str = str.substring(0, i + 1);
return whitespace.indexOf(str.charAt(0)) === -1 ? str : '';

I took this code verbatim from a web page in which the author of it claims (ironically) that it's an incredibly fast routine!

Compare with:

function trim(a) {
return a.replace(/^ +/,"").replace(/ +$/,"");

In testing, I found the shorter routine faster by 50% on very small strings with very few leading or trailing spaces, and faster by 300% or more on strings of length ~150 with ten to twenty leading or trailing spaces.

The better performance of the shorter function has nothing to do with it being shorter, of course. It has everything to do with the fact that the built-in JavaScript "replace( )" method (on the String pseudoclass) is implemented in C++ and runs at compiled-C speed.

This is an important point. Interpreters are written in C++ (Spidermonkey) or Java (Rhino). The built-in functions of the ECMAScript language are implemented in C++ in your browser. Harness that power! Use the built-in functions of the language. Never hand-parse strings with "indexOf" inside for-loops (etc.) when you can use native methods that run at compiled speed. Why walk if you can ride the bullet train?

The implications here for client/server web-app design are quite far-reaching. If you are using server-side JavaScript, and your server runtimes are Java-based, it means your server-side scripts are running (asymptotically, at least) at Java speed. Well-written client-side JavaScript runs (asymptotically) at C++ speed. Therefore, any script logic you can move to the client should be moved there. It's madness to waste precious server cycles.

Madness, I say.


  1. You're making some good points, but using the trim function examples you gave weakens it. The page you linked to (did you actually read all of it?) explains exactly which types of strings the approach you bashed is faster with and why, and provides a performance test page for verification.

  2. Anonymous5:26 PM

    It also depends on the string size.
    a replace() replaces the string, makes a copy of it and returns the copy this copy is taken by the second replace() and another copy is made. so 3 copies in memory (two of them for the Garbage Collector to clean)
    Say you then peruse each start, then end and get 2 numbers, you then use substr() and return a copy of the string: 2 copies in memory (only one for the GC).
    Now, we do have enough RAM, but it's the memcopy that could be slower (given a sufficient large string) than a loop checking each charAt().



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