Saturday, March 26, 2011

Lazy Logging on Javascript

One of the things that I like about C/C++ is compile time macros. What they mean is that if I have debug print statements, not only will they be compiled out, but also the expressions passed to the printing statement will not be evaluated.

I have code like this in javascript:
var logging = false;
function log() {
  if (logging)
    console.log.apply(console, arguments);
}

var xml = an_xml_stanza();
log("XML Stanza:", xml.toString());

Even if logging is false, the expression xml.toString() will be evaluated, which can be quite costly in a production setup (I'm talking about node.js and not on a browser).

The way I've solved this is by making sure that my log() function can accept functions as well. Hence, code such as this becomes possible:

var xml = an_xml_stanza();
log(function() {
  return ["XML Stanza:", xml.toString()];
});

The log() function needs to be patched to look like this:
log = function() {
  if (!logging)
    return;

  if (arguments.length == 1 && typeof arguments[0] == "function")
    arguments = arguments[0]();

  console.log.apply(console, arguments);
}

This essentially means that you've gotten rid of the runtime cost of calling xml.toString() when logging is disabled.

2 comments:

XIX said...

We are kind of at the point where js should be "compiled" anyway.

Why not just run the js files through the c pre preprocessor...

:)

dhruv said...

haha!! Yes, I guess people already do that in the "minification" step, so that is a very real possibility...