javascript log

alert() is ok, but console.log() is more powerful.
You can use console to debug Javascript with either Firebug for Firefox, Javascript console in WebKit browsers, Opera or anything with Firebug Lite installed.

Wrapper console to avoid breakdown in IE.

if (window.console && console.log) {
    console.log("debug info");
}

or add following code first

if ( ! window.console ) {

    (function() {
      var names = ["log", "debug", "info", "warn", "error",
          "assert", "dir", "dirxml", "group", "groupEnd", "time",
          "timeEnd", "count", "trace", "profile", "profileEnd"],
          i, l = names.length;

      window.console = {};

      for ( i = 0; i < l; i++ ) {
        window.console[ names[i] ] = function() {};
      }
    }());

}

see the discussion in StackOverflow what is console log

see firebug, and firebug lite

see a javascript log lib, log4javascript