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