What are the best practices for JavaScript error handling?

Question

I\'m looking to start making my JavaScript a bit more error proof, and I\'m finding plenty of documentation on using try, catch, finally,

Answer 1

Nicholas Zakas of Yahoo! fame did a talk on Enterprise Error Handling (slides) at Ajax Experience 2008, in which he proposed something like this:

function log(sev,msg) {
    var img = new Image();
    img.src = "log.php?sev=" +
        encodeURIComponent(sev) +
        "&msg=" + encodeURIComponent(msg);
}

// usage
log(1, "Something bad happened.")

// Auto-log uncaught JS errors
window.onerror = function(msg, url, line) {
    log(1, msg);
    return true;
}

A year later, Nicholas Zakas posted an update on his blog which included a clever pattern to inject error handling code automatically on your production environment (using aspect-oriented programming).

When you start logging window.error calls, you're going to notice two things:

1. If your site is fairly complex, you're going to log a lot of errors
2. You'll be seeing a bunch of useless "window.error in undefined:0" messages

Reducing the torrent of log entries is as simple as testing for severity and/or a random number before logging to the server:

function log(sev,msg) {
    if (Math.random() > 0.1) return; // only log some errors

    var img = new Image();
    img.src = "log.php?sev=" +
        encodeURIComponent(sev) +
        "&msg=" + encodeURIComponent(msg);
}

Handling the useless window.error in undefined:0 errors depends on your site architecture, but can try identifying all Ajax calls and throwing an exception when something fails (possibly returning a stack trace using stacktrace.js).