[removed] How do I print a message to the error console?

Question

How can I print a message to the error console, preferably including a variable?

For example, something like:

print(\'x=%d\', x);

Answer 1

console.error(message); //gives you the red errormessage
console.log(message); //gives the default message
console.warn(message); //gives the warn message with the exclamation mark in front of it
console.info(message); //gives an info message with an 'i' in front of the message

You also can add CSS to your logging messages:

console.log('%c My message here', "background: blue; color: white; padding-left:10px;");

Answer 2

This does not print to the Console, but will open you an alert Popup with your message which might be useful for some debugging:

just do:

alert("message");

Answer 3

As always, Internet Explorer is the big elephant in rollerskates that stops you just simply using console.log().

jQuery's log can be adapted quite easily, but is a pain having to add it everywhere. One solution if you're using jQuery is to put it into your jQuery file at the end, minified first:

function log()
{
    if (arguments.length > 0)
    {
        // Join for graceful degregation
        var args = (arguments.length > 1) ? Array.prototype.join.call(arguments, " ") : arguments[0];

        // This is the standard; Firebug and newer WebKit browsers support this.
        try {
            console.log(args);
            return true;
        } catch(e) {
            // Newer Opera browsers support posting erros to their consoles.
            try {
                opera.postError(args);
                return true;
            } 
            catch(e) 
            {
            }
        }

        // Catch all; a good old alert box.
        alert(args);
        return false;
    }
}

Answer 4

The simplest way to do this is:

console.warn("Text to print on console");

Answer 5

Here is a solution to the literal question of how to print a message to the browser's error console, not the debugger console. (There might be good reasons to bypass the debugger.)

As I noted in comments about the suggestion to throw an error to get a message in the error console, one problem is that this will interrupt the thread of execution. If you don't want to interrupt the thread, you can throw the error in a separate thread, one created using setTimeout. Hence my solution (which turns out to be an elaboration of the one by Ivo Danihelka):

var startTime = (new Date()).getTime();
function logError(msg)
{
  var milliseconds = (new Date()).getTime() - startTime;
  window.setTimeout(function () {
    throw( new Error(milliseconds + ': ' + msg, "") );
  });
}
logError('testing');

I include the time in milliseconds since the start time because the timeout could skew the order in which you might expect to see the messages.

The second argument to the Error method is for the filename, which is an empty string here to prevent output of the useless filename and line number. It is possible to get the caller function but not in a simple browser independent way.

It would be nice if we could display the message with a warning or message icon instead of the error icon, but I can't find a way to do that.

Another problem with using throw is that it could be caught and thrown away by an enclosing try-catch, and putting the throw in a separate thread avoids that obstacle as well. However, there is yet another way the error could be caught, which is if the window.onerror handler is replaced with one that does something different. Can't help you there.

Answer 6

The WebKit Web Inspector also supports Firebug's console API (just a minor addition to Dan's answer).