javascript - How do you send console messages and errors to alert?

Question

I would like to pass errors to an alert to warn the user they made mistake in their code even if they don't have console open.

    var doc=(frame.contentWindow.document || obj.contentDocument|| obj.contentWindow);
    var head = doc.getElementsByTagName('head')[0];
    var scriptElement = doc.createElement('script');
    scriptElement.setAttribute('type', 'text/javascript');
    scriptElement.text = scripts;

    try{
        head.appendChild(scriptElement);
    }
     catch(e){ alert("error:"+e.message +"  linenumber:"+e.lineNumber);}

The appendChild throws an error when the scripts contain an error. It goes straight to the console though, and I want it to display in an alert, because it is for kids and they might not check the console. The try catch block does not catch the error. I tried it with eval(scripts).

   try{
   eval(scripts);} catch(e){ alert("error:"+e.message +"  linenumber:"+e.lineNumber);}

this does work but it means that the code is executed twice, and that is very inconvenient in some cases.

I tried monkey patching the console.error:

       console.log=function(){alert("taking over the log");}
       console.error=function(){alert("taking over the log");}

but that only works when I literally use console.error. Not when an actual error is thrown. What function sends the error to the console in the case of a real error,if it isn't console.error? and can I access it and change it? Any ideas? Help would be really appreciated. Thanks Jenita

Answer 1

Whilst try ... catch will work on the code that the script runs initially, as Jenita says it won't catch Syntax Errors, and also it won't catch errors thrown by callback functions which execute later (long after the try-catch has finished). That means no errors from any functions passed to setTimeout or addEventListener.

However, you can try a different approach. Register an error listener on the window.

window.addEventListener("error", handleError, true);

function handleError(evt) {
    if (evt.message) { // Chrome sometimes provides this
      alert("error: "+evt.message +" at linenumber: "+evt.lineno+" of file: "+evt.filename);
    } else {
      alert("error: "+evt.type+" from element: "+(evt.srcElement || evt.target));
    }
}

This will be called when an exception is thrown from a callback function. But it will also trigger on general DOM errors such as images failing to load, which you may not be interested in.

It should also fire on Syntax Errors but only if it was able to run first so you should put it in a separate script from the one that may contain typos! (A Syntax Error later in a script will prevent valid lines at the top of the same script from running.)

Unfortunately, I never found a way to get a line number from the evt in Firefox. (Edit: Poke around, I think it might be there now.)

---

I discovered this when trying to write FastJSLogger, an in-page logger I used back when the browser devtools were somewhat slow.

Desperate to catch line numbers, I started to experiment with wrappers for setTimeout and addEventListener that would re-introduce try-catch around those calls. For example:

var realAddEventListener = HTMLElement.prototype.addEventListener;

HTMLElement.prototype.addEventListener = function(type,handler,capture,other){
    var newHandler = function(evt) {
        try {
            return handler.apply(this,arguments);
        } catch (e) {
            alert("error handling "+type+" event:"+e.message +"  linenumber:"+e.lineNumber);
        }
    };

    realAddEventListener.call(this,type,newHandler,capture,other);
};

Obviously this should be done before any event listeners are registered, and possibly even before libraries like jQuery are loaded, to prevent them from grabbing a reference to the real addEventListener before we have been able to replace it.