How can I override/extend ReferenceError in Chrome's JavaScript?

Question

To make debugging easier, I'm capturing all of the console logs in Chrome so that users who submit a feedback entry will also submit all of the logs to our server. When someone encounters a problem in production, I can first and foremost get them back to work so that I can then sit down and more thoroughly go through all of the logs to determine the root cause of whatever issue the user encountered in production.

The technique I use to capture the logs involves overriding console.log so that all text entered in the first argument gets stored in an array while simultaneously invoking the legacy function so that I can still see the logs in the console too.

The problem is when there's the occasional uncaught exception. These aren't included in the uploaded logs, so it's not always clear what caused the problem. So I tried overriding ReferenceError by writing a JavaScript function that takes a function as an argument, then returns a new function that does stuff with it, like storing data in a variable, and then invoking the legacy function as the last step:

function overrideException(legacyFn) {  

    /** arguments for original fn **/
    return function() {

        var args = [];

        args[0] = arguments[0];

        // pass in as arguments to original function and store result to 
          // prove we overrode the ReferenceError
        output = ">> " + legacyFn.apply(this, args).stack;

        return legacyFn.apply(this, arguments);
    }           

}

To test the overrideException function, I ran the following code on the console:

ReferenceError = overrideException(ReferenceError);

Afterwards, I tested the returned function, the new ReferenceError, by manually throwing a ReferenceError:

throw new ReferenceError("YES!! IT WORKS! HAHAHA!");

The resulting output on the console is:

ReferenceError: YES!! IT WORKS! HAHAHA!

And checking the global variable output from the overrideException function shows that it did indeed run:

output
  ">> ReferenceError: YES!! IT WORKS! HAHAHA!
  at ReferenceError (<anonymous>)
  at new <anonymous> (<anonymous>:18:35)
  at <anonymous>:2:7
  at Object.InjectedScript._evaluateOn (<anonymous>:562:39)
  at Object.InjectedScript._evaluateAndWrap (<anonymous>:521:52)
  at Object.InjectedScript.evaluate (<anonymous>:440:21)"

Now, here's where things start to fall apart. In our code, we're not going to know when an uncaught exception occurs, so I tested it by attempting to run a function that doesn't exist:

ttt();

Which results in:

ReferenceError: ttt is not defined

However, unlike the case where we explicitly throw an error, in this case, the function doesn't fire, and we're left with only the legacy functionality. The contents of the variable output is the same as in the first test.

So the question seems to be this: How do we override the ReferenceError functionality that the JavaScript engine uses to throw errors so that it's the same one we use when we throw a ReferenceError?

Keep in mind that my problem is limited only to Chrome at this time; I'm building a Chrome Packaged app.

Answer 1

I have done quite a bit of research for the same reason: I wanted to log errors and report them.

"Overriding" a native type (whether ReferenceError, String, or Array) is not possible.

Chrome binds these before any Javascript is run, so redefining window.ReferenceError has no effect.

You can extend ReferenceError with something like ReferenceError.prototype.extension = function() { return 0; }, or even override toString (for consistency, try it on the page, not the Dev Tools).

That doesn't help you much.

But not to worry....

(1) Use window.onerror to get file name, 1-indexed line number, and 0-indexed position of uncaught errors, as well as the error itself.

var errorData = [];
onerror = function(message, file, line, position, error) {
    errorData.push({message:message, file:file, line:line, position:position, error:error});
};

See the fiddle for an example. Since the OP was Chrome-specific, this has only been tested to work in Chrome.

(2) Because of improvements to (1), this is no longer necessary, but I leave this second technique here for completeness, and since onerror is not guaranteed to work for all errors on all browsers. You will also sometimes see the following:

var errors = [];
function protectedFunction(f) {
    return function() {
        try {
            f.apply(this, arguments);
        } catch(e) {
            errors.push(e);
            throw e;
        }
    };
}
setTimeout = protectedFunction(setTimeout);
setInterval = protectedFunction(setInterval);
etc...

FYI, all this is very similar to what has been done in the Google Closure Compiler library, in goog.debug, created during Gmail development with the intent of doing exactly this. Of particular interest is goog.debug.ErrorHandler and goog.debug.ErrorReporter.