Poor Man's Javascript Error Reporter

If you want to track errors in your production Javascript (which you should), you should use a fully-baked service like BugSnag. If you can’t, there’s at least one other thing you could try: a poor man’s javascript error reporter.

Use BugSnag

Use Bugsnag or something like it if you can. There are many other Javascript and entire-app error monitoring, reporting and alerting systems. Please use these.

An Alternative

If you’re looking for a poor man’s javascript error reporting, you can make use of the onerror handler like a blunt hammer.

I DO NOT RECOMMEND DOING THIS but I’ve had to do it at a point or two in my career.

Here’s the Javascript code you should put at the very top of your document:

function errorReporter(message, source, line_number, column_number, error_object) {
  const params = new URLSearchParams({
    message,
    source,
    line_number,
    column_number,
    user_agent: navigator.userAgent
});

  const i = new Image();
  i.src = '/error-reporting?' + params.toString();
}

window.onerror = errorReporter;

When you directly assign the onerror handler to a function, you’re basically wiping out any other error handling functionality. In addition, you’re not following best practices like using event listeners instead of defining handlers. It’s better to add an event listener in most Javascript cases. However, in this particular one we want to override the entire handler.

The message is a programmer-readable version of the error message. The source indicates what file the error is in. The line_number and column_number indicate where the last functioning instruction exists that caused this error. Remember, if you’re compiling your Javascript with any build tools, this information won’t be as useful. The error_object is the Javascript Error object that was thrown.

Then, we take those values and build an URLSearchParams object (you’ll have to do this the hard way if you’re supporting Internet Explorer, though) to create parameters for a query. Then, make a new Image object. Finally, assign the src attribute to the end point of yours that can accept a GET request with error details. Javascript will automatically start to load the source for the Image if you assign the src attribute. You don’t need to place the image anywhere for this to happen. Now, your end point will receive something like this:

http://my-app.test/error-reporting?message=Uncaught+ReferenceError%3A+params+is+not+defined&source=http%3A%2F%2Fmy-app.test%2Ftest.html&line_number=33&column_number=25&user_agent=Mozilla%2F5.0+%28Macintosh%3B+Intel+Mac+OS+X+10_15_7%29+AppleWebKit...

Now, you can parse this in your backend and report on the errors yourself.

If, for some reason, you want to stop the error from appearing in the console, you could end your error handler with return true.

Finally DO NOT DO THIS AND INSTEAD USE BUGSNAG or a tool like it.

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