How to Detect Object Changes in Javascript

Recently I’ve run into a frustrating issue: I worked with another team and we were supposed to exchange some objects between us. The problem was that the objects were supposed to be changed/updated between the start and the end of the journey. And of course, something was changing the objects in a bad way.

Third-party libraries were used by both ends, a lot of configurations and settings done by many people for many purposes. This was not a walk in the park.

I controlled only the start of the journey, someone else was using the objects at the end. So we asked each other: Who is the culprit? Because each of us was using a different JavaScript library, we had to put the objects on window. Every JavaScript piece of code has access to window so finding the culprit is not easy.

So, at the start of the journey I put an object on window like this:

window.pageData = {
    pageName: 'Payment',
    userStatus: 'anonymous',
    event: ['payByCard']
}

For some reason they receive:

window.pageData = {
    pageName: '',
    userStatus: '',
    event: []
}

What can be done in this case? Blaming each other :) is one option, but not very helpful.

The first thing I did was to freeze the object, to see if they get it that way:

window.pageData = Object.freeze({
    pageName: 'Payment',
    userStatus: 'anonymous',
    event: ['payByCard']
})

That helped, it was clear that I was sending the correct data in some cases. But there were still some cases that broke some legit functionality that needed to update the object.

The second thing I did was to create a Proxy around the data, and log each change to it:

window.pageData = new Proxy({
    pageName: 'Payment',
    userStatus: 'anonymous',
    event: ['payByCard']
    }, {
      set: function (target, key, value) {
        target[key] = value;

        try {
          throw new Error('window.pageData changed by something!');
        } catch (err) {
          console.warn(err);
        }
        return true;
      }
    });

Each time something was changing the object, a console.warn was triggered indicating the JavaScript source that attempted the change. Busted! We now understood what happened and were able to fix the issues.

Bottom line: Being able to understand what happens in a complex system is very important. Most problems can be fixed correctly only if you understand what’s going on.