Welcome to the ClassDojo
Engineering Blog


How we build and scale ClassDojo.
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At ClassDojo, our frontend web apps are large single-page applications running in React and Redux. Millions of teachers, parents, and students use these apps every day, and we want to ensure a quality experience for them. However, effectively testing frontend apps is hard.

We have a good number of unit tests, but found that apart from a few complicated components with lots of stateful interactions, these tests are tedious to write and usually ineffective at preventing bugs. This is because most of our frontend bugs have occurred in the boundary between components or layers of our app – for example, between the API and the data layer, the data layer and our components, or between a parent and child component. These boundaries are explicitly not tested by standard unit tests, which mock out external dependencies. It’s always very frustrating to find that an API change or refactor had broken our app, when all of our unit tests continue to pass.

To address this issue, we decided to look into integration testing our frontend apps. The standard way to do this is with a browser automation tool like Selenium. However, these tools are legendary for their difficulty of use and unreliability. Tests written this way are brittle, breaking when class names change or if API calls or renders take too long. We wanted an easier, more efficient method. Luckily, React and Redux provided us the tools to do so.

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At ClassDojo, we’ve been working on getting visibility into bugs that get deployed to our production React webapp. Catching errors in our data layer is pretty straightforward, but the view layer presents more of a challenge.

Errors in your React code can happen in a variety of places such as

  • render functions
  • lifecycle methods (componentDidMount, componentWillUpdate)
  • event callbacks (onClick , onChange)

Our goal was to cover all of these cases without adding boilerplate to every component.

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Class Story is one of our core product features at ClassDojo. It allows teachers to post photos and videos that parents can view, like, and comment on, and teachers around the world use it to share cute, funny, or exciting moments from their classrooms. Since we’re approaching the end of the school year, we wanted to provide teachers with a something memorable and engaging to send home. But with a ton of new projects we’d like to ship before back-to-school in August, we also wanted something we could build quickly.

We settled on a photo collage of a classroom’s top posts throughout the year. We generated collages for all classrooms with at least 9 posts this year and surfaced them in-app for teachers to share with their parents. We hope parents, teachers, and students everywhere will enjoy seeing a year’s worth of memories!

Here’s an example of one of the collages, using posts from our internal Class Story:

An example with the ClassDojo team

In the process, we wrote a simple npm module, photo-collage, which generates photo collages. You can view it here, and see the source code here.

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