Intro to time travel

What is time travel?

Replay Test Suites bring time-traveling DevTools to your existing Cypress, Playwright or any other test suites


Software development should feel magical, but when you’re stuck and feeling alone it does not. Replay restores that feeling of magic by making it easier to see and understand how software works and by making debugging faster and collaborative.

Time travel debugging is not a new idea. In fact it’s one of the oldest. Ever since the first bugs were created, folks have been trying to figure out how to record and replay them, so they can capture them once, put them under the microscope, and figure out how they occurred.

The reason why debugging a bug is 2X harder than writing the software in the first place is that, bugs can come from anywhere, and fixing them requires tracing the sequence of events backwards from where something is obviously wrong to the root cause.

If you’re debugging a bug with browser DevTools and Console logs, you have two problems. The first one is obvious. When you’re inspecting the bug with browser DevTools, there’s no way to go backwards without refreshing and redoing your steps. The second is that every time you add console logs in your code, you need to replicate the bug again, in order to see the messages in the Console.

Adding console logs

This is where time travel comes in. When you’re in Replay DevTools, you’re able to add console logs in your code and “automagically” see the messages appear in the Console. And when you want to go forwards or backwards to a new point in time, all you have to do is click the “jump” button and you’re immediately brought there.

Viewing console logs

Time travel is one of those radical ideas which sounds simple when you first hear them, but then take awhile to sink in. The first time you’re in Replay DevTools, the UI will look familiar to Chrome DevTools and VS Code, but time travel will feel weird and uncomfortable. That’s okay. Give it five minutes and you’ll become a time traveler for life.

Bringing time travel mainstream

Replay started as an R&D project at Mozilla called WebReplay alongside Rust and WebAssembly because when we built the first time travel debugger for C++ ten years ago it made some of the most difficult bugs in Firefox’s Graphics and JS engine tractable.

We knew we had something valuable when six months into starting Replay some of the top open source developers were using Replay to fix some of the most difficult bugs in React, Redux, and other popular open source libraries. It didn’t matter if Replay crashed every five minutes, and was as slower than a dial-up modem, the ability to slow down and study a problem with Replay, was so great that some problems simply necessitated time travel.

Sebastian testimonial

Today Replay is used today in many of the top Open Source projects such as NextJS and Typescript. It’s hard to work on a web app and not use a tool that’s better because of Replay. Replay also is far faster. You can record in real time and forget it’s on and Replay DevTools is as responsive as Chrome DevTools.

With our recent move from Firefox to Chrome, you can use Replay as your development browser to capture the perfect bug report, and to record and replay your browser tests. It turns out that flaky test failures are timing issues and time travel debugging is perfect for understanding them. You’ll laugh the first time you fix a flaky test and then try to imagine how you would have done it before!