It works on my machine. Why?

We’ve all been here. Something works just fine on your computer but not on someone else’s. The root problem might be tricky, so I listed possible things you can investigate. Next time it happens, you have a list of things to check.

You have a different environment

In my experience, this is the most common reason. You have something set up differently in your environment (i.e., computer) that makes the code work. Here’s a list of things to check, in no particular order:

• Different execution environment: you’re running the code in development/debug mode, while the other person is running in production/release mode.
• Different env vars: maybe you have a different value for an environment variable. Check if your project .env files and system environment variables match those of the other person.
• Your database schema is different: your database might have different columns, indexes, tables, or constraints. This could be because you ran a migration that the other person didn’t.
• You have different data: your database doesn’t have enough records to trigger a particular bug, or you have a record that the other person doesn’t. Check if you both have run the same seed files.
• You have different dependencies: you have a missing (or extra) system dependency, like libxml2. (Insert meme about Docker and shipping your computer here.)
• Different browsers: it’s no secret that browsers behave differently. Check if the issue is browser-specific. Can I Use and MDN Web Docs are super helpful for this.
• Different email clients: very similar to the previous point. Email clients are known for being hard to work with. Can I Email is a friend for this scenario.
• Background services not running: maybe it’s your database, maybe it’s Redis, or perhaps a background job processor like Sidekiq. This is particularly common if you’re running your server manually with rails s instead of bin/dev.
• You have cached data: indeed one of the hardest problems in computer science. You have cached data in your browser, project, or package manager. Reloading with Ctrl/Cmd + Shift + R or running something like rails tmp:clear might just do the trick.
• You have different cookies: you were working on a feature in a different branch that changed some cookies, and now something works differently for you. Try clearing your cookies.
• Different security settings: check if you have different security settings. That might be in your browser configuration or extensions (like uBlock Origin), or in your project (like Content Security Policy), or maybe the non-working machine is behind a firewall or VPN.
• CORS: it often happens that something works in development but not in production because of CORS.
• You have different Docker image versions: even you, Docker! Yes, even with Docker things might behave differently. For one, Docker works differently depending on the OS you have. That aside, you might be using :latest for your image, but you ran that a while ago and then there are new versions available. So you can be running under a different environment than your colleague.
• Different resources available: the non-working machine is running other software that is using resources (ports, processes, files) in a way that makes them unavailable. Looking at you, AirPlay Receiver running on port 5000.
• Different operating systems: there’s just too much to list here. Windows, MacOS, iOS, Android, all the Linux distributions… They’re all awesome and terrible in their ways. Check if the software you want to run is compatible with the OS you have.
• Different computer hardware: again, there are countless possibilities here, but one that has become more common is the ARM vs Intel processors. Ever since Apple’s M1 chips dropped, this has become a more common issue. Check if the software you want to run is compatible with the architecture you have.

You have different code

If your environment isn’t the issue, having different code might be. Things to check:

• You’re on a different branch: very common and easy to overlook. This is sort of a catch-all for many of the other points. Your branch might be ahead with a bug fix or behind before a bug was introduced. Make sure you’re on the same branch.
• You have a modified file that you forgot to commit: maybe you were splitting work into multiple PRs and forgot to commit a change. This might cause tests to pass on your machine but fail on CI. Another possibility is that you have your changes hidden by a global .gitignore or you used something like .git/info/exclude or git update-index --assume-unchanged (Oh My ZSH has a ghide alias for this) to hide your changes locally and forgot about it.
• You’ve touched a dependencies code and forgot to revert it: I often use bundle open to open a gem’s code and debug it. If you forget to revert these changes, you might run into trouble. Fortunately, you can run bundle pristine to revert them all.
• You have updated dependencies: maybe you’re running a newer version of some dependency that has fixed bugs (or introduced new ones). Perhaps you installed them with different package managers (yarn vs manually), or the lockfile isn’t committed, so you are running different versions. Ensure that your versions match those of the other person to debug this.

Other reasons

• Different user permissions: if you have an authentication system based on user roles (say, Auth0), you might have different permissions than the other person.
• Different feature flags: similar to the previous point, you might have different flags turned on, so you have access to a feature that the other person doesn’t.
• You’re in a different physical location: if you and your colleague don’t live in the same physical location (region/country/continent), you might run into issues with time zones and language settings, for example. I’ve had to have different setup instructions for a client project because I was in Brazil and they were in the US.
• Things happen too fast/too slow for you to notice the error: you have a fancy new chip or a very fast fiber internet connection. That might make the error happen so fast (like something blinking in the UI) that you don’t even notice it. Who would’ve thought that having a faster computer could be a problem?

No luck?

If you’ve checked all of these and still can’t find the issue, give a shot at the good old turn it off and on again. Sometimes, simply restarting your computer can resolve the problem. Or if your issue is that you change code but nothing happens, check out Rails: When Changing Code Doesn’t Change Behavior, where I describe possible reasons for that.