Don't trust git conflicts

tl;dr — Rebase and run tests before every merge (solution).

# Study case

Here’s an example that generates zero git conflicts and yet introduces a bug.

1. We test master, tests pass ✅

   app.js

   const config = require('config.js');
   
   const greeting = () => {
     if (config.index % 3 === 0) { return 'hi' };
     else { return 'bye' };
   }
   
   module.exports = { greeting }

   config.js

   module.exports = { index: 6 };

   test.js

   const app = require('app.js');
   assert(app.greeting() === 'hi');

2. We test Branch A, tests pass ✅, no git conflicts ✅, we merge 🔽

   Branch A

   @@ -1,7 +1,7 @@
   const config = require('config.js');
   
   const greeting = () => {
   

• if (config.index % 3 === 0) { return 'hi' };

• if (config.index === 6) { return 'hi' }; else { return 'bye' }; }

1. We test Branch B, tests pass ✅, no git conflicts ✅, we merge 🔽

   Branch B

   @@ -1 +1 @@
   -module.exports = { index: 6 };
   +module.exports = { index: 3 };

2. Now, we test master, and tests fail! ❌

   Expected values to be strictly equal:
   
   'bye' !== 'hi'

# What happened?

It’s a bit counter-intuitive. Take a minute to study the case above.

# Solution

Rebase and run tests immediately before every merge. On Github you can set up a branch protection rule like this:

Enabled option "Require branches to be up to date before merging"