Module Federation shared dependencies need explicit version constraints

Module Federation has this wonderful feature where it lets two different versions of React run simultaneously in your app. I say “wonderful” because it will absolutely not tell you it’s doing this. Everything works. Until it doesn’t. And then you’re staring at a hook error that makes zero sense.

The fix: stop being lazy with the shared config.

// This looks clean. It's a trap.
shared: ['react', 'react-dom']

// This is what you actually want.
shared: {
  react: { 
    singleton: true, 
    requiredVersion: deps.react,
    strictVersion: true 
  },
  'react-dom': { 
    singleton: true, 
    requiredVersion: deps['react-dom'],
    strictVersion: true 
  },
}

singleton: true = one React to rule them all. strictVersion: true = blow up loudly if versions don’t match. Loud failure beats two hours of “why is useState returning undefined.”

The deps reference is just require('./package.json').dependencies. Nothing fancy. The fancy part was the debugging session that led me here.