Implement server-side code hot-swapping on file change

[damassi]

This PR sets up code hot-swapping on the server without a full restart (but still requiring a refresh of the browser window). Crazy-fast development speed!

Overview of process:

• reloadable is called with a path to an app (client, api, etc) as well as a callback function that is executed whenever files with that appPath change
• When a source-code change is detected an internal lookup is made to Node, scanning its require cache
• If found, it is cleared via delete require.cache[id]
• When the browser is reloaded the callback re-requires the initial app and since the module is no longer in the cache node re-caches it, effectively hot-reloading the code during runtime.

Example:

import { createReloadable, isDevelopment } from 'lib/reloadable'

... 

if (isDevelopment) {
  const reloadAndMount = createReloadable(app)

  // Note that if you need to mount an app at a particular root (`/api`), pass 
  // in `mountPoint` as an option.
  app.use('/api', reloadAndMount(path.resolve(__dirname, 'api'), {
    mountPoint: '/api'
  }))

  // Otherwise, just pass in the path to the express app and everything is taken care of 
  reloadAndMount(path.resolve(__dirname, 'client'))
} else {
  app.use('/api', require('./api')
  app.use(require('./client')
}

[kanaabe]

Ah right this comment can probably be removed since it's pretty old and we actually benefit from not sharing req.user because it keeps our API stateless.

[damassi]

Oh yeah, gotta fix this

[kanaabe]

Love this! Very clear and thoughtful error handling

[kanaabe]

👍

[kanaabe]

👍

[kanaabe]

This is 🔥