const Domain = require('async-hook-domain')

// instantiating a Domain attaches it to the current async execution
// context, and all child contexts that come off of it.  You don't have
// to call d.enter() or d.run(cb), just instantiate and it's done.
// Pass an error-handling function to the constructor.  This function
// will be called whenever there is an uncaught exception or an
// unhandled Promise rejection.

const d = new Domain(er => {
  console.log('caught an error', er)
  // if you re-throw, it's not going to be caught, and will probably
  // cause the process to crash.
})

setTimeout(() => {
  throw new Error('this is caught by the domain a few lines up')
})

process.nextTick(() => {
  const d2 = new Domain(er => {
    console.log('any contexts spawned from this nextTick are caught here', er)
    // only catch one error.  The next one will go to the parent
    d2.destroy()
  })
  fs.readFile('does not exist', (er, data) => {
    if (er)
      throw er
  })
  fs.readFile('also does not exist', (er, data) => {
    if (er)
      throw er
  })
})

// Adding a new domain here in the same context as the d above will
// take over for this current context, as well as any that are created
// from now on.  But it won't affect the setTimeout above, because that
// async context was created before this domain existed.
const d3 = new Domain(er => console.log('d3', er))

// Unhandled promise rejections are handled, too.
Promise.reject(new Error('this will be handled by d3'))

// since a Promise rejection is an async hop, if we destroyed it right
// now, it would not be there to catch the Promise.reject event.
setTimeout(() => {
  // destroying d3 makes it like it never happened, so this will
  // be handled by the parent domain we created at the outset.
  d3.destroy()
  throw new Error('this will be handled by the parent')
})

// When all domains are destroyed either manually or by virtue of their
// async execution contexts being completed, or if no domain is active
// for the current execution context, then it reverts back to normal
// operation, with all event handlers removed and everything cleaned up.
setTimeout(() => {
  d.destroy()
  throw new Error('this crashes the process like normal')
}, 500) // time for the other contexts to wrap up