FastifyWarning not an instance of Error

[jamestalton]

I am running into an issue using typescript, fastify, and jest.
A plugin I am using is throwing a FastifyWarning.
Jest blows up with:

    TypeError [ERR_INVALID_ARG_TYPE]: The "warning" argument must be of type string or an instance of Error. Received an instance of FastifyWarning
        at process.emitWarning (internal/process/warning.js)

[jamestalton]

On further investigation... it seems that this is thrown when anything other than a string is passed to process.emitWarning
process.emitWarning(new Error('test'))

This might be a JEST thing...

[jamestalton]

https://github.com/nodejs/node-auto-test/blob/b067260648eb34c5553856729db47482cc7b412d/lib/internal/process/warning.js

if (typeof warning === 'string') {
    warning = createWarningObject(warning, type, code, ctor, detail);
  } else if (!(warning instanceof Error)) {
    throw new ERR_INVALID_ARG_TYPE('warning', ['Error', 'string'], warning);
  }
  if (warning.name === 'DeprecationWarning') {
    if (process.noDeprecation)
      return;
    if (process.throwDeprecation) {
      // Delay throwing the error to guarantee that all former warnings were
      // properly logged.
      return process.nextTick(() => {
        throw warning;
      });
    }
  }

Apparently if the warning name in nodejs is 'DeprecationWarning' it handles differently.
Since many of the fastify warnings are deprecations, they should be updated to that style?
that way they can be ignored with process.noDeprecation or turned into thrown errors using process.throwDeprecation.

This works in my environment:

function emitWarning (code) {
  if (codes[code] === undefined) throw new Error(`The code '${code}' does not exist`)
  if (emittedWarnings.has(code) === true) return
  emittedWarnings.set(code, true)
  const error = new codes[code]()
  process.emitWarning(error.message, "DeprecationWarning", error.code)
}

(node:9771) [FSTDEP003] DeprecationWarning: You are using the legacy Content Type Parser function signature. Use the one suggested in the documentation instead.