Bug Description

MockAgent allows us to generate a reply dynamically. The request's options are passed into that function. Undici's type definitions specify that a Headers object will be used for options.headers, but instead a flattened array is given. This is entirely unusable without splitting it into two-tuple chunks first and manually wrapping it in a new Headers() call.

Note that it is also possible to access the headers inside a matcher function passed to intercept(). This was a useful workaround for me, but keep in mind that for some reason, the matcher gets called twice which you can also see in the output below.

Reproducible By

import { fetch, MockAgent, setGlobalDispatcher } from 'undici'

const mockAgent = new MockAgent()
const mockPool = mockAgent.get('http://localhost')

setGlobalDispatcher(mockAgent)

mockPool.intercept({
  path: '/foo',
  method: 'GET',
  headers: (headers) => {
    console.log('headers in intercept:', Object.prototype.toString.call(headers), headers)
    return true
  }
}).reply(200, (options) => {
  console.log('headers in reply:', Object.prototype.toString.call(options.headers), options.headers)
  return {}
})

await fetch('http://localhost/foo')

Expected Behavior

The included type definitions specify that the parameter to the headers matcher in intercept() should receive Record<string, string>. For reply(), they specify options.headers to be a Headers object.
As such I would expect the following output:

headers in intercept: [object Object] {
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate'
}
headers in reply: [object Headers] HeadersList {
  [Symbol(headers map)]: Map(5) {
    'accept' => '*/*',
    'accept-language' => '*',
    'sec-fetch-mode' => 'cors',
    'user-agent' => 'undici',
    'accept-encoding' => 'gzip, deflate'
  },
  [Symbol(headers map sorted)]: null
}

Logs & Screenshots

Instead of the above, the following is logged:

headers in intercept: [object Object] {
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate'
}
headers in reply: [object Array] [
  'accept',
  '*/*',
  'accept-language',
  '*',
  'sec-fetch-mode',
  'cors',
  'user-agent',
  'undici',
  'accept-encoding',
  'gzip, deflate'
]
headers in intercept: [object Object] {
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate'
}

Environment

• Windows 11, additionally tried in WSL running Ubuntu 20.04.4 LTS with same behavior
• Node v18.6.0
• npm 8.13.2
• Undici 5.8.0

Additional context

• Relevant type definitions for options.headers in reply():

  undici/types/mock-interceptor.d.ts

  Line 77 in 784c6b4

  headers: Headers;

• I didn't yet have the time to figure out where in the source code the headers array is coming from, unfortunately.