ErrorHandling with responseError interceptor when using MockAgent

[blephy]

Bug Description

When using responseError interceptor, the error thrown is a ResponseError which always include an empty property body: '' and an empty property headers: {}. This always happens when using MockAgent.

Reproducible By

https://github.com/blephy/undici-issue-mock

Expected Behavior

Body, headers, and statusCode should be populated in the error thrown and this error should be an instance of ResponseError when using MockAgent.

Environment

• undici 7.3.0
• node 22.13.0

Additional context

I'm trying to make a library where an option exist : throwOnError. Depending on the value, i'm setting the interceptor. When this is set to true, i want to be able to catch the error and to return the response body / statusCode / headers.
Actually, the only way i can do it is by always settings it to false, and use a code like :

if (option.throwOnError && !response.ok) {
  throw new ResponseError('', {
    body: await response.json()
    headers: response.headers,
    statusCode: response.statusCode
    // ...
  })
}

So i'm losing the ability to use the responseError interceptor

[metcoder95]

Can you aggregate a Minimum Reproducible Example to support you better?

[blephy]

Yes there is :)

https://github.com/blephy/undici-issue-mock

[blephy]

Ok.

Got this.

To be able to get it working :

• you must set headers manually returned by the mock. I can understand that for a mock as it cannot know what a real server can return.
• a body passed to reply can be returned in the error thrown only if 'content-type': 'application/json' is set. Unless, it's returned as '' (empty string)

So a working code is :

mockAgent
 .get('http://localhost:3000')
 .intercept({
        path: '/error',
        method: 'POST',
      })
 .replyContentLength()
 .reply(
        400,
        { data: 'error occurred' },
        {
          headers: {
            'content-type': 'application/json',
            connection: 'keep-alive',
            'keep-alive': 'timeout=5',
          },
        },
      )

// calling this mock is then throwing as expected : 

new errors.ResponseError('Response Error', 400, {
  body: { data: 'error occurred' },
  headers: expect.objectContaining({
    connection: 'keep-alive',
    'content-length': '25',
    'content-type': 'application/json',
    'keep-alive': 'timeout=5',
  }),
});

So this is not really a bug, this is a lack of documentation.

MockAgent could read the body passed through reply and set a content-type automatically but real dispatcher is not doing it when using undici. So it's cannot be a feature request too.