mocking-request.md

Mocking Request

Undici have its own mocking utility. It allow us to intercept undici HTTP request and return mocked value instead. It can be useful for testing purposes.

Example:

// bank.mjs
import { request } from 'undici'

export async function bankTransfer(recepient, amount) {
  const { body } = await request('http://localhost:3000/bank-transfer',
    {
      method: 'POST',
      headers: {
        'X-TOKEN-SECRET': 'SuperSecretToken',
      },
      body: JSON.stringify({
        recepient,
        amount
      })
    }
  )
  return await body.json()
}

And this is what the test file looks like:

// index.test.mjs
import { strict as assert } from 'assert'
import { MockAgent, setGlobalDispatcher, } from 'undici'
import { bankTransfer } from './bank.mjs'

const mockAgent = new MockAgent();

setGlobalDispatcher(mockAgent);

// Provide the base url to the request
const mockPool = mockAgent.get('http://localhost:3000');

// intercept the request
mockPool.intercept({
  path: '/bank-transfer',
  method: 'POST',
  headers: {
    'X-TOKEN-SECRET': 'SuperSecretToken',
  },
  body: JSON.stringify({
    recepient: '1234567890',
    amount: '100'
  })
}).reply(200, {
  message: 'transaction processed'
})

const success = await bankTransfer('1234567890', '100')

assert.deepEqual(success, { message: 'transaction processed' })

// if you dont want to check whether the body or the headers contain the same value
// just remove it from interceptor
mockPool.intercept({
  path: '/bank-transfer',
  method: 'POST',
}).reply(400, {
  message: 'bank account not found'
})

const badRequest = await bankTransfer('1234567890', '100')

assert.deepEqual(badRequest, { message: 'bank account not found' })

Explore other MockAgent functionality here

Debug Mock Value

When the interceptor we wrote are not the same undici will automatically call real HTTP request. To debug our mock value use mockAgent.disableNetConnect()

const mockAgent = new MockAgent();

setGlobalDispatcher(mockAgent);
mockAgent.disableNetConnect()

// Provide the base url to the request
const mockPool = mockAgent.get('http://localhost:3000');

mockPool.intercept({
  path: '/bank-tanfer',
  method: 'POST',
}).reply(200, {
  message: 'transaction processed'
})

const badRequest = await bankTransfer('1234567890', '100')
// Will throw an error
// MockNotMatchedError: Mock dispatch not matched for path '/bank-transfer':
// subsequent request to origin http://localhost:3000 was not allowed (net.connect disabled)

Reply with data based on request

If the mocked response needs to be dynamically derived from the request parameters, you can provide a function instead of an object to reply

mockPool.intercept({
  path: '/bank-transfer',
  method: 'POST',
  headers: {
    'X-TOKEN-SECRET': 'SuperSecretToken',
  },
  body: JSON.stringify({
    recepient: '1234567890',
    amount: '100'
  })
}).reply(200, (opts) => {
  // do something with opts

  return { message: 'transaction processed' }
})

in this case opts will be

{
  method: 'POST',
  headers: { 'X-TOKEN-SECRET': 'SuperSecretToken' },
  body: '{"recepient":"1234567890","amount":"100"}',
  origin: 'http://localhost:3000',
  path: '/bank-transfer'
}