Proper Jest Testing Azure Functions

Question

I am wondering how to properly test Azure Functions with Jest. I have read the online documentation provided by MSoft but it's very vague, and brief. There are also some outdated articles I found that don't really explain much. Here is what I understand: I understand how to test normal JS async functions with Jest. And I understand how to test very simple Azure Functions. However I am not sure how to go about properly testing more complex Azure Functions that make multiple API calls, etc.

For example I have an HTTP Function that is supposed to make a few API calls and mutate the data and then return the output. How do I properly mock the API calls in the test? We only have one point of entry for the function. (Meaning one function that is exported module.exports = async function(context,req). So all of our tests enter through there. If I have sub functions making calls I can't access them from the test. So is there some clever way of mocking the API calls? (since actually calling API's during tests is bad practice/design)

Here is a sample of code to show what I mean

module.exports = async function (context, req)
{
    let response = {}

    if (req.body && req.body.id)
    {
        try
        {
            //get order details
            response = await getOrder(context, req)
        }
        catch (err)
        {
            response = await catchError(context, err);
        }
    }
    else
    {
        response.status = 400
        response.message = 'Missing Payload'
    }

    //respond
    context.res =
    {
        headers: { 'Content-Type': 'application/json' },
        status: response.status,
        body: response
    }
};

async function getOrder(context, req)
{
   //connection to db
   let db = await getDb() // <- how to mock this 

   //retrieve resource
   let item = await db.get...(id:req.body.id)... // <- and this

   //return 
   return {'status':200, 'data':item}
}

the answer would depend on where does the `getDb` comes from. Probably form some module, so it will be a good idea to include the import statements — Teneff, Nov 18 '20 at 14:31

Max Ivanov · Answer 1 · 2020-11-20T14:03:19.597

Consider this (simplified) example.

src/index.js (Azure Function entry point):

const { getInstance } = require('./db')

module.exports = async function (context) {
  // assuming we want to mock getInstance and db.getOrder
  const db = await getInstance()
  const order = await db.getOrder()

  return order
}

src/db.js:

let db

async function getInstance() {
  if (db === undefined) {
    // connect ...
    db = new Database()
  }

  return db
}

class Database {
  async getOrder() {
    return 'result from real call'
  }
}

module.exports = {
  getInstance,
  Database,
}

src/__tests__/index.test.js:

const handler = require('../index')
const db = require('../db')

jest.mock('../db')

describe('azure function handler', () => {
  it('should call mocked getOrder', async () => {
    const dbInstanceMock = new db.Database() // db.Database is already auto-mocked
    dbInstanceMock.getOrder.mockResolvedValue('result from mock call')

    db.getInstance.mockResolvedValue(dbInstanceMock)

    const fakeAzureContext = {} // fake the context accordingly so that it triggers "getOrder" in the handler
    const res = await handler(fakeAzureContext)

    expect(db.getInstance).toHaveBeenCalledTimes(1)
    expect(dbInstanceMock.getOrder).toHaveBeenCalledTimes(1)
    expect(res).toEqual('result from mock call')
  })
})

> jest --runInBand --verbose

 PASS  src/__tests__/index.test.js
  azure function handler
    ✓ should call mocked getOrder (4 ms)

For a complete quickstart, you may want to check my blog post

If we're mocking the db call then that actual line of db code is not executed, right? for example if in my `db.js` i misspelled my collection name instead of `apples` it's `appls` then how does that error get caught by the mock function if it never called my real db to check for a collection called apples instead of appls. Also you are exporting every function called by your main. What happens when you cant export them? Like lets say I have a `api.js` file that gets a connection but then in my AZ func i make 3 api calls ,w/ that singleton, those can't be exported, so how would that look like? — search-learn, Nov 18 '20 at 22:24
You have a class defined with methods and all. What if someone isn't using a class @Max Ivanov? — search-learn, Nov 18 '20 at 22:26
I made an assumption about the internals of the module to be mocked. The example shows how to mock a class and a function and how to use it in a test for an Azure function. Looks like you have a few questions about mocking with Jest in general and I believe those should be addressed separately. — Max Ivanov, Nov 18 '20 at 22:43

Proper Jest Testing Azure Functions

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