Proper Jest Testing Azure Functions - node.js

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 There are also some outdated articles I found that don't really explain much.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}
}

Answer 1

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