How to increase code coverage for Mocha testing on basic Typescript example?

Question

I want to get a better understanding of how to test these kinds of functions in Typescript using mocha. Namely, I have the following small file:

import { Request, Response } from 'express';

export const ping = (req: Request, res: Response) => {
    res.send('pong');
};

export const health = (req: Request, res: Response) => {
    res.send('OK');
};

This exercise may be trivial since there's not much to test here but below I have some basic mocha tests:

 describe('Health Tests', () => {
    it('Check ping', () => {
        expect(ping).to.not.be.null;
    })
    it('Check health', () => {
        expect(health).to.not.be.null;
    })
})

When I run code coverage on this test I get: 50% Stmts | 100% Branch | 0% Funcs | 50% Lines. Especially because this is such a small bit of code, I want to get 100% coverage on all of the categories if possible. Would anyone have any advice on how to accomplish this? Also, could someone please explain why I have 0% coverage for functions? By checking ping or health, am I not also then calling a function and hence, testing that as well.

Any advice would be much appreciated!

I think that what you're missing is to test the results of calling those functions. Those tests are just checking that the functions (the variable holding the reference to the function) exist - not its execution and actual results — Gonzalo.-
– Gonzalo.-, Commented Jul 24, 2020 at 2:42
When the function has res.send() as it's result, how am I able to access that result in mocha testing? — ENV
– ENV, Commented Jul 24, 2020 at 3:25
Those functions don't return anything (technically they return undefined). It is always easier to test a function that returns a value based on its parameters so write as many functions in this manner as possible for maximal testability. You can't always do that, but in this case, you still have to call them, and your assertions will need to be performed on their side effects, which is to say on how they impact res. — Aluan Haddad
– Aluan Haddad, Commented Jul 24, 2020 at 3:33

Lin Du · Accepted Answer · 2020-07-24 04:12:36Z

You need to use some mock/stub library like sinonjs. Here is unit test solution with 100% coverage:

index.ts:

import { Request, Response } from 'express';

export const ping = (req: Request, res: Response) => {
  res.send('pong');
};

export const health = (req: Request, res: Response) => {
  res.send('OK');
};

index.test.ts:

import { ping, health } from './';
import sinon from 'sinon';

describe('63065938', () => {
  describe('#ping', () => {
    it('should pass', () => {
      const mReq = {};
      const mRes = { send: sinon.stub() };
      ping(mReq, mRes as any);
      sinon.assert.calledWith(mRes.send, 'pong');
    });
  });

  describe('#health', () => {
    it('should pass', () => {
      const mReq = {};
      const mRes = { send: sinon.stub() };
      health(mReq, mRes as any);
      sinon.assert.calledWith(mRes.send, 'OK');
    });
  });
});

unit test result with 100% coverage:

  63065938
    #ping
      ✓ should pass
    #health
      ✓ should pass


  2 passing (12ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 index.ts |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------

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