Test callback function with jest

Question

I'm trying to test a function with a callback inside. I set up a mock function, but I also need to test a callback.

I've tried to separate it as another mock function, but it doesn't counted as covered.

Function I'm trying to test:

export const checkDescription = async page => {
    const metaDescription = await page.$eval(
      'meta[name="description"]',
      description => description.getAttribute("content")
    );
    return metaDescription;
};

I've mocked the page function :

const page = {
  $eval: jest.fn(() => "Value")
};

my test :

test("Should return description", async () => {
  expect(await checkDescription(page)).toBe("Value"); 
  expect(page.$eval).toHaveBeenCalled();
});

I've tried to separate description :

const description = {
  getAttribute: jest.fn(() => "Value")
};

but I don't think that it's a correct way to cover description inside $eval.

Are there any side effects of that function being called that you could observe? What's the behaviour it implements? — jonrsharpe, May 02 '19 at 19:00

score 11 · Accepted Answer · answered May 03 '19 at 06:08

You're close!

The description arrow function is passed to your page.$eval mock function so you can use mockFn.mock.calls to retrieve it.

Once you've retrieved it, you can call it directly to test it and get full code coverage:

test("Should return description", async () => {
  expect(await checkDescription(page)).toBe("Value");  // Success!
  expect(page.$eval).toHaveBeenCalled();  // Success!

  const description = page.$eval.mock.calls[0][1];  // <= get the description arrow function
  const getAttributeMock = jest.fn(() => 'mock content');
  expect(description({ getAttribute: getAttributeMock })).toBe('mock content');  // Success!
  expect(getAttributeMock).toHaveBeenCalledWith('content');  // Success!
  // Success!  checkDescription now has full code coverage
});

score 0 · Answer 2 · answered Jan 24 '23 at 11:50

I receive async messages from serial port via callbacks. Try to read here: https://jest-bot.github.io/jest/docs/asynchronous.html

import { InpassTerminal } from "../src/main.js"

jest.setTimeout(45000);
describe('Basic tests', () => {
test('1. Host connection', async (done) => {     
    await new Promise( resolve => setTimeout(resolve, 500) ); 

    const commandTest = {actionCode: '12345', terminalId: '1019****'}

    function cb (data) { 
      if (data.operationCode == 12345) {
        const actualStatus = Buffer.from(data.status, "ascii")          
        const expectedStatus = '1'

        expect(actualStatus.toString()).toBe(expectedStatus)      
        done()
      }
    }  
    const terminal = new InpasTerminal()
    terminal.exec('/dev/ttyPos0', commandTest, cb)
})

Test callback function with jest

2 Answers2

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