How to verify console.log was called in componentDidMount using Jest and Enzyme?

Question

I'm trying test for cases when my axios call does not get an HTTP response of 200. When axios does not get a successful response, it throws an error. I want to verify that console.log gets called twice in this case.

Here's a snippet of the class I'm testing:

class App extends React.Component {
    ...  
    async componentDidMount() {
        let url = "/api/info/tmp"
        try {
            let response = await axios.get(url);
            ...do stuff
            this.setState(...);
        } catch (e) {
            console.log("Could not get " + url);
            console.log(e);
        }
    }
    ...
}

And here's a snippet of my jest test

let mockAxios = new MockAdapter(axios);
...
describe("App - componentDidMount() behavior test", () => {
    beforeEach(() => {
        app = shallow(<App />);
    })

    afterEach(() => {
        app = undefined;
        mockAxios.reset();
    });
    ...
    describe("Get " + url + " HTTP response status is not 200", () => {
        beforeAll(() => {
            mockAxios.onGet(url).reply(302, mockData);
        });
        it("Does not set state regardless of response body", () => {
            console.log = jest.fn();
            const state = app.state();
            expect(console.log).toHaveBeenCalledTimes(2);
            expect(state.solutions).toEqual({});
            expect(state.username).toEqual("");
        });
    });
});

I know the console.log = jest.fn() bit is doing something because the console does not log the fake error anymore when I set it. However, the test fails because Expected mock function to have been called two times, but it was called zero times.

I've tried moving the console.log = jest.fn() into the "beforeEach", "beforeAll", and as a global variable.

UPDATE

I am pretty sure it's something to do with all the async that is going on. If I do this:

    it("Does not set state regardless of response body", async () => {
        console.log = jest.fn();
        await app.instance().componentDidMount();
        expect(console.log).toHaveBeenCalledTimes(2);

        const state = app.state();
        expect(state.solutions).toEqual({});
        expect(state.username).toEqual("");
    });

Then the test still fails but my reason changed: Expected mock function to have been called two times, but it was called four times. Now I just got to figure out why it was called four times not twice.

UPDATE 2

I figured out why console.log was being called 4 times! Now I just need to figure out how I should refactor my tests. If I comment out my jest mock, and even the whole unit test

    it("Does not set state regardless of response body", async () => {
        //const state = app.state();
        //expect(state.solutions).toEqual({});
        //expect(state.username).toEqual("");
        //expect(console.log).toHaveBeenCalledTimes(2);
    });

Then I can count in my console that there are already indeed two different console.log calls. shallow(<App />) must be already calling componentDidMount() or something. When I add app.instance().componentDidMount(), I can visually see that it is logging 4 times.

Answer 1

Updated Answer

Since it looks like you already know what you're doing with mocks, perhaps the issue has to do with componentDidMount().

I believe that your call to shallow(<App />) will already call App's componentDidMount() one time (which means your console.log will get called twice there).

Then, you subsequently call app.instance().componentDidMount() - that is, you call componentDidMount() again (which means your console.log will get called twice there again).

So, total... 4 calls to console.log.

Hope that points you in the right direction...

Original Answer

Actually, your question looks quite similar to [this StackOverFlow question on how to "How to mock console when it is used by a third-party library?"

You can use Jest mock functions to spyOn the global.console object.

For example, your test may look like this:

// Setup jest to spy on the console
const consoleSpy = jest.spyOn(global.console, 'log')

describe('App - componentDidMount() behavior test', () => {
  beforeEach(() => {
    jest.resetAllMocks()  // reset your consoleSpy state back to initial
    app = shallow(<App />)
  })

  ...

      it('Does not set state regardless of response body', () => {
        const spy = jest.spyOn(global.console, 'log')
        const state = app.state()
        expect(consoleSpy).toHaveBeenCalledTimes(2)
        expect(state.solutions).toEqual({})
        expect(state.username).toEqual('')
      })
  ...

Answer 2

Ideally, you'd move your API call outside of componentDidMount and into its own class method. Thay way it can be manually invoked from a lifecycle method or from an event callback. Also, you should anticipate the response to affect your UI state in some fashion (example: displaying a message to the user that the request failed and to try again).

The following example can be done with .then/.catch instead of async/await. Either way, you're working with Promises that are asynchronous and therefore they need asynchronous tests.

Note: The below assumes disableLifecycleMethods is true in the enzyme adapter. Also, just testing state changes (or a console.log) is a bit superfluous; instead, you would test if a component is rendered based upon the current state.

Working example: https://codesandbox.io/s/939w229l9r (includes both end to end and integration tests --- you can run the tests by clicking on the Tests tab located near the bottom left of the sandbox)

---

App.js (this will be a container that holds all relevant state and disperses it to its children as needed)

import React, { Component } from 'react';

class App extends Component {
  state = = {
    error: "", 
    isLoading: true,
    solutions: {}, 
    username: ""
  };

  componentDidMount() {
    this.fetchData("/api/info/tmp");
  }

  fetchData = async (url) => {
    try {
      const res = await axios.get(url);

      ...do stuff

      this.setState({ 
        error: "", 
        isLoading: false, 
        solutions: res.data.solutions, 
        username: res.data.username 
      });
    } catch (err) {
        this.setState({ 
          error: err, 
          isLoading: false, 
          solutions: {}, 
          username: "" 
        });
    }
  }

  render() { ... }
}

App.test.js (this assumes you'd want an end to end test)

import { shallow } from 'enzyme';
import App from './App';

const timeout = () =>
  new Promise(resolve => {
    setTimeout(() => {
      resolve();
    }, 2000);
  });

const initialState = {
  error: "", 
  isLoading: true,
  solutions: {}, 
  username: ""
};

describe("App", () => {
  let wrapper;
  beforeEach(() => {
    wrapper = shallow(<App />);
    wrapper.setState({ ...initialState });
  });

  afterAll(() => {
     wrapper.unmount();
  });

  it("sets data to state based upon successful API call", async () => { 
   wrapper.instance().fetchData("/api/info/tmp");
   await timeout();

   wrapper.update();
   expect(wrapper.state('isLoading')).toBeFalsy();
   expect(wrapper.state('solutions')).toEqual({ somedata });
   expect(wrapper.state('username')).toEqual("Some User");
  });

  it("displays an error upon unsuccessful API call", async () => { 
   wrapper.instance().fetchData("/api/bad/url");
   await timeout();

   wrapper.update();
   expect(wrapper.state('isLoading')).toBeFalsy();
   expect(wrapper.state('solutions')).toEqual({});
   expect(wrapper.state('username')).toEqual("");
   expect(wrapper.state('error')).toEqual("No data found.");
  });
});

App.test.js (this assumes you'd want an integration test)

import axios from "axios";
import MockAdapter from "axios-mock-adapter";
import React from "react";
import { shallow } from "enzyme";
import App from "../App";

const solutions = [{ ... }, { ... }];
const username = "Some User"

const mockAxios = new MockAdapter(axios);

const initialState = {
  error: "", 
  isLoading: true,
  solutions: {}, 
  username: ""
};

describe("App", () => { 
  let wrapper;
  beforeEach(() => {
    wrapper = shallow(<App />);
    wrapper.setState({ ...initialState });
  });

  afterEach(() => {
    mock.reset();
  });

  afterAll(() => {
    mock.restore();
    wrapper.unmount();
  });

  it("displays an error upon unsuccessful API call", async () => { 
    try {
       mockAxios.onGet("/users").networkErrorOnce();

       await axios.get("users");

     } catch (err) {
       const error = err.toString();
       wrapper.setState({ 
         error, 
         isLoading: false,
         solutions: {}, 
         username: ""
       });

       wrapper.update();
       expect(wrapper.state('isLoading')).toBeEqual(error);
       expect(wrapper.state('isLoading')).toBeFalsy();
       expect(wrapper.state('solutions')).toEqual({});
       expect(wrapper.state('username')).toEqual("");
     } 
  });

  it("sets data to state based upon successful API call", async () => {
    try {
      mockAxios.onGet("/users").reply(200, { solutions, username });

      const res = await axios.get("users");

      wrapper.setState({ 
        error: "", 
        isLoading: true,
        solutions: res.data.solutions, 
        username: res.data.username
      });

      wrapper.update();
      expect(wrapper.state('isLoading')).toBeFalsy();
      expect(wrapper.state('solutions')).toEqual(solutions);
      expect(wrapper.state('username')).toEqual(username);
    } catch (e) {
        console.log(e);
    } 
  });
});

Answer 3

I figured it out! Kind of... I am not certain why it works like this, but setting the mock in the actual "it" did not work. The solution was making a beforeEach and afterEach

describe("Get " + url + " HTTP response status is not 200", () => {
    beforeAll(() => {
        mockAxios.onGet(url).reply(302, mockData);
    });
    beforeEach(() => {
        console.log = jest.fn();
    });

    afterEach(() => {
        jest.resetAllMocks();
    });
    it("Does not set state regardless of response body", async () => {
        const state = app.state();
        expect(state.solutions).toEqual({});
        expect(state.username).toEqual("");
        expect(console.log).toHaveBeenCalledTimes(2);
    });
});