Testing with React's Jest and Enzyme when async componentDidMount

Question

• react:16.3.0-alpha.1
• jest: "22.3.0"
• enzyme: 3.3.0
• typescript: 2.7.1

code:

class Foo extends React.PureComponent<undefined,undefined>{
   bar:number;
   async componentDidMount() {
     this.bar = 0;
     let echarts = await import('echarts'); // async import
     this.bar = 100;
   }
}

test:

describe('...', () => {
  test('...', async () => {
    const wrapper = shallow(<Foo/>);
    const instance = await wrapper.instance();
    expect(instance.bar).toBe(100);
  });
});

Error:

Expected value to be:
  100
Received:
  0

Approval

Answer 1

Solution:

1: use the async/await syntax.

2: Use mount (no shallow).

3: await async componentLifecycle.

For ex:

    test(' ',async () => {
      const wrapper = mount(
         <Foo />
      );
      await wrapper.instance().componentDidMount();
    })

Approval

This is not what you were trying to check in your question posted above.My answer is the right solution to your problem. – Gynecologist

This will cause the method to run twice. – Pluvious

@Gynecologist has the right solution. This will run "componentDidMount" twice. Any await after mounting will probably "solve" the problem, but it's not the right solution in my opinion. – Patrica

Answer 2

Something like this should work for you:-

 describe('...', () => {
   test('...', async () => {
     const wrapper = await mount(<Foo/>);
     expect(wrapper.instance().bar).toBe(100);
   });
 });

Gynecologist

This was the best solution for me, particularly as I didn't want to shallow mount – Delanadelancey

Indeed; and if you have a setState in the component's didMount, you'll need to add wrapper.update(); before the expect. – Sallyanne

Answer 3

Try this:

it('should do something', async function() {
  const wrapper = shallow(<Foo />);
  await wrapper.instance().componentDidMount();
  app.update();
  expect(wrapper.instance().bar).toBe(100);
});

Crysta

After looking through google on this problem and trying to implement async componentDidMount through mount's enzyme. And "mount" way is not fit for react native project - I had a lot mistakes. This method helped me. And you don't need to install jsdom and use mount. "react": "16.4.1", "react-native": "0.56.0", "enzyme": "^3.4.4", "jest": "23.5.0" – Clump

what's app here? – Arjan

@Arjan I think wrapper.update() was intended. --ref – Longcloth

btw, await won't do anything there unless your componentDidMount() returns a promise – Longcloth

Answer 4

None of the solutions provided here fixed all my issues. At the end I found https://medium.com/@lucksp_22012/jest-enzyme-react-testing-with-async-componentdidmount-7c4c99e77d2d which fixed my problems.

Summary

function flushPromises() {
    return new Promise(resolve => setImmediate(resolve));
}

it('should do someting', async () => {
    const wrapper = await mount(<Foo/>);
    await flushPromises();

    expect(wrapper.instance().bar).toBe(100);
});

Bibbie

For anybody looking at this, use setTimeout instead of setImmediate. – Purvis

Answer 5

Your test also needs to implement async, await.
For ex:

  it('should do something', async function() {
    const wrapper = shallow(<Foo />);
    const result = await wrapper.instance();
    expect(result.bar).toBe(100);
  });

Joyejoyful

edited, have you tried the await when you execute the instance?, The await basically operates on a promise – Joyejoyful

if you can share your complete test – Joyejoyful

I don't think .instance() returns a promise, so I don't think await is doing anything there. TypeError: wrapper.instance(...).then is not a function – Longcloth

Answer 6

If you are using jest.useFakeTimers(); then you need to use

await jest.runAllTimersAsync();

instead of the flushPromises solution

Gabbro