Using Jest to mock a React component with props

Question

I have a React component which contains some other components that depend on access to a Redux store etc., which cause issues when doing a full Enzyme mount. Let's say a structure like this:

import ComponentToMock from './ComponentToMock';

<ComponentToTest>
  ...some stuff
  <ComponentToMock testProp="This throws a warning" />
</ComponentToTest>

I want to use Jest's .mock() method to mock out the sub-component, so that it is not a concern for the test.

I'm aware that I can mock out a straight component with something like:

jest.mock('./ComponentToMock', () => 'ComponentToMock');

However, as this component would normally receive props, React gets upset, giving a warning about unknown props (in this case, testProp) being passed to <ComponentToMock />.

I've tried to return a function instead, however you can't return JSX (from what I could tell) in a Jest mock, due to it being hoisted. It throws an error in this case.

So my question is how can I either

a) get ComponentToMock to ignore props passed to it, or

b) return a React component that can be used to mock the child component that I'm not worried about testing.

Or... is there a better way?

Solution 1

There's a note at the bottom of the docs for jest.mock() for preventing the hoisting behavior:

Note: When using babel-jest, calls to mock will automatically be hoisted to the top of the code block. Use doMock if you want to explicitly avoid this behavior.

Then you can do as you described: return a function that is a stub of the component you don't need to test.

jest.doMock('./ComponentToMock', () => {
  const ComponentToMock = () => <div />;
  return ComponentToMock;
});

const ComponentToTest = require('./ComponentToTest').default;

It's helpful to name the stub component since it gets rendered in snapshots.

Solution 2

I've learned a little more since I asked this question. Here's an alternative (better?) way of dealing with mocking components that need to be passed props: using module mock files.

First create a file with the same name as the component to mock in a __mocks__ folder under the component's folder e.g.

.
|- /ComponentToMock.js
└- /__mocks__/ComponentToMock.js <-- create this folder/file!

Note: It seems as of the time of writing, the folder must be called __mocks__ (you will need to create __mocks__ in each folder you need to mock components for. If the underscores upset you, just pretend they aren't there ;) )

Next, in this mock file, you can write the file as you wish, e.g.

// This code would live in ./__mocks__/ComponentToMock.js
import React from 'react';
const ComponentToMock = ({ testProp }) => <div>A mock with '{testProp}' passed!</div>;
export default ComponentToMock;

Then in the test file, change the Jest mock statement to: jest.mock('./ComponentToMock');

When Jest encounters a .mock() without the second function parameter, it automatically looks for a __mocks__ folder. However, even though the mock statement gets hoisted in the component being tested, this doesn't affect the imports of the mock itself - hence why it's able to import and compile a React component!

This seems to work well for mocked components that need to be passed props, and would otherwise produce prop warnings if a nulled function was returned (but which is perfectly acceptable to continue using if the component does not receive props). I hope this helps some people out there.

Solution 3

Addition to accepted answer, If you are using multiple exports then mocking can be done as follows:

// Component.jsx

export { A, B };

// Component-test.js

jest.mock("../src/Component", () => {
  return {
    A: true,
    B: () => {
      return <></>;
    },
  };
})

Reference Blog: https://thoughtbot.com/blog/mocking-react-components-with-jest

Comments

[Mike Hopkins]

This is good to know! The docs mention it a little off-handedly. However, I tried this and it still didn't work out. Because the import of <ComponentToMock /> happens when the <ComponentToTest /> is brought in, unless Jest's mock is hoisted to be first, it happens after the fact and the mock does not work.

[Mike Reifman]

I updated the snippet: you have to do the mocking and then require the component to test.

[Mike Hopkins]

I've managed to get this working in a basic sense. It does require the use of... require, ahem, not import. That may be worth considering for anyone with a similar situation. I've got a more complex setup that seems to not be working with the fix, but that could just be some interfering HOCs. Thanks for your help!

[Christof Kälin]

Same here, this solution does not work out, it throws a Invariant Violation: getNodeFromInstance: Invalid argument. Seems that we should live with the warnings upon test output.

[aoh]

Depending on the way your component is exported/imported you might need to do return {ComponentToMock}

[Filip Kowal]

The part about pretending not to see the underscores was very helpful. I will!