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Codesanbox link - includes working component Child2.js and working test Child2.test.js

Child2.js

import React, { useRef } from "react";

export default function Child2() {
  const divRef = useRef();

  function getDivWidth() {
    if (divRef.current) {
      console.log(divRef.current);
    }
    return divRef.current ? divRef.current.offsetWidth : "";
  }

  function getDivText() {
    const divWidth = getDivWidth();

    if (divWidth) {
      if (divWidth > 100) {
        return "ABC";
      }
      return "123";
    }

    return "123";
  }

  return (
    <>
      <div id="myDiv" ref={divRef}>
        {getDivText()}
      </div>
      <p>Div width is: {getDivWidth()}</p>
    </>
  );
}

Child2.test.js

import React from "react";
import Enzyme, { shallow } from "enzyme";
import Adapter from "enzyme-adapter-react-16";
import Child2 from "../src/Child2";

Enzyme.configure({ adapter: new Adapter() });

it("div text is ABC when div width is more then 100 ", () => {
  const wrapper = shallow(<Child2 />);
  expect(wrapper.find("#myDiv").exists()).toBe(true);
  expect(wrapper.find("#myDiv").text()).toBe("ABC");
});

it("div text is 123 when div width is less then 100 ", () => {
  const wrapper = shallow(<Child2 />);
  expect(wrapper.find("#myDiv").exists()).toBe(true);
  expect(wrapper.find("#myDiv").text()).toBe("123");
});

When i run the tests, obvisouly the offsetWidth of the div is 0, hence i need to find a way to mock the useRef to return a div element with width or mock the getDivWidth function to return a desired number for the width.

How could I achieve that? I have searched for a solution, but I am stuck. There are some examples with class components or using typescript which I have not managed to use.

Answers

You can use jest.mock(moduleName, factory, options) and jest.requireActual(moduleName) APIs to mock useRef hook except others. Which means other functions and methods of react are still original version.

E.g.

index.jsx:

import React, { useRef } from 'react';

export default function Child2() {
  const divRef = useRef();

  function getDivWidth() {
    if (divRef.current) {
      console.log(divRef.current);
    }
    return divRef.current ? divRef.current.offsetWidth : '';
  }

  function getDivText() {
    const divWidth = getDivWidth();

    if (divWidth) {
      if (divWidth > 100) {
        return 'ABC';
      }
      return '123';
    }

    return '123';
  }

  return (
    <>
      <div id="myDiv" ref={divRef}>
        {getDivText()}
      </div>
      <p>Div width is: {getDivWidth()}</p>
    </>
  );
}

index.test.jsx:

import React, { useRef } from 'react';
import { shallow } from 'enzyme';
import Child2 from './';

jest.mock('react', () => {
  const originReact = jest.requireActual('react');
  const mUseRef = jest.fn();
  return {
    ...originReact,
    useRef: mUseRef,
  };
});

describe('61782695', () => {
  it('should pass', () => {
    const mRef = { current: { offsetWidth: 100 } };
    useRef.mockReturnValueOnce(mRef);
    const wrapper = shallow(<Child2></Child2>);
    expect(wrapper.find('#myDiv').text()).toBe('123');
    expect(wrapper.find('p').text()).toBe('Div width is: 100');
  });

  it('should pass - 2', () => {
    const mRef = { current: { offsetWidth: 300 } };
    useRef.mockReturnValueOnce(mRef);
    const wrapper = shallow(<Child2></Child2>);
    expect(wrapper.find('#myDiv').text()).toBe('ABC');
    expect(wrapper.find('p').text()).toBe('Div width is: 300');
  });

  it('should pass - 3', () => {
    const mRef = {};
    useRef.mockReturnValueOnce(mRef);
    const wrapper = shallow(<Child2></Child2>);
    expect(wrapper.find('#myDiv').text()).toBe('123');
    expect(wrapper.find('p').text()).toBe('Div width is: ');
  });
});

Unit test results with 100% coverage:

 PASS  stackoverflow/61782695/index.test.jsx (9.755s)
  61782695
    ✓ should pass (111ms)
    ✓ should pass - 2 (15ms)
    ✓ should pass - 3 (1ms)

  console.log
    { offsetWidth: 100 }

      at getDivWidth (stackoverflow/61782695/index.jsx:8:15)

  console.log
    { offsetWidth: 100 }

      at getDivWidth (stackoverflow/61782695/index.jsx:8:15)

  console.log
    { offsetWidth: 300 }

      at getDivWidth (stackoverflow/61782695/index.jsx:8:15)

  console.log
    { offsetWidth: 300 }

      at getDivWidth (stackoverflow/61782695/index.jsx:8:15)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |     100 |      100 |     100 |     100 |                   
 index.jsx |     100 |      100 |     100 |     100 |                   
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        10.885s

package versions:

"react": "^16.13.1",
"react-dom": "^16.13.1",
"enzyme": "^3.11.0",
"enzyme-adapter-react-16": "^1.15.2",
"jest": "^25.5.4",
"jest-environment-enzyme": "^7.1.2",
"jest-enzyme": "^7.1.2",