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Reactjs – How to cover the catch block in a function with optional chaining using Jest in react?

I’m working on a React project where I have a function that uses optional chaining and a try-catch block to assign a navigate function. I’m trying to write a Jest test to cover the catch block, but I’m having difficulty triggering it. Here’s the code:

    import { useNavigate } from 'react-router-dom';

let navigate;

function useNavigateMFE() {
  try {
    // Assign the navigate function based on the availability of the hfe properties.
    navigate = window?.hfe?.testdevice
      ? window?.hfe?.histr
      : useNavigate(); // Fallback to the standard useNavigate function if conditions are not met.
  } catch (e) {
    console.log("An error occurred:", e);
    // Optionally, provide a fallback action or handling in case of an error.
  }
}

// Call the function to set the navigate variable.
useNavigateMFE();

Note : this will trigger whenever component renders

What I’ve Tried
I’ve tried the following approaches to cover the catch block:

Mocking window.hfe to be null or undefined: This doesn’t trigger the catch block, likely because optional chaining prevents an error from being thrown.
Mocking the useNavigate function to throw an error:

jest.mock('react-router-dom', () => ({
  ...jest.requireActual('react-router-dom'),
  useNavigate: () => {
    throw new Error("Mocked error");
  },
}));

Despite this, the catch block still isn’t covered in the test.

My Goal
I want to simulate an error that triggers the catch block in my useNavigateMFE function. Here’s an example of my current test case:

import { useNavigate } from 'react-router-dom';

jest.mock('react-router-dom', () => ({
  ...jest.requireActual('react-router-dom'),
  useNavigate: jest.fn(() => {
    throw new Error("Mocked error");
  }),
}));

describe('useNavigateMFE', () => {
  it('should enter the catch block when an error occurs', () => {
    // Spy on console.log to verify it was called
    const consoleSpy = jest.spyOn(console, 'log').mockImplementation(() => {});

    // Act: Call useNavigateMFE to trigger the catch block
    useNavigateMFE();

    // Assert: Check that console.log was called
    expect(consoleSpy).toHaveBeenCalledWith(expect.any(Error));

    // Cleanup
    consoleSpy.mockRestore();
  });
});

The Issue
Even with the mocked implementation throwing an error, the catch block isn’t being triggered. How can I ensure that the catch block is covered in my tests?

Additional Info
I’m using Jest for testing.
The code is part of a React project.
Any advice or suggestions would be greatly appreciated!

Tags:

2

Answers


  1. 0

    To answer your question, by looking at the code, I believe your approach should work, although of course, to get an error thrown, you need to both mock window.hfe to be null and mock useNavigate to throw an error. It isn’t clear whether you did that. Also, for the test to pass, you need to add "An error occurred:" to .toHaveBeenCalledWith.

    BUT

    You need to recosider your approach to testing. Basically, the test you are describing is testing whether

    try {
  throw new Error();
} catch (e) {
  console.log(e);
}

    prints the error. Yes, it does. It is in the language specification, you don’t need to worry about that.

    Additional thoughts

    1. Your hook is invalid. The rules of hooks states that you can’t call Hooks inside conditions or loops, which you are doing with useNavigate. A valid alternative would be
    const nav = useNavigate();
navigate = window?.hfe?.testdevice
  ? window?.hfe?.histr
  : nav;
    1. React embraces a function approach, having navigate as a module scoped variable outside of the function violates that and would make it hard to reason about if another function also were to modify the variable. It would be better to return navigate from the function.
    const nav = useNavigate();
return window?.hfe?.testdevice
  ? window?.hfe?.histr
  : nav;
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  2. 0

    The problem is that; optional chaining prevents errors from being thrown in the way you initially tried.

    modify the useNavigateMFE function slightly to make it more testable:

    import { useNavigate } from 'react-router-dom';

export function useNavigateMFE() {
  try {
    return window?.hfe?.testdevice
      ? window?.hfe?.histr
      : useNavigate(); 
  } catch (e) {
    console.log("An error occurred:", e);
    return null; 
  }
}

    Test that can trigger the catch block:

    import { useNavigate } from 'react-router-dom';

jest.mock('react-router-dom', () => ({
  ...jest.requireActual('react-router-dom'),
  useNavigate: jest.fn(),
}));

describe('useNavigateMFE', () => {
  let originalWindow;

  beforeEach(() => {
    originalWindow = global.window;
    global.window = { ...originalWindow };
  });

  afterEach(() => {
    global.window = originalWindow;
    jest.restoreAllMocks();
  });

  it('should use window.hfe.histr when testdevice is true', () => {
    global.window.hfe = {
      testdevice: true,
      histr: jest.fn(),
    };
    const result = useNavigateMFE();
    expect(result).toBe(global.window.hfe.histr);
  });

  it('should use useNavigate when testdevice is false', () => {
    global.window.hfe = {
      testdevice: false,
    };
    const mockNavigate = jest.fn();
    useNavigate.mockReturnValue(mockNavigate);
    const result = useNavigateMFE();
    expect(result).toBe(mockNavigate);
  });

  it('should enter the catch block when an error occurs', () => {
    // Setup: Make window.hfe a getter that throws an error
    Object.defineProperty(global.window, 'hfe', {
      get: () => {
        throw new Error('Simulated error');
      },
    });

    // Spy on console.log to verify it was called
    const consoleSpy = jest.spyOn(console, 'log').mockImplementation(() => {});

    // Act: Call useNavigateMFE to trigger the catch block
    const result = useNavigateMFE();

    // Assert: Check that console.log was called and the function returned null
    expect(consoleSpy).toHaveBeenCalledWith("An error occurred:", expect.any(Error));
    expect(result).toBeNull();

    // Cleanup
    consoleSpy.mockRestore();
  });
});

    The key to triggering the catch block is to use Object.defineProperty to create a getter for window.hfe that throws an error. This simulates a scenario where accessing window.hfe causes an error, which will be caught by the try-catch block in your function.

    I believe this solution should give you full coverage of your useNavigateMFE function, including the catch block.

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