如何测试Get请求处理函数的.json()异常捕获分支？

Great question! To trigger the catch block in your get function, there are two key scenarios you can mock—let's break them down, with a focus on forcing the json() method to fail (since that's what you're asking about):

1. Mock json() to Reject (Simulate Invalid JSON) #

The json() method of a Fetch Response returns a Promise that rejects if the response body isn't valid JSON. To replicate this realistically in your test, you need to mock the fetch function to return a Response-like object where the json() method returns a rejected Promise.

Here's a concrete Jest implementation:

test('get function catches JSON parsing errors', async () => {
  // Mock fetch to resolve with a fake Response whose json() rejects
  global.fetch = jest.fn(() => 
    Promise.resolve({
      json: jest.fn(() => Promise.reject(new Error('Invalid JSON payload')))
    })
  );

  const result = await get('https://test-url.com');
  // Verify the catch block returned the error as a string
  expect(result).toBe('Error: Invalid JSON payload');
  // Ensure fetch was called with the correct URL
  expect(fetch).toHaveBeenCalledWith('https://test-url.com');
});

Why your previous attempts didn't work #

Mocking the response as a raw string/array doesn't mimic real Fetch API behavior: actual fetch returns a Response object, not a primitive value. When you tried that, calling rawResponse.json() would throw a TypeError (since strings/arrays don't have a json() method)—which would trigger the catch block, but it's not a realistic simulation of invalid JSON parsing. The approach above matches real-world failure scenarios.

2. Mock fetch to Reject (Simulate Network Errors) #

If you also want to test the scenario where the fetch request itself fails (e.g., no internet connection), you can mock fetch to return a rejected Promise directly:

test('get function catches network errors', async () => {
  global.fetch = jest.fn(() => Promise.reject(new Error('Network connection failed')));

  const result = await get('https://test-url.com');
  expect(result).toBe('Error: Network connection failed');
});

Cleanup Tip #

Don't forget to reset your mocks after each test to avoid cross-test contamination:

afterEach(() => {
  jest.restoreAllMocks();
});

Alternatively, use jest.spyOn(global, 'fetch') instead of overwriting global.fetch directly—this makes restoring the original function easier.

内容的提问来源于stack exchange，提问作者LazioTibijczyk