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Jest-Fetch-Mock - Understanding It For Efficient API Testing

If you are tired of writing complex code for API testing then Jest-Fetch-Mock can make your life much easier. Jest-Fetch-Mock is a powerful tool for mocking fetch requests in your Jest tests. It provides a simple API for creating mock responses, so you can test your code without relying on a live API.

Kelvin Farr
Apr 27, 202334 Shares928 Views
If you are tired of writing complex code for API testing then Jest-Fetch-Mock can make your life much easier. Jest-Fetch-Mockis a powerful tool for mocking fetch requests in your Jest tests. It provides a simple API for creating mock responses, so you can test your code without relying on a live API.

What Is Jest-Fetch-Mock?

Jest-Fetch-Mock is a package that allows you to mock the fetch API in Jest tests. With Jest-Fetch-Mock, you can simulate responses from a server without actually sending requests over the network. This makes it easy to test your code in isolation and ensure that it works as expected in a variety of scenarios.

Why Use Jest-Fetch-Mock?

There are several reasons why Jest-Fetch-Mock is a great choice for API testing:
  • Speed- Jest-Fetch-Mock allows you to test your code without actually making network requests. This can greatly reduce the time it takes to run your tests, making your development process faster and more efficient.
  • Isolation- By mocking API responses, you can test your code in isolation from the rest of your system. This means you can focus on testing specific functionality without worrying about dependencies.
  • Consistency- With Jest-Fetch-Mock, you can ensure that your tests always receive the same response, regardless of the state of the server. This makes it easier to write reliable tests that don't fail unexpectedly.

Installing Jest-Fetch-Mock

To get started with Jest-Fetch-Mock, you'll need to install it in your project. You can do this using npm:
  • npm install jest-fetch-mock --save-dev
Once Jest-Fetch-Mock is installed, you can start using it in your tests.

Setting Up Jest-Fetch-Mock

To use Jest-Fetch-Mock, you'll need to set it up in your test file. Here's an example of how to do this:
import fetchMock from 'jest-fetch-mock';
fetchMock.enableMocks();
This code sets up Jest-Fetch-Mock and enables it for use in your tests.

Creating Mock Responses

Once Jest-Fetch-Mock is set up, you can start creating mock responses. Here's an example of how to create a mock response for a GET request:
  • fetchMock.mockResponse(JSON.stringify({ data: 'hello world' }));
This code creates a mock response that returns the JSON object { data: 'hello world' }. When your code makes a GET request using the fetch API, Jest-Fetch-Mock will intercept the request and return this mock response instead.

Handling Different HTTP Methods

Jest-Fetch-Mock can handle different HTTP methods, including GET, POST, PUT, DELETE, and more. Here's an example of how to create a mock response for a POST request:
fetchMock.mockResponse((req) => {
if (req.method === 'POST') {
return Promise.resolve({ body: 'ok' });
}
});
This code creates a mock response that only applies to POST requests. When a POST request is made using the fetch API, Jest-Fetch-Mock will intercept the request and return the object { body: 'ok' }.

Handling Query Parameters

Jest-Fetch-Mock can also handle query parameters. Here's an example of how to create a mock response that includes query parameters:
fetchMock.mockResponse((req) => {
if (req.url.includes('search=')) {
return Promise.resolve({ body: 'results' });
}
});
This code creates a mock response that applies to any request that includes the query parameter search. When a request with the search parameter is made using the fetch API, Jest-Fetch-Mock will intercept the request and return the object { body: 'results' }.

Handling Request Headers

Jest-Fetch-Mock can also handle request headers. Here's an example of how to create a mock response that only applies to requests with a specific header:
fetchMock.mockResponse((req) => {
if (req.headers.get('Authorization') === 'Bearer myToken') {
return Promise.resolve({ body: 'ok' });
}
});
This code creates a mock response that only applies to requests with an Authorization header that matches the string 'Bearer myToken'. When a request with this header is made using the fetch API, Jest-Fetch-Mock will intercept the request and return the object { body: 'ok' }.
Jest Fetch Mock Github Post
Jest Fetch Mock Github Post

Handling Multiple Responses

Jest-Fetch-Mock can also handle multiple responses for the same request. Here's an example of how to create a mock response that returns different responses based on the number of times the request has been made:
fetchMock.mockResponses(
[JSON.stringify({ data: 'first' }), { status: 200 }],
[JSON.stringify({ data: 'second' }), { status: 404 }]
);
This code creates a mock response that returns different responses for the first and second times a request is made. The first request will receive the JSON object { data: 'first' } and a status code of 200, while the second request will receive the JSON object { data: 'second' } and a status code of 404.

Assertions With Jest-Fetch-Mock

Jest-Fetch-Mock also allows you to make assertions about the requests that are made in your tests. Here's an example of how to check that a request was made with the correct URL:
it('fetches the correct URL', async () => {
fetchMock.mockResponseOnce(JSON.stringify({ data: 'hello world' }));
await myFunction();
expect(fetchMock).toHaveBeenCalledWith('/api/data');
});
This code sets up a mock response and calls a function called myFunction() that makes a fetch request to /api/data. After the request is made, the test uses expect to check that fetchMock was called with the correct URL.

Mocking API Responses With Different Content Types

Jest-Fetch-Mock can be used to mock API responses with different content types. This is useful if you need to test how your application handles different types of responses, such as JSON, XML, or text. Here's an example of how to create a mock response with a JSON content type:
fetchMock.mockResponse(JSON.stringify({ data: 'hello world' }), {
headers: { 'Content-Type': 'application/json' },
});
This code creates a mock response that returns a JSON object with the property data set to 'hello world'. The headers option is used to set the Content-Type header to 'application/json'. When a request is made using the fetch API, Jest-Fetch-Mock will intercept the request and return this response.
To create a mock response with a different content type, you can simply change the Content-Type header. For example, to create a mock response with a text content type, you can do the following:
fetchMock.mockResponse('hello world', {
headers: { 'Content-Type': 'text/plain' },
});
This code creates a mock response that returns the string 'hello world' with a content type of 'text/plain'.

Debugging Jest-Fetch-Mock Errors

When using Jest-Fetch-Mock, it's important to know how to debug errors that may occur. One common error is when a request is made but the mock response is not returned. This can happen if the URL or request options do not match the mock response.
To debug this, you can use the console.log function to output the request object and the mock response object. Here's an example:
fetchMock.mockResponseOnce(JSON.stringify({ data: 'hello world' }));
const response = await myFunction();
console.log(fetchMock.lastCall());
console.log(response);
expect(response).toEqual({ data: 'hello world' });
This code sets up a mock response and calls a function called myFunction() that makes a fetch request. After the request is made, the console.log function is used to output the last request object and the response object. This can help you identify any mismatches between the request and the mock response.
Another common error is when a mock response is returned, but it does not have the expected content. To debug this, you can use the console.log function to output the response object. Here's an example:
fetchMock.mockResponseOnce(JSON.stringify({ data: 'hello world' }));
  • const response = await myFunction();
  • console.log(response);
  • expect(response).toEqual({ data: 'hello' });
This code sets up a mock response and calls a function called myFunction() that makes a fetch request. After the request is made, the console.log function is used to output the response object. This can help you identify any differences between the actual response and the expected response.

Mocking Fetch Using jest-fetch-mock

Using Jest-Fetch-Mock With Server-Side Rendering

Jest-Fetch-Mock can also be used with server-side rendering. When rendering a component server-side, you can use Jest-Fetch-Mock to mock API responses in the same way as you would in a client-side test. Here's an example of how to use Jest-Fetch-Mock with server-side rendering:
import fetchMock from 'jest-fetch-mock';
fetchMock.enableMocks();
describe('My Component', () => {
it('renders correctly', async () => {
fetchMock.mockResponseOnce(JSON.stringify({ data: 'hello world' }));
const html = await renderToStringAsync(<MyComponent />);
expect(html).toMatchSnapshot();
});
});
This code sets up a mock response and then renders a component called MyComponent using renderToStringAsync. The expect function is used to ensure that the rendered HTML matches a previously saved snapshot.
One thing to keep in mind when using Jest-Fetch-Mock with server-side rendering is that you need to call fetchMock.enableMocks() before running your tests. This enables Jest-Fetch-Mock to intercept fetch requests made by the server rendering code.

People Also Ask

Can You Use Jest-Fetch-Mock With TypeScript?

Yes, Jest-Fetch-Mock is compatible with TypeScript.

How Do You Mock A Response With A Specific HTTP Status Code Using Jest-Fetch-Mock?

You can mock a response with a specific status code by calling fetchMock.mockResponseOnce with the status property set to the desired code.

How Do You Mock A Response With A Delay Using Jest-Fetch-Mock?

You can mock a response with a delay by passing a delay option to fetchMock.mockResponseOnce.

How Do You Check That A Mock Response Was Called A Certain Number Of Times In Jest-Fetch-Mock?

You can use the fetchMock.calls property to check how many times a mock response was called.

Conclusion

Jest-Fetch-Mock is a powerful tool for API testing that can save you time and effort. By mocking API responses, you can test your code in isolation and ensure that it works as expected in a variety of scenarios.
With Jest-Fetch-Mock, you can handle different HTTP methods, query parameters, request headers, and multiple responses. You can also make assertions about the requests that are made in your tests. By using Jest-Fetch-Mock, you can improve the quality and reliability of your code, and speed up your development process.
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