Homex-jest-mock-axios NPM

What's this?

This is a light-weight, easy to use synchronous Axios mock for unit testing with Jest.

Why do I need homex-jest-mock-axios?

@homex/jest-mock-axios gives you extra power apart from only using jest-mock-axios original functionalities, but also patch method and changing global default header in axios request.

Why would I use it?

Because it works synchronously, meaning that your tests will be easier to write, read and understand.

Can it be used with Jasmine/Mocha?

Unfortunately out of the box this mock works only with Jest.

However, if you look at the source code, you can see that it uses Jest only to define spies (for methods post, get, put, delete). This means that it can easily be modified to use any other testing framework - go to GitHub, clone it, modify it, play with it :)

What's in this document?

Installation

Installation is simple - just run:

npm i --save-dev jest-mock-axios

Next you need to setup a manual Jest mock for Axios (we'll explain why a bit later):

create __mocks__ directory in your project root
inside this new directory create a files named axios.js
copy & past the following snippets to axios.js file

// ./__mocks__/axios.js
import mockAxios from 'jest-mock-axios';
export default mockAxios;

Why do we need to manually create the mock?

It's because Jest expects mocks to be placed in the project root, while packages installed via NPM get stored inside node_modules subdirectory.

Basic example

Let's consider that we want to test a component which uses Axios. This component returns a promise, which will be resolved after Axios is done communicating with the server.

Here's a Jest snippet, which explains how we would test this component:

// ./src/__tests__/UppercaseProxy.spec.js
import mockAxios from 'jest-mock-axios';

afterEach(() => {
    // cleaning up the mess left behind the previous test
    mockAxios.reset();
});

it('UppercaseProxy should get data from the server and convert it to UPPERCASE', () => {

    let catchFn = jest.fn(),
        thenFn = jest.fn();

    // using the component, which should make a server response
    UppercaseProxy('client is saying hello!')
        .then(thenFn)
        .catch(catchFn);

    // simulating a server response
    mockAxios.mockResponse({ data: 'server says hello!' });

    // since `post` method is a spy, we can check if the server request was correct
    // a) the correct method was used (post)
    // b) went to the correct web service URL ('/web-service-url/')
    // c) if the payload was correct ('client is saying hello!')
    expect(mockAxios.post).toHaveBeenCalledWith('/web-service-url/', 'client is saying hello!' );

    // checking the `then` spy has been called and if the
    // response from the server was converted to upper case
    expect(thenFn).toHaveBeenCalledWith('SERVER SAYS HELLO!');

    // catch should not have been called
    expect(catchFn).not.toHaveBeenCalled();
});

To make this example complete and easier to understand, let's have a look at a (verbose) implementation of component we are testing:

// ./src/UppercaseProxy.js
import axios from 'axios';

const UppercaseProxy = (clientMessage) => {

    // requesting data from server
    let axiosPromise = axios.post('/web-service-url/', { data: clientMessage });

    // converting server response to upper case
    axiosPromise = axiosPromise.then(serverData => serverData.toUpper());

    // returning promise so that client code can attach `then` and `catch` handler
    return(axiosPromise);
};

export default UppercaseProxy;

At the bottom of this page you can find additional examples.

Axios mock API

In addition to standard Axios methods (get, post, put, delete), which are exposed as spies, Axios mock has three additional public methods, which are intended to facilitate mocking:

mockResponse - simulates a server (web service) response
mockError - simulates a (network/server) error
lastReqGet - returns a last promise which was given (useful if multiple Axios requests are made within a single it block)
reset - reset the Axios mock - prepare it for the next test (typically used in afterEach)

axios.mockResponse(response, promise)

After a request request has been made to the server (web service), this method resolves that request by simulating a server response.

Arguments: `response`

The first argument of this method is the a response object returned by the server, with a structure illustrated by the snippet below. Each of the properties is optional, meaning that if missing it will be replaced by the a default value (defaults are shown in the snippet).

response = {
    data: {},
    status: 200,
    statusText: 'OK',
    headers: {},
    config: {},
}

The given response object will get passed to then even handler function.

Arguments: `promise`

The second argument is a promise, which was given when the server request was made. This argument is also optional - it defaults to the promise made when the last server request was made.

We can use it to pinpoint an exact server request we wish to resolve, which is useful only if we're making multiple server requests before calling axios.reset.

At the end of this document you can find an example which demonstrates how this parameter can be used.

axios.mockError(err, promise)

This method simulates an error while making a server request (network error, server error, etc ...). It is in fact very similar to mockResponse method: it receives to optional parameters: error object and a promise.

Arguments: `err`

Error object will get passed to catch event handler function. If omitted it defaults to an empty object.

Arguments: `promise`

The second argument is a promise object, which works the same way as described part about the mockResponse method.

axios.lastReqGet()

lastReqGet method returns a promise given when the most recent server request was made. We can use this method if we want to pass the promise object to mockResponse or mockError methods.

axios.reset()

reset method clears state of the Axios mock to initial values. It should be called after each test, so that we can start fresh with our next test (i.e. from afterEach method).

Additional examples

Since this is a simple mock, most of the functionality was covered in basic test, at the begining of this page. In this section we'll explore features not covered by that initial example.

Using `lastReqGet` method

In the following example we'll have a look at use case for the lastReqGet method. In this example we'll create two consecutive requests before simulating a server response to the first.

it('when resolving a request it should call the appropriate handler', () => {

    let thenFn1 = jest.fn(),
        thenFn2 = jest.fn();
    
    // creating the first server request
    UppercaseProxy('client is saying hello!').then(thenFn1);
    let firstReqPromise = mockAxios.lastReqGet();

    // creating the second server request
    // BEFORE the first had chance to be resolved
    UppercaseProxy('client says bye bye!').then(thenFn2);

    // simulating a server response to the FIRST request
    mockAxios.mockResponse({ data: 'server says hello!' }, firstReqPromise);

    // only the first handler should have been called
    expect(thenFn1).toHaveBeenCalled();
    expect(thenFn2).not.toHaveBeenCalled();

    // simulating a server response to the SECOND request
    // NOTE: here we don't need to provide the request object,
    //       since we want to reolve the request which was made last
    mockAxios.mockResponse({ data: 'server says bye bye!' });

    // the first `then` handles should be called only once
    expect(thenFn1).toHaveBeenCalledTimes(1);
    // now the second `then` handler should be called
    expect(thenFn2).toHaveBeenCalled();
});

Although this might not be the most realistic use-case of this functionality, It's good enough to illustrate how lastReqGet method can be used.

Synchronous promise

Tha magic which enables axio mock to work synchronously is hidden away in jest-mock-promise, which enables promises to be settled in synchronous manner.

The jest-mock-promise can be used to mock any asyc component which uses promises.