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What's this?

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

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, create). 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
  • Why do we need to manually create the mock?
• Basic example
• Axios mock API
  • axios.mockResponse
  • axios.mockError
  • axios.lastReqGet
  • axios.lastPromiseGet
  • axios.reset
• Additional examples
  • Values returned by lastReqGet and lastPromiseGet methods
  • Resolving requests out of order
• Missing features
• Synchronous promise

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:

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

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
    let clientMessage = 'client is saying hello!';

    UppercaseProxy(clientMessage)
        .then(thenFn)
        .catch(catchFn);

    // 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/', {data: clientMessage });

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

    // 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.data.toUpperCase());

    // 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 (post, get, put, delete, create), 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 extended info about the most recent request
• lastPromiseGet - returns promise created when the most recent request was made
• reset - resets the Axios mock object - prepare it for the next test (typically used in afterEach)

axios.mockResponse(response[, requestInfo])

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. All the properties are optional, meaning that if a property is ommitted it will be replaced by 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: (optional) requestInfo

The second argument enables us to pinpoint an exact server request we wish to resolve. This can be useful if we're making multiple server requests and are planing to resolve them in a different order from the one in which they were made.

We supply two different objects:

• a extended request info object, which can be accessed by calling lastReqGet method
• a promise object, which can be accessed by calling the lastPromiseGet method

If ommited this argument defaults to the latest request made (internally the lastReqGet method is called).

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

axios.mockError(err[, requestInfo])

This method simulates an error while making a server request (network error, server error, etc ...).

Arguments: err

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

Arguments: (optional) requestInfo

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

axios.lastReqGet()

lastReqGet method returns extended info about the most recent request. The returned value can be used to pinpoint exact server request we wish to resolve (the value is passed as the second param of mockResponse or mockError methods).

The returned info contains all the data relevant to the request. It has the following structure (an example):

let requestInfo = {
    // promise created while
    promise: SimplePromise,
    // URL passed to the get/post/head/delete method
    url: "https://github.com/",
    // data which was pased to the get/post/head/delete method
    data: { text: "this is payload sent to the server" },
    // config which was pased to the get/post/head/delete method
    config: {
        ... something ...
    }
}

Additional examples at the end of this document illustrate how this method can be used.

NOTE: this is a sibling method to the lastPromiseGet (which returns only the promise portion of this the request object).

axios.lastPromiseGet()

lastPromiseGet method returns a promise given when the most recent server request was made. The returned value can be used to pinpoint exact server request we wish to resolve (the value is passed as the second param of mockResponse or mockError methods).

The promise object returned by this function corresponds to the one returned by post, get, put or delete method inside the code we wish to test.

Additional examples at the end of this document illustrate how this method can be used.

NOTE: This is a sibling method to the lastReqGet, which in addition to promise returns object containing extended info about the request.

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 AxiosMock is relatively simple, most of its functionality was covered in basic example at the begining of this document. In this section we'll explore features not covered by that initial example.

Values returned by lastReqGet and lastPromiseGet methods

The following example illustrates the meaning of the values returned by lastReqGet and lastPromiseGet methods.

The first snippet shows a component which will be tested. The component makes a post request to the server and stores the promise returned by Axios.

// ./src/MyComponent.js
import axios from '../lib/index';

class MyComponent {

    CallServer () {
        // making a `post` requst and storing the given promise
        this.axiosPromise = axios.post('/web-service-url/', { data: clientMessage });
    }
}

export default MyComponent;

In our spec file we will compare promise stored inside the MyComponent with values returned by lastReqGet and lastPromiseGet methods:

// ./test/MyComponent.spec.js
    import MyComponent from '../src/SomeSourceFile';

    let myComp = new MyComponent();
    
    myComp.CallServer();

    // getting the extended info about the most recent request
    let lastReqInfo = MockAxios.lastReqGet();
    // getting the promise made when the most recent request was made
    let lastPromise = MockAxios.lastPromiseGet();
    
    // the following expression will write `true` to the console
    // > here we compare promise stored in the `MyComponent` to the one
    //   returned by the `lastPromiseGet` method
    console.log(myComp.axiosPromise === lastPromise);

    // the following expression will also write `true` to the console
    // > here we compare promise stored in the `MyComponent`
    //   to the one in the request info, which was returned by the
    //   `lastReqGet` method
    console.log(myComp.axiosPromise === lastReqInfo.promise);

    // the following will also write "true" to console,
    // since it't the same object
    console.log(lastPromise ===  lastReqInfo.promise);

Resolving requests out of order

In the following example we'll have a look at how to resolve requests at desired order by using lastReqGet method.

In this example we'll create two consecutive requests before simulating a server response to the first one.

it('when resolving a request an appropriate handler should be called', () => {

    let thenFn1 = jest.fn(),
        thenFn2 = jest.fn();
    
    // creating the FIRST server request
    UppercaseProxy('client is saying hello!').then(thenFn1);
    // storing the request info - we'll need it later to pinpoint the request
    let firstRequestInfo = 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
    // -> we're using request info object to pinpoint the request
    // ... IF the info object is ommited, the method would automatically
    // resolve to the newest request from the internal queue (the SECOND one)
    mockAxios.mockResponse({ data: 'server says hello!' }, firstRequestInfo);

    // 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 info,
    // since there is only one unresolved request left
    // -> `mockResponse` resolves the last request in the
    //     queue if request info is ommited
    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 does illustrate how lastReqGet method can be used to alter the default behaviour of the mockResponse method.

NOTE: the identical effect can be achived by using the lastPromiseGet method. These two methods perform a similar task, as described in the corresponding documentation.

Missing features

AxiosMock coveres the most popular parts of Axios API, meaning that some of the features are not missing (i.e. interceptors).

If you need an additional feature, you can request it by creating a new issue on project's GitHub page.

Also you are welcome to implement the missing feature yourself and make a pull request :)

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.

Inspiration

This mock is loosely based on the following gist: tux4/axios-test.js

License

MIT License, http://www.opensource.org/licenses/MIT