`eventsourcemock`

A dependency free mock for EventSource objects.

Example (with Jest)

Setup

To setup the mock create a new file where the mock is assigned to the window object.

// __test__/configJSDom.js
import EventSource from 'eventsourcemock';

Object.defineProperty(window, 'EventSource', {
  value: EventSource,
});

Then instruct Jest to use that file during setup in package.json.

"jest": {
  "setupFiles": [
    "./__test__/configJSDom.js"
  ]
}

Usage

Suppose we want to test this component, which updates its state when a message is received from an EventSource instance.

export default class Component extends React.Component {
  props: Props;
  state: State;
  source: $PropertyType<window, 'EventSource'>;

  constructor(props: Props) {
    super(props);
    this.state = {
      counter: 0,
    };
  }

  componentDidMount() {
    this.source = new window.EventSource('http://example.com/events');
    this.source.addEventListener('foo', messageEvent => {
      this.setState({ counter: parseInt(messageEvent.data, 10) });
    });
  }

  componentWillUnmount() {
    this.source.close();
  }

  render() {
    return <div className="Component">{this.state.counter}</div>;
  }
}

We can write a test file that grabs the source from the global sources object and simulates messages:

// @flow
import React from 'react';
import { mount } from 'enzyme';

import Component from './Component';
import { sources } from 'eventsourcemock';

const messageEvent = new MessageEvent('foo', {
  data: '1',
});

describe('update counter on SSE', () => {
  let wrapper;
  beforeAll(() => {
    wrapper = mount(<Component />);
  });

  it('should initialise counter to 0', () => {
    expect(wrapper.state('counter')).toBe(0);
  });

  it('should display "0"', () => {
    expect(wrapper.text()).toBe('0');
  });

  it('should update the counter to 1', () => {
    sources['http://example.com/events'].emit(
      messageEvent.type,
      messageEvent
    );
    expect(wrapper.state('counter')).toBe(1);
  });

  it('should close the EventSource on unmount', () => {
    wrapper.unmount();
    expect(sources['http://example.com/events'].readyState).toBe(2);
  });
});

API

`sources: { [key: string]: EventSource }`

sources holds the EventSource instances created.

import { sources } from 'eventsourcemock';

`EventSource`

EventSource mocks window.EventSource, providing methods to simulate messages and errors from the network.

import EventSource from 'eventsourcemock';

Constructor

EventSource(
  url: string,
  options?: {
    withCredentials: boolean
  }
)

Properties

`__emitter`

A reference to the node EventEmitter instance used internally.

Methods

`emit(eventName: string, messageEvent?: MessageEvent)`

Calls each of the listeners registered for the event named eventName, providing messageEvent as argument.

Example

const messageEvent = new MessageEvent('type', {
  data: 'message event data'
});
source.emit(messageEvent.type, messageEvent);

`emitError(error: Error)`

Simulates dispatching an error event on the EventSource instance. Causes onerror to be called.

Example (jest)

const onErrorSpy = jest.fn();
const error = new Error('Something went wrong.');
eventSource.onerror = onErrorSpy;
eventSource.emitError(error);
expect(onErrorSpy).toHaveBeenCalledWith(error);

`addEventListener(eventName: string, listener: Function)`

See EventTarget.addEventListener.

`close()`

See EventSource.close.

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