How to mock API client created inside a test class? - node.js

Question

I have a component class as below which create the rest and websocket connections using a third party npm module. I could change the Component.constructor to accept the module as a dependency so that I can inject a mock version during my Jest testing. But I read about Mocks with Jest, I thought I want to try it, but I cannot seem to understand how to intercept Api.Rest() and Api.Websocket return values.
// component.ts
import * as Api from 'npm-module'
import * as wait from 'wait-for-stuff' // actual npm module
export class Component {
private _rest:any;
private _websocket:any;
public events = new EventEmitter();
constructor() {
// I want to intecept the return value of
// Api.Rest() and Api.Websocket() to use mock versions.
this._rest = new Api.Rest();
this._websocket = new Api.Websocket();
this._init();
}
private _init() {
// so that when do stuff with this._rest and this._websocket;
// I can control what is the expected results during test
this._websocket.onUpdate((data) => {
events.emit('update', data);
});
var value = wait.for.promise(this._rest.getSomething());
}
}
Do I have to use another test library like Sinon or Jasmine?

Answer 1

Here is a simplified working example to get you started:
// #ts-ignore
import * as Api from 'npm-module'; // <= (ts-ignore since "npm-module" doesn't exist)
import EventEmitter from 'events';
jest.mock('npm-module', () => {
const getSomethingMock = jest.fn(); // <= always return...
const onUpdateMock = jest.fn(); // <= ...the same mocks...
return {
Rest: () => ({ getSomething: getSomethingMock }),
Websocket: () => ({ onUpdate: onUpdateMock })
}
},
{ virtual: true }); // <= (use virtual since "npm-module" doesn't exist)
class Component {
private _rest: any;
private _websocket: any;
public events = new EventEmitter();
constructor() {
this._rest = new Api.Rest();
this._websocket = new Api.Websocket();
this._init();
}
private _init() {
this._websocket.onUpdate((data) => { // <= ...so that this onUpdate...
this.events.emit('update', data);
});
}
}
test('Component', () => {
const component = new Component();
const listener = jest.fn();
component.events.on('update', listener);
const onUpdate = new Api.Websocket().onUpdate; // <= ...is the same as this one
const onUpdateArrowFunction = onUpdate.mock.calls[0][0]; // <= get the arrow function passed to it
onUpdateArrowFunction('mock data'); // <= now call the function
expect(listener).toHaveBeenCalledWith('mock data'); // Success!
});
Details
Jest takes over the require system and allows you to specify what you want it to return when a module is required (note that TypeScript import statements get compiled to require calls).
One way to mock a module is to create a manual mock by creating a file at __mocks__/npm-module.ts that contains your mock.
Another way (show above) is to use jest.mock and pass it a module factory function.
Whenever the module is required during the test Jest will return the mocked module instead.
Note that the example above always returns the same mock for getSomething and onUpdate so those mock functions can be retrieved during the test.
Also note the use of mockFn.mock.calls to retrieve this arrow function:
(data) => {
this.events.emit('update', data);
}
...which gets passed to onUpdate. Once it has been retrieved, it can be called directly which triggers the listener as expected.