Make partial mock of class with Jest in NodeJS - node.js

Question

I'm new to NodeJS and came from PHP, where creating partial mock was easy. But I'm not able to accomplish the same with Jest in NodeJS.
I have a function extractPayloadDates which accept an instance of dialogflow Agent and taking and parsing data from it. I want to mock only single method getParameter of Agent, because no more methods are used in the tested function. I found this code online but it doesn't work
import { Agent } from '../../src/dialogflow/Agent';
import { extractPayloadDates } from '../../src/intents/extractPayloadDates';
describe('extractPayloadDates', () => {
it('tests extracting string', () => {
const AgentMock = jest.fn<Agent, []>(() => ({
getParameter: () => {
return 'some date';
}
}));
const agent = new AgentMock();
expect(extractPayloadDates(agent)).toEqual('some date');
});
});
This code produce following error:
Type '{ getParameter: () => string; }' is missing the following properties from type 'Agent': payload, webhookClient, chatter, getOriginalRequest, and 13 more.ts(2740)
index.d.ts(124, 53): The expected type comes from the return type of this signature.
I also tried to use jest.spyOn, but the problem is, that I cannot create Agent instance as it needs many other objects.
Edit 3.9.2019 with more code
Agent.ts
export class Agent {
private payload: DialogFlowPayload[] = [];
constructor(readonly webhookClient: WebhookClient, private readonly chatter: Chatter) {}
...
}
WebhookClient and Chatter have more dependencies in their constructor as well...
extractPayloads.spec.ts
import { Agent } from '../../src/dialogflow/Agent';
import { extractPayloadDates } from '../../src/intents/extractPayloadDates';
describe('extractPayloadDates', () => {
it('tests extracting string', () => {
const webhookMock = jest.fn();
const chatter = jest.fn();
const agent = new Agent(webhookMock, chatter);
expect(extractPayloadDates(agent)).toEqual('some date');
});
});
This produce another error:
Argument of type 'Mock' is not assignable to parameter of type 'Chatter'.
Property 'getMessage' is missing in type 'Mock' but required in type 'Chatter'
Do I really have to create also WebhookClient and all its dependencies and do the same with Chatter? If I do, I have to create instances of multiple classes just to mock 1 method in Agent, which then will not use any of dependency.

Answer 1

From the docs of jest
If you want to overwrite the original function, you can use jest.spyOn(object, methodName).mockImplementation(() => customImplementation) or object[methodName] = jest.fn(() => customImplementation);
jest.spyOn call jest.fn internally. So you can only mock getParameter method of agent like this:
extractPayloadDates.ts:
function extractPayloadDates(agent) {
return agent.getParameter();
}
export { extractPayloadDates };
Agent.ts:
interface DialogFlowPayload {}
interface WebhookClient {}
interface Chatter {
getMessage(): any;
}
class Agent {
private payload: DialogFlowPayload[] = [];
constructor(readonly webhookClient: WebhookClient, private readonly chatter: Chatter) {}
public getParameter() {
return 'real data';
}
public otherMethod() {
return 'other real data';
}
}
export { Agent, Chatter };
Unit test, only mock getParameter method of agent, keep the original implementation of otherMethod
import { extractPayloadDates } from './extractPayloadDates';
import { Agent, Chatter } from './Agent';
const webhookMock = jest.fn();
const chatter: jest.Mocked<Chatter> = {
getMessage: jest.fn()
};
const agent = new Agent(webhookMock, chatter);
describe('extractPayloadDates', () => {
it('should only mock getParameter method of agent', () => {
agent.getParameter = jest.fn().mockReturnValueOnce('mocked data');
const actualValue = extractPayloadDates(agent);
expect(actualValue).toEqual('mocked data');
expect(agent.otherMethod()).toBe('other real data');
});
});
PASS src/stackoverflow/57428542/extractPayloadDates.spec.ts
extractPayloadDates
✓ should only mock getParameter method of agent (7ms)
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 2.484s, estimated 3s