How to mock event and event handler for node.js module? - jestjs

Question

Here is my function
async function remoteCSVToJSON<T>(url: string, options: csv.ConverterOptions) {
const defaultOptions = {
noheader: false,
delimiter: ',',
workerNum: os.cpus().length,
};
const finalOptions = defaultsDeep(options, defaultOptions);
const datas: T[] = [];
return new Promise<T[]>((resolve, reject) => {
request.get(url).then(res => {
csv(finalOptions)
.fromString(res)
.on('json', (jsonObj: T) => datas.push(jsonObj))
.on('error', err => reject(err))
.on('end', () => {
logger.info('convert csv to json done');
resolve(datas);
});
});
});
}
I can mock fromString for csvtojson module like this:
jest.mock('csvtojson', () => {
return {
__esModule: true,
fromString: jest.fn(),
};
});
But how do I mock the .on(event) method and event handlers?

Answer 1

Here is the unit test solution:
index.ts:
import request from 'request-promise';
import csv from 'csvtojson';
import os from 'os';
import { defaultsDeep } from 'lodash';
export async function remoteCSVToJSON<T>(url: string, options: any) {
const defaultOptions = {
noheader: false,
delimiter: ',',
workerNum: os.cpus().length
};
const finalOptions = defaultsDeep(options, defaultOptions);
const datas: T[] = [];
return new Promise<T[]>((resolve, reject) => {
request.get(url).then(res => {
csv(finalOptions)
.fromString(res)
.on('json', (jsonObj: T) => datas.push(jsonObj))
.on('error', err => reject(err))
.on('end', () => {
console.info('convert csv to json done');
resolve(datas);
});
});
});
}
index.spec.ts:
import request from 'request-promise';
import csv from 'csvtojson';
import os from 'os';
import { remoteCSVToJSON } from './';
jest.mock('csvtojson', () => {
const mCsv = {
on: jest.fn(),
fromString: jest.fn().mockReturnThis()
};
return jest.fn(() => mCsv);
});
jest.mock('request-promise', () => {
return {
get: jest.fn()
};
});
describe('remoteCSVToJSON', () => {
afterEach(() => {
jest.restoreAllMocks();
});
test('should request and convert csv to json', async () => {
const events: any = {};
csv()
.fromString()
.on.mockImplementation(function(this: any, ...args: any[]) {
const [event, handler] = args;
events[event] = handler;
return this;
});
const mGetResponse = { data: 'fake data' };
(request.get as jest.MockedFunction<typeof request.get>).mockResolvedValueOnce(mGetResponse);
const infoSpy = jest.spyOn(console, 'info');
const actualValue = remoteCSVToJSON('url', {});
const mJsonObj = { id: 1, name: 'mrdulin' };
expect(jest.isMockFunction(csv)).toBeTruthy();
await new Promise(resolve => setTimeout(resolve, 0));
expect(csv).toBeCalledWith({
noheader: false,
delimiter: ',',
workerNum: os.cpus().length
});
expect(csv().fromString).toBeCalledWith(mGetResponse);
events['json'](mJsonObj);
events['end']();
await expect(actualValue).resolves.toEqual([mJsonObj]);
expect(request.get).toBeCalledWith('url');
expect(infoSpy).toBeCalledWith('convert csv to json done');
});
test('should throw error when convert error', async () => {
const events: any = {};
csv()
.fromString()
.on.mockImplementation(function(this: any, ...args: any[]) {
const [event, handler] = args;
events[event] = handler;
return this;
});
const mGetResponse = { data: 'fake data' };
(request.get as jest.MockedFunction<typeof request.get>).mockResolvedValueOnce(mGetResponse);
const actualValue = remoteCSVToJSON('url', {});
const mJsonObj = { id: 1, name: 'mrdulin' };
expect(jest.isMockFunction(csv)).toBeTruthy();
await new Promise(resolve => setTimeout(resolve, 0));
expect(csv).toBeCalledWith({
noheader: false,
delimiter: ',',
workerNum: os.cpus().length
});
expect(csv().fromString).toBeCalledWith(mGetResponse);
events['json'](mJsonObj);
const mError = new Error('mock error');
events['error'](mError);
await expect(actualValue).rejects.toThrowError(mError);
expect(request.get).toBeCalledWith('url');
});
});
Unit test result with 100% coverage:
PASS src/stackoverflow/57423762/index.spec.ts (6.995s)
remoteCSVToJSON
✓ should request and convert csv to json (16ms)
✓ should throw error when convert error (22ms)
console.info node_modules/jest-mock/build/index.js:860
convert csv to json done
----------|----------|----------|----------|----------|-------------------|
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files | 100 | 100 | 100 | 100 | |
index.ts | 100 | 100 | 100 | 100 | |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests: 2 passed, 2 total
Snapshots: 0 total
Time: 8.114s
Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/57423762