Jest Mock FS File Stream - node.js

Question

I am very new to jest and mocking, I have a fs module read stream, which has 2 events on and data, i am trying to mock below code
ReadFile.js
const csv = require('csv-parser');
let storeData=[];
csvFileReader() {
fs.createReadStream(path.resolve(__dirname, "./abc.csv"))
.pipe(csv())
.on('data', async (row) => {
storeData[0] = row.postcode;
})
.on('end', () => {
console.log('Done')
});
}
ReadFileTest.js
import ReadFile from './readFile.js';
const fs = require('fs');
jest.mock('fs');
describe('Load File', () => {
const readFile= new ReadFile();
test('Test data handler', async () => {
const mockPipeOn = { on: jest.fn().mockImplementation(function(this, event, handler) {
if (event === 'data') {
jest.fn.mockReturnValueOnce("Reading Data")
}
if (event === 'end') {
jest.fn.mockReturnValueOnce("Completed Reading Data")
}
return this;
}), };
const mockReadStream = { pipe: jest.fn().mockReturnValueOnce(mockPipeOn) };
const createReadStream = jest.fn().mockReturnValueOnce(mockReadStream);
await readFile.csvFileReader();
});
});
I am getting error on 'this' key word its not going into 'data' and 'end' error handler

Answer 1

You should use mockFn.mockReturnThis() to return the context.
E.g.
ReadFile.js:
import fs from 'fs';
import path from 'path';
import csv from 'csv-parser';
class ReadFile {
csvFileReader() {
fs.createReadStream(path.resolve(__dirname, './abc.csv'))
.pipe(csv())
.on('data', async (row) => {
console.log('Storing Data');
})
.on('end', () => {
console.log('Done');
});
}
}
export default ReadFile;
ReadFile.test.js
import ReadFile from './ReadFile';
import fs from 'fs';
jest.mock('fs');
describe('67216891', () => {
const readFile = new ReadFile();
it('should store', () => {
const mReadStream = {
pipe: jest.fn().mockReturnThis(),
on: jest.fn().mockImplementation(function (event, handler) {
handler();
return this;
}),
};
fs.createReadStream.mockReturnValueOnce(mReadStream);
readFile.csvFileReader();
expect(fs.createReadStream).toBeCalledTimes(1);
expect(mReadStream.pipe).toBeCalledTimes(1);
expect(mReadStream.on).toBeCalledWith('data', expect.any(Function));
expect(mReadStream.on).toBeCalledWith('end', expect.any(Function));
});
});
test result:
PASS examples/67216891/ReadFile.test.js (8.891 s)
67216891
✓ should store (20 ms)
console.log
Storing Data
at ReadFile.<anonymous> (examples/67216891/ReadFile.js:10:17)
console.log
Done
at examples/67216891/ReadFile.js:13:17
-------------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
-------------|---------|----------|---------|---------|-------------------
All files | 100 | 100 | 100 | 100 |
ReadFile.js | 100 | 100 | 100 | 100 |
-------------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 10.159 s