How can I mock return value of chained function of a constructor function?

Question

I need to mock the value of csvJsonArray using jest

const csv = require('csvtojson')

const csvJsonArray = await csv().fromFile(csvFilePath)

I have tried the following

jest.mock('csvtojson', () => jest.fn())
const fromFile = jest.fn().mockReturnValue(commTemplateCsvJsonArray)
csv.mockImplementation(() => ({fromFile}))

But here csvJsonArray value is null

How can I mock the return value for a chained function of a constructor function?

Answer 1

Since the return value of csv().fromFile() method is a promise , you should mock the resolved value using mockFn.mockResolvedValue(value) .

Eg

main.js :

const csv = require('csvtojson');

async function main() {
  const csvFilePath = './test.csv';
  const csvJsonArray = await csv().fromFile(csvFilePath);
  return csvJsonArray;
}

module.exports = { main };

main.test.js :

const { main } = require('./main');
const csv = require('csvtojson');

jest.mock('csvtojson');

describe('65620607', () => {
  it('should pass', async () => {
    const commTemplateCsvJsonArray = [1, 2, 3];
    const fromFile = jest.fn().mockResolvedValue(commTemplateCsvJsonArray);
    csv.mockImplementation(() => ({ fromFile }));
    const actual = await main();
    expect(actual).toEqual([1, 2, 3]);
    expect(csv).toBeCalledTimes(1);
    expect(fromFile).toBeCalledWith('./test.csv');
  });
});

unit test result:

 PASS  examples/65620607/main.test.js
  65620607
    ✓ should pass (4 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 main.js  |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        5.705 s