Mocking nested functions in jestjs

Question

I have a very complex object i am using to mock out a DataTable() object for the purposes of testing.

const obj = {
  DataTable: () => {
    return {
      columns: () => {
        return {
          data: () => {
            return {
              eq: () => {
                return {
                  indexOf: jest.fn(),
                };
              },
            };
          },
          visible: () => {
            return jest.fn();
          },
        };
      },
    };
  },
};

Inside my testing code I am trying to spy on some of these functions but it always returns undefined. Is there a way to mock out the return value of deeply nested functions?

    jest.spyOn(obj.DataTable().columns().data().eq(), 'indexOf').mockReturnValue('test');
    console.log(obj.DataTable().columns().data().eq().indexOf()); // returns undefined, should return 'test'

Lin Du · Accepted Answer · 2020-04-24 02:56:07Z

11

Here is the unit test solution:

index.ts:

import { obj } from './obj';

export function main() {
  return obj.DataTable().columns().data().eq().indexOf();
}

obj.ts:

export const obj = {
  DataTable: () => {
    return {
      columns: () => {
        return {
          data: () => {
            return {
              eq: () => {
                return {
                  indexOf: () => 'real data',
                };
              },
            };
          },
        };
      },
    };
  },
};

index.test.ts:

import { main } from './';
import { obj } from './obj';

describe('61396089', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  it('should pass', () => {
    const mockDataTable = {
      columns: jest.fn().mockReturnThis(),
      data: jest.fn().mockReturnThis(),
      eq: jest.fn().mockReturnThis(),
      indexOf: jest.fn().mockReturnValueOnce('fake data'),
    };
    jest.spyOn(obj, 'DataTable').mockImplementationOnce(() => mockDataTable);
    const actual = main();
    expect(actual).toBe('fake data');
    expect(mockDataTable.columns).toBeCalledTimes(1);
    expect(mockDataTable.data).toBeCalledTimes(1);
    expect(mockDataTable.eq).toBeCalledTimes(1);
    expect(mockDataTable.indexOf).toBeCalledTimes(1);
  });
});

unit test results with coverage report:

 PASS  stackoverflow/61396089/index.test.ts (20.978s)
  61396089
    ✓ should pass (8ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |   44.44 |      100 |   16.67 |   44.44 |                   
 index.ts |     100 |      100 |     100 |     100 |                   
 obj.ts   |   16.67 |      100 |       0 |   16.67 | 3-10              
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        23.889s

source code: https://github.com/mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/61396089

answered Apr 24, 2020 at 2:56

Lin Du

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2 Comments

MrHIDEn Over a year ago

I was looking for that 2 days... At the beginning I could not understand why I can not mock function, but they are isolated behind parent function... I slightly modified that as

const mockDataTable = (some return values or even whole jest.fn().. spy) => ({ ..., indexOf: () => return value or indexOf: indexOfSpy passed by mockDataTable(params ...)

. Thx

kursus Over a year ago

Much more elegant solution than the nest of mockImplementationOnce that seems to be prevalent.

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