sinon wont replace dependency

Question

I am trying to write a unit test for the class in the example below.

const DependencyClass = require('../../../../Dependency/src/index').DependencyClass;
const string = 'test';

class FirstClass {
    async getResult() {
        const dependency = new DependencyClass();
        const result = dependency.getResult(string);
        return result;
    }
}

module.exports = {
    FirstClass
};

I am trying to stub the getResult() method of the DependencyClass class so it returns a predefined value when called from my unit tests but cant figure out how to do it.

const FirstClass = require('../../lib/FirstClass ').FirstClass;

describe('FirstClass.js', function() {
    describe('getResult()', function() {
        it('throws an exception if the result is not returned', async function() {
            const firstClass = new FirstClass();
            sinon.replace(firstClass.getResult, 'DependencyClass.getResult', function() {
                    const fakeResult =
                    [
                        'test1',
                        'test2'
                    ];
                    return fakeResult;
                });

            const expectedResult =
                [
                    'test1',
                    'test2'
                ];

            const result = await firstClass.getResult();
            expect(result).to.deep.eq(expectedResult);
        });
    });
});

afterEach(function() {
    sinon.restore();
});

I understand that DependencyClass.getResult is not a property of the firstClass.getResult object but I am struggling to understand how sinon should be used in this context.

Answer 1

If you want to stub method getResult() from class DependencyClass, then you need to create stub from it: sinon.stub(DependencyClass.prototype, 'getResult');.

Here the complete example.

Note: I remove all async await, because it is unnecessary for this simple example.

File DependencyClass.js

// @file DependencyClass.js
class DependencyClass {
  getResult() {
    return 'xxx';
  }
}

module.exports = { DependencyClass };

File FirstClass.js

// @file FirstClass.js
const { DependencyClass } = require('./DependencyClass.js');

const string = 'test';

class FirstClass {
  getResult() {
    const dependency = new DependencyClass();
    return dependency.getResult(string);
  }
}


module.exports = { FirstClass };

Test spec file

// @file stackoverflow.spec.js
const sinon = require('sinon');
const { expect } = require('chai');

const { DependencyClass } = require('./DependencyClass');
const { FirstClass } = require('./FirstClass');

describe('FirstClass.js', function () {
  describe('getResult()', function () {
    it('throws an exception if the result is not returned', function () {
      // Create fake response.
      const fakeResult = ['test1', 'test2'];
      // Create stub DependencyClass method getResult().
      const stubDependencyGetResult = sinon.stub(DependencyClass.prototype, 'getResult');
      stubDependencyGetResult.returns(fakeResult);

      // Initiate first class.
      const firstClass = new FirstClass();
      // Call firstClass method getResult.
      const result = firstClass.getResult();
      // Check whether the result is correct.
      expect(result).to.deep.equal(fakeResult);
      // Verify stub get called.
      expect(stubDependencyGetResult.calledOnce).to.equal(true);
      // Restore stub.
      stubDependencyGetResult.restore();
    });
  });
});

When I run it using mocha:

$ mocha stackoverflow.spec.js 


  FirstClass.js
    getResult()
      ✓ throws an exception if the result is not returned


  1 passing (6ms)

$

Hope this helps.