How mocking can be dangerous sometime, missing the integration between functions.

mocking-bug.js

function Person(firstName, lastName, address) {
    let self = this;
    this.firstName = firstName;
    this.lastName = lastName;
    this.address = address;
    /**
     * @return {string}
     */
    this.getInfo = function() {
        return {
            address: address,
            tax: new TaxCalculator(self).getTaxBy() // Must parameter to this getTaxBy function, which is wrong in this line!
        }
    };
}

// tax-calculator.service / module.
function TaxCalculator(param) {
    let person = param;
    this.getTaxBy = function(year) {
        if(!year || person.address.country) {
            return new Error('Required info is missing')
        }
    }
}


describe('Testing  Person', function() {
    var person;
    var firstName = 'Imdadul';
    var lastName = 'Huq';
    var address = { country: 'Germany', city: 'Darmstadt' };
    beforeEach(function() {
        person = new Person(firstName, lastName, address);
    });

    describe('Test of getInfo', function() {
        beforeEach(function() {
            // For example let's mock a function of the class.
            spyOn(TaxCalculator, 'getTaxBy').and.callFake(function() {
                return 100;
            });
        });

        expect(person.getInfo()).toEqual({ // This test will pass! even though undefined variable has been passed!
            address:address,
            tax:100
        });
    });
});