Mocha: testing function used in constructor - node.js

Question

I am using mocha, chai and sinon for my testing purposes. I've got a class like below:
class ClassToTest {
person;
constructor(person) {
this.setPerson(person);
}
setPerson(person) {
if (typeof person.firstName === 'undefined') {
throw Error('Person has to have first name.');
}
this.person = person;
}
}
How I can test setPerson function? When I create new ClassToTest object setPerson gets called by constructor and Error gets thrown immediately. I've tried creating stubs with Sinon but no luck so far.
Should I even test setPerson function to begin with? I was thinking about:
1. moving validation (typeof if) to other function (e.g. validatePerson) and test that
2. testing only if constructor throws Error or sets person

Answer 1

Here is the unit test solution:
index.ts:
export class ClassToTest {
person;
constructor(person) {
this.setPerson(person);
}
setPerson(person) {
if (typeof person.firstName === 'undefined') {
throw new Error('Person has to have first name.');
}
this.person = person;
}
}
index.test.ts:
import { ClassToTest } from './';
import sinon from 'sinon';
import { expect } from 'chai';
describe('57091171', () => {
afterEach(() => {
sinon.restore();
});
describe('#constructor', () => {
it('should set person', () => {
const setPersonStub = sinon.stub(ClassToTest.prototype, 'setPerson');
const person = { firstName: 'sinon' };
new ClassToTest(person);
sinon.assert.calledWithExactly(setPersonStub, person);
});
});
describe('#setPerson', () => {
it('should set person', () => {
const stub = sinon.stub(ClassToTest.prototype, 'setPerson').returns();
const person = { firstName: 'sinon' };
const ins = new ClassToTest(person);
stub.restore();
ins.setPerson(person);
expect(ins.person).to.deep.equal(person);
});
it('should handle error if firstName is not existed', () => {
const stub = sinon.stub(ClassToTest.prototype, 'setPerson').returns();
const person = {};
const ins = new ClassToTest(person);
stub.restore();
expect(() => ins.setPerson(person)).to.throw('Person has to have first name.');
});
});
});
unit test results with 100% coverage:
57091171
#constructor
✓ should set person
#setPerson
✓ should set person
✓ should handle error if firstName is not existed
3 passing (43ms)
----------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files | 100 | 100 | 100 | 100 |
index.ts | 100 | 100 | 100 | 100 |
----------|---------|----------|---------|---------|-------------------