Testing private method using spyOn and Jest

Question

My functions in Calculator class is private and I can't test in this way,

describe('calculate', function() {
  it('add', function() {
    let result = Calculator.Sum(5, 2);
    expect(result).toBe(7);   
  });

  it('substract', function() {
    let result = Calculator.Difference(5, 2);
    expect(result).toBe(3);
  });
});

my class:

export default class calculator {
  private Sum(a: number, b: number): number {
    let c = a + b;
    return c;
  }

  private Difference(a: number, b: number): number {
    let c = a - b;
    return c;
  }
}

how can I modify this test with using spyOn on private methods?

thanks for any help

Answer 1

The easiest solution, to spy on your 'Sum' method of Calculator class, I came around would be:

jest.spyOn(calculator as any, 'Sum').<do-something>();

Answer 2

In Typescript it doesn't allow you to test the private method. So you need something to escape from this behavior. What you can do is something like this

let result = (Calculator as any).Sum(2,3);
expect(result).toBe(5);

I hope that will solve your problem.