How to fix "No overload matches this call" error in jest.spyOn on a private method?

Question

Unit testing

private getPosOnScale(currentPos: number): number {
  const sliderRect = this.slider.element!.getBoundingClientRect();

  return this.settings.isVertical
    ? currentPos - sliderRect.top
    : currentPos - sliderRect.left;
}

describe('some method', () => {
  test('should return smth', () => {
    const view = new View('range-slider', settings);
    jest.spyOn(view, 'getPosOnScale').mockReturnValue(100);
    // code
  });
});

on the private method getPosOnScale I get an error

No overload matches this call.
Overload 1 of 4, '(object: View, method: FunctionPropertyNames>): SpyInstance', gave the following error.
Argument of type '"getPosOnScale"' is not assignable to parameter of type 'FunctionPropertyNames>'.
Overload 2 of 4, '(object: View, method: never): SpyInstance', gave the following error.
Argument of type 'string' is not assignable to parameter of type 'never'.ts(2769)

if method make public no error

Answer 1

jest.spyOn has 4 overloads that separate ordinary fields (these overloads have a 3rd parameter 'get' | 'set'), methods (functional type on the instance) and static methods (functional type on the class itself).
Your call implies just a method.
The problem is that private and protected fields don't match as function types. Private fields have no type at all outside the class, while protected fields have a type only in the descendant.
It remains only to cheat the type system:

declare class ViewHack {
  getPosOnScale(currentPos: number): number;
}
describe('some method', () => {
  test('should return smth', () => {
    const view = new View('range-slider', settings);
    jest.spyOn(view as unknown as ViewHack, 'getPosOnScale').mockReturnValue(100);
  });
});