Angular jasmine test private function

Question

I have a private function mutateSectionIds() that deletes id key if it's value is negative, from an array with objects called sections[].

sections = [
            {
              id: 1,
            },
            {
              id: -1,
            },
          ]

makeTree(sections) {
    this.mutateSectionIds(sections);
    return this.mutateSectionIds;
  }
 
  private mutateSectionIds(sections) {
    sections.forEach((section) => {
      if (section.id < 0) {
        delete section.id;
      }
      this.mutateSectionIds(sections);
    });
  }

Testing only the makeTree() function, how can I access the private mutateSectionIds(), and test if sections[1].id, which is a negative value, was deleted?

So far, I wrote this test without luck:

it('should mutate section ids', () => {
    const key = sections[1].id;
    const obj = [{ id: -1 }];
    service.makeTree(sections);
    expect(key).toBeLessThan(0);
    expect(obj[0].id).toBeLessThan(0);
    expect(sections[1].id).not.toContain(key);
  })

EDIT:

I keep on trying and finally was able too made a solution by myself, but I have to test the private function directly using service['mutateSectionIds'](sections).

   it('should mutate section ids', () => {
        service['mutateSectionIds'](sections);
        expect(sections[1].id).toBeUndefined;
      });

If anyone knows how to achieve the same goal without testing directly the private function I would appreciate.