old_contents: describe("analizOfNumber", function() {
    it("numbers divisible by 3 become 'ping'", function() {
      expect(analizOfNumber(6)).to.equal("ping");
    });
  
    it("numbers divisible by 5 become 'pong'", function() {
      expect(analizOfNumber(10)).to.equal("pong");
    });
  
    it("numbers divisible by 3 and 5 become 'pingpong'", function() {
      expect(analizOfNumber(15)).to.equal("pingpong");
    });
  
    it("number divisible by 3 and 5 have to be only 'pingpong'", function() {
      expect(analizOfNumber(30)).to.equal("pingpong");
    });
  
    it("number not divisible by 3 or 5 supposed to stay number", function() {
      expect(analizOfNumber(26)).to.equal(26);
    });
  });
  
  describe("pingPongSequence", function() {
    it("chosen number must be equal to sequence of elements according rulls", function() {
      expect(pingPongSequence(16)).to.eql([1, 2, "ping", 4, "pong", "ping", 7, 8, "ping", "pong", 11, "ping", 13, 14, "pingpong", 16]);
    });
  });
  
new_contents: describe("analizOfNumber", function() {
    it("numbers divisible by 3 become 'ping'", function() {
      expect(analizOfNumber(6)).to.equal("ping");
    });
  
    it("numbers divisible by 5 become 'pong'", function() {
      expect(analizOfNumber(10)).to.equal("pong");
    });
  
    it("numbers divisible by 3 and 5 become 'pingpong'", function() {
      expect(analizOfNumber(15)).to.equal("pingpong");
    });
  
    it("number divisible by 3 and 5 have to be only 'pingpong'", function() {
      expect(analizOfNumber(30)).to.equal("pingpong");
    });
  
    it("number not divisible by 3 or 5 supposed to stay number", function() {
      expect(analizOfNumber(26)).to.equal(26);
    });
  });
  
  describe("pingPongSequence", function() {
    it("array length must be equal to user input", function () {
      expect(pingPongSequence(103)).to.have.length(103);
    });
  
    it("chosen number must be equal to sequence of elements according rulls", function() {
      expect(pingPongSequence(16)).to.eql([1, 2, "ping", 4, "pong", "ping", 7, 8, "ping", "pong", 11, "ping", 13, 14, "pingpong", 16]);
    });
  });
  
subject: add test code to check array length