describe('demo tests', () => {
    it('passing test', () =>{
        expect(sum([1,2,3])).to.equal(6);
    })
    it('passing test', () =>{
        expect(sum([-1,2,3])).to.equal(4);
    })

    it('string in array', () => {
expect(sum(["m"])).to.be.NaN;
    })

})