import * as chai from 'chai';
chai.should();

describe('Array', () => {
  describe('#indexOf()', () => {
    it('should return -1 when the value is not present', () => {
      [1, 2, 3].indexOf(4).should.equal(-1);
    });
  });
});

import {expect} from 'chai';

describe('Array', () => {
  describe('#indexOf()', () => {
    it('should return -1 when the value is not present', () => {
      expect([1, 2, 3].indexOf(4)).to.equal(-1);
    });
  });
});

Use expect instead of should assertions
Use expect instead of should assertions
