text: function lookupChar(string, index) {
      if (typeof(string) !== 'string' || !Number.isInteger(index)) {
          return undefined;
      }
      if (string.length <= index || index < 0) {
          return "Incorrect index";
      }
      return string.charAt(index);
  }
  let assert = require("chai").assert;
  describe("lookupChar", function () {
      describe("Input parameters tests", function () {
          it('should return undefined with non-string first parameter', function () {
              let result = lookupChar(1, 0);
              assert.isUndefined(result);
          });
          it('should return undefined with non-integer second parameter', function () {
              let result = lookupChar("string", "non-integer");
              assert.isUndefined(result);
          });
          it('should return undefined with floating-point number as second parameter', function () {
              let result = lookupChar("string", 3.5);
              assert.isUndefined(result);
          });
      });
  
      describe("Index correctness tests", function () {
          it('should return "Incorrect index" if index is equal to string length', function () {
              let result = lookupChar("string", 6);
              assert.equal(result, "Incorrect index");
          });
          it('should return "Incorrect index" if index is greater than string length', function () {
              let result = lookupChar("string", 7);
              assert.equal(result, "Incorrect index");
          });
          it('should return "Incorrect index" if index is negative number', function () {
              let result = lookupChar("string", -7);
              assert.equal(result, "Incorrect index");
          });
      });
  
      describe("Correct results test", function () {
          it('should return correct result with correct parameters', function () {
              let result = lookupChar("string", 0);
              assert.equal(result, "s");
          });
          it('should return correct result with correct parameters', function () {
              let result = lookupChar("Pesho", 1);
              assert.equal(result, "e");
          });
      });
  });