text: 'use strict';  // must included with quotes;
  
  var s = "hello, world!";
  console.log(s);
  s = s.toUpperCase();
  console.log(s);
  s = s.toLowerCase();
  console.log(s);
  
  s.toUpperCase();  // s didn't changed unless assign result to it;
  console.log(s);
  
  var sub = s.substring(7, 10);
  console.log(sub);
  
  var i = 2344;
  console.log(i);
  if (i % 2 === 0) {
      console.log("i is even");
  } else {
      console.log("i is odd");
  }
  
  
  