old_contents: // 'use strict';
  
  // my js compiler currently didn't support arrow function;
  var fn = x => x * x;
  console.log(fn(3));
  
  
  
new_contents: // 'use strict';
  
  // node -v = v5.9.1
  
  // my js compiler currently didn't support arrow function;
  var fn = x => x * x;
  console.log(fn(3));
  
  
  var square = (x, y) => x * x + y * y;
  
  
  console.log(square(3, 4));
  
  
  var obj = {
      birth: 1990,
      getAge: function() {
          var b = this.birth;
          var fn = () => new Date().getFullYear() - this.birth;
          return fn();
      }
  }
  
  console.log(obj.getAge());
  
  
  
subject: add 14-arrow.js to learn js arrow function