code: function _findChangedCharacters(oldValue, value) {
          if (oldValue === value) {
              return undefined;
          }
          var length = oldValue.length;
          var index = 0;
  
          // find the first character that changed
          var i;
          for (i = 0; i < length; i++) {
              if (value[i] !== oldValue[i]) {
                  break;
              }
          }
          index += i;
          value = value.substr(i);
          length -= i;
  
          // find the last character that changed
          for (i = 0; i < length; i++) {
              if (value[value.length - 1 - i] !== oldValue[oldValue.length - 1 - i]) {
                  break;
              }
          }
          length -= i;
          value = value.substr(0, value.length - i);
  
          return { from: index, to: index + length, text: value };
      }
docs: Find changed characters
  @param {string} old value
  @param {string} changed value
  @return {from, to, text}