class WTF {
  ...
  @memoize
  getSerious_str() {
    ...
  }
}

--- or if ES7 not available-to/trusted-by you ---

WTF.prototype.getSerious_str = memoize(function getActualSerious_str() {
  ...
});

【 在 shaolin (漫漫当爹路) 的大作中提到: 】
: var serious_str = '';
: this.getSerious_str = function(){
:     return serious_str || (serious_str = this.getActalSerious_str());
: ...................
--
FROM 180.173.119.*