簡單來說，
!! 可以判斷跟在他後面式子的真/假 (true/false) ，並回傳判斷的結果。
式子是 falsy value 的話，也會回傳 false
像是 上一篇 有提到的這幾個值
-
undefined,null,NaN,false,0,''/""
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