JavaScript三等於和三變量比較

Question

有人可以解釋這個嗎？

1 == 1  //true, as expected 
1 === 1  //true, as expected 
1 == 1 == 1 //true, as expected 
1 == 1 == 2 //false, as expected 
1 === 1 === 2 //false, as expected 
1 === 1 === 1 //false? <-- 

也有布爾邏輯，超過兩個號碼以這種方式進行比較的名稱（我把它叫做「三變的比較」，但我認爲這會是錯誤的...）

Answer 1

該表達式：

1 === 1 === 1 

被評價爲：

(1 === 1) === 1 

評估圓括號內的表達式後：

true === 1 

並且該表達式在邏輯上是錯誤的。下面的表達式返回true如同預期：

1 === 1 === true 

Answer 2

平等是一個left-to-right precedence operation。

所以：

1 == 1 == 1 
true == 1 
true 

和：

1 === 1 === 1 
true === 1 
false // because triple-equals checks type as well