if表達式中還可以有一個可選項，即else。如果條件返回false的話，我們也可以執行后面else子句中的代碼塊。 ``` >>> var result = '',a = 3; undefined >>> if(a > 2){ ... result = 'a is greater than 2'; ... }else{ ... result = 'a is not greater than 2'; ... } a is greater than 2 ``` 我們還可以在if和else之間插入任意個else if子句，例如： ``` >>> var result = '',a = 3; undefined >>> if(a > 2 || a < -2){ ... result = 'a is not between -2 and 2'; ... }else if(a === 0 && b === 0){ ... result = 'both a and b are zeros'; ... }else if(a === b){ ... result = 'a not b are equal'; ... }else{ ... result = 'i give up'; ... } a is not between -2 and 2 ``` 還可以在當前的if代碼塊中在內嵌一個新的條件語句。