爲什麼（obj ['arr'] || obj ['arr'] = []）.push（5）;給我一個「分配中無效的左側」錯誤？

Question

我想寫看中

"Push on the array if the array exists; else, initialize the array and then push"

實施。我在我的Chrome控制檯中寫道

var obj = new Object(); 
(obj['arr'] || obj['arr'] = []).push(5); 

我有理由相信這應該起作用。

據道格拉斯Crockford的書的JavaScript：好的部分，

The || operator produces the value of its first operand if the first operand is truthy. Otherwise, it produces the value of the second operand.

如果鍵入obj['arr']到我的控制檯，返回的值是undefined，這是falsy，不truthy。如果我輸入obj['arr'] = []到我的控制檯，返回的值是數組obj['arr'] = []，這是truthy。因此，聲明

(obj['arr'] || obj['arr'] = []).push(5); 

應相當於

(obj['arr'] = []).push(5) 

預期返回obj['arr'] = [5]。

所以後來爲什麼會在Chrome瀏覽器

Uncaught ReferenceError: Invalid left-hand side in assignment(…)

錯誤，當我寫(obj['arr'] || obj['arr'] = []).push(5); ?????操作

Answer 1

爲了......你需要括號：

(obj['arr'] || (obj['arr'] = [])).push(5);