window.a 与 window[a]区别

 1 obj = {}
 2 
 3 window[a]    // a is not defined
 4 obj[b]    // b is not defined
 5 obj[a]    // a is not defined
 6 
 7 window.a = 'dpc'
 8 obj.b = 'dpc'
 9 
10 window[a]    // undefined
11 obj[b]    // b is not defined
12 obj[a]    // undefined
13 
14 但obj['a']访问不到值
15 
16 window['a']    // dpc
17 obj['a']    // undefined

疑窦:

window.a 与 window[a] 有什么含义怎么用什么时候可以互换

11 obj[b]    // b is not defined 为什么
12 obj[a]    // undefined 获得已定义变量为啥obj['a']取不到值
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