javascript - js物件 屬性的存取和創建

Question

一個有意思的問題：

1. var a = new Object();  
   var b = new Object();  
   var c = new Object();
     
   c[a] = a;  
   c[b] = b;
     
   console.log(c[a] === a); //输出什么?  ---> false
   console.log(c[b] === b); //输出什么?  ---> true

2. var a = new Object();  
   var b = new Object();  
   var c = new Object();
     
   c.a=a;  
   c.b=b;
     
   console.log(c.a === a); //输出什么?  ---> true
   console.log(c.b === b); //输出什么?  ---> true

這裡其實牽涉到的就是[]運算子 和.運算子 相關知識。

附上相關規則和網址,你們自己研究吧：

MemberExpression : MemberExpression [ Expression ]

1. Let baseReference be the result of evaluating MemberExpression.

2. Let baseValue be GetValue(baseReference).

3. ReturnIfAbrupt(baseValue).

4. #Let propertyNameReference be the result of evaluating Expression.

5. Let propertyNameValue be GetValue(propertyNameReference).

6. #ReturnIfAbrupt(propertyNameValue).

7. Let bv be RequireObjectCoercible(baseValue).

8. #ReturnIfAbrupt(bv).

9. #Let propertyKey be ToPropertyKey(propertyNameValue).

10. #ReturnIfAbrupt(propertyKey).

11. If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.

12. Return a value of type Reference whose base value is bv and whose referenced name is propertyKey, and whose strict reference flag is strict.

MemberExpression : MemberExpression . IdentifierName

1. #Let baseReference be the result of evaluating MemberExpression.

2. Let baseValue be GetValue(baseReference).

3. ReturnIfAbrupt(baseValue).

4. #Let bv be RequireObjectCoercible(baseValue).

5. #ReturnIfAbrupt(bv).

6. #Let propertyNameString be StringValue of IdentifierName

7. #If the code matched by the syntactic production that is being evaluated is strict mode code, let strict be true, else let strict be false.

8. Return a value of type Reference whose base value is bv and whose referenced name is propertyNameString, and whose strict reference flag is strict.

CallExpression : CallExpression [ Expression ]

• Is evaluated in exactly the same manner as MemberExpression : MemberExpression [ Expression ] except that the contained CallExpression is evaluated in step 1.

CallExpression : CallExpression . IdentifierName

#######Is evaluated in exactly the same manner as MemberExpression : MemberExpression . IdentifierName except that the contained CallExpression is evaluated in step 1.######### ### ###ECMAScript 2015 #sec-property-accessors#####

Answer 1

其實就是個 Object toString 的問題。