問題描述

一個有意思的問題：

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

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

這里其實涉及到的就是[]運算符 和.運算符 相關知識。

附上相關規(guī)則和網(wǎng)址,你們自己研究吧：

MemberExpression : MemberExpression [ Expression ]

Let baseReference be the result of evaluating MemberExpression.

Let baseValue be GetValue(baseReference).

ReturnIfAbrupt(baseValue).

Let propertyNameReference be the result of evaluating Expression.

Let propertyNameValue be GetValue(propertyNameReference).

ReturnIfAbrupt(propertyNameValue).

Let bv be RequireObjectCoercible(baseValue).

ReturnIfAbrupt(bv).

Let propertyKey be ToPropertyKey(propertyNameValue).

ReturnIfAbrupt(propertyKey).

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.

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

Let baseReference be the result of evaluating MemberExpression.

Let baseValue be GetValue(baseReference).

ReturnIfAbrupt(baseValue).

Let bv be RequireObjectCoercible(baseValue).

ReturnIfAbrupt(bv).

Let propertyNameString be StringValue of IdentifierName

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.

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

問題解答

其實就是個 Object toString 的問題。