本文的主題關注相反的操作：對引用類型進行那些基本類型“才該有的”操作時會怎樣？即，“拆箱操作”。

比如，如下的代碼并不會報錯：

var?a??=?1;
var?b?=?{};
console.log(a?-?b);

對普通對象進行減法操作時，對象需要轉化為數字類型。《Ecma-262 Edition 5.1》第11.6.2節對減法操作符規范如下：

The production AdditiveExpression : AdditiveExpression - MultiplicativeExpression is evaluated as follows:

1. Let lref be the result of evaluating AdditiveExpression.

2. Let lval be GetValue(lref).

3. Let rref be the result of evaluating MultiplicativeExpression.

4. Let rval be GetValue(rref).

5. Let lnum be ToNumber(lval).

6. Let rnum be ToNumber(rval).

7. Return the result of applying the subtraction operation to lnum and rnum. See the note below 11.6.3.

上述操作中第 5、6 步比較關鍵，調用了內部操作 ToNumber:

Argument Type	Result
Undefined	NaN
Null	+0
Boolean	The result is 1 if the argument is true. The result is +0 if the argument is false.
Number	The result equals the input argument (no conversion).
String	See grammar and note below.
Object	Apply the following steps: 1. Let primValue be ToPrimitive(input argument, hint Number). 2. Return ToNumber(primValue).

最后一行，處理Object時，經歷兩步：1. ToPrimitive。2. ToNumber。

而 ToPrimitive 操作正與 ToObject 相對，表示轉化為基本類型：

Input Type	Result
Undefined	The result equals the input argument (no conversion).
Null	The result equals the input argument (no conversion).
Boolean	The result equals the input argument (no conversion).
Number	The result equals the input argument (no conversion).
String	The result equals the input argument (no conversion).
Object	Return a default value for the Object. The default value of an object is retrieved by calling the [[DefaultValue]] internal method of the object, passing the optional hint PreferredType. The behaviour of the [[DefaultValue]] internal method is defined by this specification for all native ECMAScript objects in 8.12.8.

最后一行說，對象轉化為基本類型時，是獲取的對象的默認值。使用的是內部[[DefaultValue]](hint)，規范原文引用如下（補充：本文中的英文都可以不看的，我都會仔細說明的）：

When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:

1. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".

2. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.

   2. If str is a primitive value, return str.

3. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".

4. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.

   2. If val is a primitive value, return val.

5. Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:

1. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".

2. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.

   2. If val is a primitive value, return val.

3. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".

4. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.

   2. If str is a primitive value, return str.
      Throw a TypeError exception.

5. When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint were String.

When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint were String.

上述算法是說，根據 hint 值采取不同的處理方式，比如 hint 是 String 時，優先調用對象的 toString 方法，如果返回值是基本類型值，返回該值，否則調用對象的 valueOf 方法，如果返回值是基本類型值，返回該值。否則報錯。

而 hint 是 Number 時，順序是反過來的，優先調用 valueOf，如果其返回值不是基本類型，再調用 toString。另外，除了日期對象外，如果沒傳 hint 的話，其默認值是 Number，因此 JS 中類型轉化時，更偏愛 Number。

下面我們舉幾個例子看看：

var?a?=?{
??toString()?{
????return?3
??},
??valueOf()?{
????return?"30"
??}
};
console.log(a?-?5);?//?25

這里使用的是減法操作，此時 hint 是 Number，因此先調用對象 a 的 valueOf 方法，其返回值 "30" 是字符串類型，是基本類型。因此 a - 5 變成了 "30" - 5。

再看：

var?a?=?{
??toString()?{
????return?{}
??},
??valueOf:?null
};
console.log(a?-?5);?//?Uncaught?TypeError:?Cannot?convert?object?to?primitive?value

對象 a，其方法 valueOf 不是函數，因而看其 toString 方法，而該方法返回的是一個空對象，不是基本類型。因而報錯。

再如：

var?o?=?{
??toString()?{
????return?"now?is:?"
??},
??valueOf:?function()?{
????return?"時間是："
??}
};
var?d?=?new?Date();
console.log(o?+?d);?//?時間是：Mon?May?06?2019?13:56:39?GMT+0800?(中國標準時間)

這里使用了加法操作：

The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows:

1. Let lref be the result of evaluating AdditiveExpression.

2. Let lval be GetValue(lref).

3. Let rref be the result of evaluating MultiplicativeExpression.

4. Let rval be GetValue(rref).

5. Let lprim be ToPrimitive(lval).

6. Let rprim be ToPrimitive(rval).

7. If Type(lprim) is String or Type(rprim) is String, then
   Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

8. Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See the Note below 11.6.3.

其中第 5、6 步直接獲取加號兩邊的基本類型。此時沒有都傳遞 hint，o 是普通對象，因此默認 hint 是 Number，使用的 valueOf 的返回值。而 d 是日期對象，默認 hint 是 String，優先調用的是其 toString 方法。然后根據第 7 步，采用的是字符串拼接方法。

加法操作，這里再舉一例：

var?o?=?{
???toString:?function()?{
????return?2
??}
};
console.log(o?+?o);?//?4

這里不過多解釋了。

ToPrimitive 除了在四則運算中大量用到外，關系運算中也經常使用。比如 == 操作。其他類型轉換等相關知識留給后續文章吧。

至此，“拆箱”已經說完了。

本文完。

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