【JS迷你书】类型转换之拆箱操作

本文的主题关注相反的操作：对引用类型进行那些基本类型“才该有的”操作时会怎样？即，“拆箱操作”。

比如，如下的代码并不会报错：

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

对普通对象进行减法操作时，对象需要转化为数字类型。《Ecma-262 Edition 5.1》第11.6.2节对减法操作符规范如下：

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 步比较关键，调用了内部操作 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，处理Object时，经历两步：1. ToPrimitive。2. ToNumber。

而 ToPrimitive 操作正与 ToObject 相对，表示转化为基本类型：

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.

最后一行说，对象转化为基本类型时，是获取的对象的默认值。使用的是内部[[DefaultValue]](hint)，规范原文引用如下（补充：本文中的英文都可以不看的，我都会仔细说明的）：

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.

上述算法是说，根据 hint 值采取不同的处理方式，比如 hint 是 String 时，优先调用对象的 toString 方法，如果返回值是基本类型值，返回该值，否则调用对象的 valueOf 方法，如果返回值是基本类型值，返回该值。否则报错。

而 hint 是 Number 时，顺序是反过来的，优先调用 valueOf，如果其返回值不是基本类型，再调用 toString。另外，除了日期对象外，如果没传 hint 的话，其默认值是 Number，因此 JS 中类型转化时，更偏爱 Number。

下面我们举几个例子看看：

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

这里使用的是减法操作，此时 hint 是 Number，因此先调用对象 a 的 valueOf 方法，其返回值 "30" 是字符串类型，是基本类型。因此 a - 5 变成了 "30" - 5。

再看：

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

对象 a，其方法 valueOf 不是函数，因而看其 toString 方法，而该方法返回的是一个空对象，不是基本类型。因而报错。

再如：

var o = {
  toString() {
    return "now is: "
  },
  valueOf: function() {
    return "时间是："
  }
};
var d = new Date();
console.log(o + d); // 时间是：Mon May 06 2019 13:56:39 GMT+0800 (中国标准时间)

这里使用了加法操作：

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 步直接获取加号两边的基本类型。此时没有都传递 hint，o 是普通对象，因此默认 hint 是 Number，使用的 valueOf 的返回值。而 d 是日期对象，默认 hint 是 String，优先调用的是其 toString 方法。然后根据第 7 步，采用的是字符串拼接方法。

加法操作，这里再举一例：

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

这里不过多解释了。

ToPrimitive 除了在四则运算中大量用到外，关系运算中也经常使用。比如 == 操作。其他类型转换等相关知识留给后续文章吧。

至此，“拆箱”已经说完了。

本文完。

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