我看过这个很有趣 screencast昨天关于 Ruby 和 Javascript 等语言的一些奇怪之处，那个人表明:

[] + [] -> ""  // returns empty string

不是很明显...

我决定去 ECMAscript language specification获取更多信息。我从 + 运算符实现(第 75 页)开始，它说:

11.6.1 The Addition operator ( + )

The addition operator either performs string concatenation or numeric addition.
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
a. 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.

NOTE1 No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given. Host objects may handle the absence of a hint in some other manner.

NOTE 2 Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the logical-or operation instead of the logical-and operation.

我的猜测是第 7 点是通过之前描述的不同评估/转换以某种方式达到的，但我无法弄清楚到底发生了什么......

有人可以对此提供更直接的解释吗？

我愿意更好地理解这一切，以便尝试回答原因:

[] + {} -> [object Object]
{} + [] -> 0

最佳答案

1. Let lprim be ToPrimitive(lval).
2. Let rprim be ToPrimitive(rval).

由于没有向 ToPrimitive 提供提示:

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.

所以这与使用数字提示调用相同:

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, a. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and  an empty argument list. b. 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, a. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and  an empty argument list. b. If str is a primitive value, return str.
5. Throw a TypeError exception.

由于 [].valueOf() 的返回值不是原始值，它会转到返回 ""< 的="">[].toString()/.

这同样适用于对象对象，除了对象的 .toString 返回 [object Object] 而数组的 .toString 返回.join(",")

{} + []

{} 在这里不被视为一个对象，而是一个空 block 。所以运行的代码是:

{}
+[]; //Same as +[] ( Number([]) ) which is 0

要解决歧义并从 {} + [] 获得正常结果，请使用

({}) + [] 或 ({} + [])

有趣的代码:

{} ! [] //false
({}) ! [] //SyntaxError

关于javascript - 为什么 Array + Array 等于空字符串？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/8951677/