为什么{} + []在Javascript中返回0？ [英] Why does {} + [] return 0 in Javascript?

问题描述

可能重复：

我知道当 [] 被强制转换为字符串时它会返回空字符串（），当 {} 被强制转换为字符串时，它返回[object Object]。

I know that when [] is coerced to a string it returns the empty string (""), and when {} is coerced to a string it returns "[object Object]".

当我运行 [] + {} 在我的浏览器的Javascript控制台中，它会按照我的预期返回：

When I run [] + {} in my browser's Javascript console, it returns as I would expect:

>> [] + {}
"[object Object]"

但是当我运行 {} + [] ，它会返回一个完全意外的值：

But when I run {} + [], it returns a completely unexpected value:

>> {} + []
0

什么可能导致它返回 0 ？

推荐答案

当有 {在语句的开头，它将被解释为一个块，它可能包含零个或多个语句。其中没有语句的块将具有空的延续值。

When there is a { at the beginning of a statement, it will be interpreted as a block, which may contain zero or more statements. An block with no statements in it will have an empty continuation value.

换句话说，在这种情况下， {} 被解释为空代码块。

In other words, in this case, {} is interpreted as an empty code block.

语句在结束括号} 之后结束，这意味着接下来的三个字符 + [] 包含他们自己的声明。

The statement ends after the ending brace }, which means that the next three characters +[] comprise a statement of their own.

在表达式或语句的开头， + 是一元加号运算符，它将其操作数强制转换为数字。

At the beginning of an expression or statement, + is the unary plus operator, which coerces its operand to a number.

所以 + [] 与 Number（[]）相同，其计算结果为 0 。

So +[] is the same as Number([]), which evaluates to 0.

简而言之， {} + [] 是一个空代码块，后跟一个数组强迫一个号码。

In short, {} + [] is an empty code block followed by an array coerced to a number.

所有这些都说，如果你评价 {} + [] 在表达式中，它会返回您的预期：

All that said, if you evaluate {} + [] inside an expression, it will return what you expect:

>> ({} + []) 
"[object Object]" 

另一件有趣的事情是你不能用对象文字开始一个语句，因为解释器会尝试将它解析为一个语句。这样做

Another interesting thing is that you cannot begin a statement with an object literal because the interpreter will try to parse it as a statement. Doing this

{ "object": "literal" };

会抛出语法错误。

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