javascript - 为什么 "typeof + ' '"returns '号'？

Question

让我们尝试在控制台中输入以下代码:

typeof + ''

这会返回“number”，而没有参数的 typeof 本身会抛出错误。为什么？

Answer 1

unary plus operator在字符串上调用内部 ToNumber 算法。 +'' === 0

typeof + ''
// The `typeof` operator has the same operator precedence than the unary plus operator,
// but these are evaluated from right to left so your expression is interpreted as:
typeof (+ '')
// which evaluates to:
typeof 0
"number"

与 parseInt 不同，+ 运算符调用的内部 ToNumber 算法计算空字符串(以及仅包含空白的字符串)到数字 0。从 ToNumber spec 向下滚动一点:

A StringNumericLiteral that is empty or contains only white space is converted to +0.

这是在控制台上的快速检查:

>>> +''
<<< 0
>>> +' '
<<< 0
>>> +'\t\r\n'
<<< 0
//parseInt with any of the strings above will return NaN

供引用:

• Whats the significant use of Unary Plus and Minus operators? (所以)
• ES5 #9.3.1 ToNumber Applied to the String Type (ES5 规范)
• Operator Precedence (MDN)