Question

An interesting thing I've never seen before was posted in another question. They had something like:

var i = + +1;

They thought the extra + converted it to a string, but they were simply adding to a string which is what caused it to convert.

This, however, lead me to the question: what is going on here?

I would have actually expected that to be a compiler error, but JavaScript (at least in Chrome) is just fine with it... it just basically does nothing.

I created a little JSFiddle to demonstrate: Demo

var i = 5;
var j = + +i;
document.body.innerHTML = i === j ? 'Same' : 'Different';

Anyone know what's actually occurring and what JavaScript is doing with this process?

I thought maybe it would treat it like ++i, but i doesn't increment, and you can even do it with a value (e.g., + +5), which you can't do with ++ (e.g., ++5 is a reference error).

Spacing also doesn't affect it (e.g., + + 1 and + +1 are the same).

My best guess is it's essentially treating them as positive/negative signs and putting them together. It looks like 1 == - -1 and -1 == + -1, but that is just so weird.

Is this just a quirky behavior, or is it documented in a standard somewhere?

Answer 1

通过AST Explorer发表您的声明，我们可以看到我们在这里得到的是两个嵌套的一元表达式，带有一元+运算符。

这是由+和+i组成的一元表达式，+i本身是由+和i组成的一元表达式。

使用一元+运算符的一元表达式将表达式部分转换为数字。所以你实际上是将i转换为数字，然后再将其结果转换为数字（这是一个无操作）。

为了完成，它可以在您添加的级别上运行：

var i = 5;
console.log(+ + + + + +i); // 5
console.log(i); // still 5

Answer 2

它在规范中。

通过挖掘，我们可以从§14.6.2.2看到增量和减量运算符在一元运算符之前（并且应该是首选）列出。因此，优先权本身并不能解释这一点。

查看the punctuation table in §11.7，我们可以看到规范中++（运算符）的每个实例都显示了两个实例，没有空格。这不是决定性的，直到你检查......

The whitespace rules in §11.2，具体来说：

空格代码点可能出现在StringLiteral，RegularExpressionLiteral，Template或TemplateSubstitutionTail中，它们被视为构成文字值一部分的重要代码点。它们也可能出现在注释中，但不能出现在任何其他类型的令牌中。

JS不允许任意空格中间操作符。

PegJS和Esprima中的JS语法证实了这一点，匹配文字的双字符串++。

Answer 3

对我来说非常清楚;

var a = +3;
var b = +a; // same as a, could be -a, for instance
var c = + +a; // same as above, same as +(+a)

Answer 4

如果您执行++variable，则javascript解释程序会将其视为increment operator。

如果您执行+ +variable，则javascript解释程序会将其视为Unary plus，将值强制转换为数字，两次。

所以

var a = 1;
var b = +a;
var c = +b;

console.log(c);// still 1

与

相同

var c = + +1;

所以简单的答案就是两个加号不能被一个空格分开来解释为增量，这个空间使得解释器看到两个单独的空格，这就是它的真正含义

Answer 5

+运算符转换为数字，两个+运算符之间有空格，不做任何额外的操作。

Answer 6

尽管它看起来非常相似，但+ +和++与AST解释器完全不同。这同样适用于令牌分离：varfoo与var foo不同。

在表达式+ + +i中，每个+都被视为不同的一元运算符，它只是将您的变量转换为数字。对于增量操作，即++，在+和变量令牌之间不允许使用空格。在下面的示例中，最后一行无效：

var x = "4";
console.log(+ + +x);
console.log(+ + ++x);
console.log(+ ++ +x);

What does JavaScript interpret `+ +i` as?

6 个答案: