JavaScript BlockStatement混乱 [英] JavaScript BlockStatement confusion

问题描述

{1 + ''} + 10 // 10
{1 + ''} + '' // 0

为什么会这样？ BlockStatements返回0，为什么？

Why does this happen? Do BlockStatements return 0, and why?

推荐答案

BlockStatements是否返回0 ......？

Do BlockStatements return 0...?

不，块返回其中最后一个表达式的值。您可以通过以下方式看到：

No, blocks return the value of the last expression within them. You can see this by just doing:

{1 + 8}

...在JavaScript控制台中，将显示 9 。

...in the JavaScript console, which will show 9.

{1 +''} + 10 // 10

{1 +''} +''// 0

为什么会发生这种情况？

{1 + ''} + 10 // 10
{1 + ''} + '' // 0
Why does this happen?

因为尽管块确实返回了一个值，但是不使用该值。 {1 +''} + 10 // 10 代码被评估为两个不同的项目：

Because although the block does return a value, that value is not used. {1 + ''} + 10 // 10 code is evaluated as two distinct items:

{1 + ''} // "1"
+10      // 10

...或者用标准缩进和分号书写：

...or writing those with standard indentation and semicolons:

{
    1 + '';
}
+10;

...你看到了第二个结果，好像第一个没有在那里。 + 没有加法运算符，它是 一元 + （类似于一元 - ，但它不会更改其操作数的符号）。 +10 当然是 10 ;和 +''是 0 因为将字符串应用于字符串会将字符串转换为数字，并且数字（''）是 0 。

...and you're seeing the result of the second one, as though the first one weren't there at all. The + there isn't the addition operator, it's the unary + (similar to the unary -, but it doesn't change the sign of its operand). +10 is, of course, 10; and +'' is 0 because applying the operator to a string converts the string to a number, and Number('') is 0.

你可以证明你看到了一元 + 而不是加法运算符：

You can prove that you're seeing the unary + rather than the addition operator by trying this:

{1 + ''} * 10

......这真的是

...which is really

{
    1 + '';
}
*10;

因语法错误而失败，因为没有一元 * 。

It fails with a syntax error because there is no unary *.

正如Felix在下面的评论中指出的那样，对于你的例子中的 + 是加法运算符（在你的情况下最终会连接字符串），它必须是两个表达式之间，一个块是声明，而不是表达式。

As Felix kindly points out in the comments below, for the + in your example to be the addition operator (which would have ended up concatenating strings, in your case), it would have to be between two expressions, and a block is a statement, not an expression.

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