将JavaScript语句括在括号中有什么意义？ [英] What is the point of wrapping JavaScript statements in parentheses?

问题描述

我发现用括号括起来的不同语句将返回最后一个：

I have discovered that wrapping different statements in parentheses will return the last one:

(34892,47691876297,2000)                => 2000
('test',73,document.createElement('p')) => <p></p>

而且我还发现所有语句仍然被执行：

And I also found out that all the statements are executed anyway:

(console.log('test'), console.log('test2'), console.log('test3'), 6)

Will log：

test
test2
test3

结果为6。

但是，我也发现某些语句无法使用：

However, I've also found that some statements can't be used:

(throw new Error(), 10)         => SyntaxError: Unexpected token throw
(if (1) console.log('test'), 5) => SyntaxError: Unexpected token if

，则出现意外的标记那么，此括号逗号表示法的意义是什么？您可以轻松执行所有语句，然后使用最后一条语句的值。这个是来做什么的？我使用不正确吗？

So, what is the point of this parenthesis-comma notation? You could easily execute all the statements and then use the last statement's value. What is this for? Am I using it incorrectly?

推荐答案

这是逗号运算符：）

它使您可以从左到右评估表达式，并返回最后一个操作数的结果（在您的情况下，该结果没有存储在任何地方，并且完全有效）。

It lets you evaluate expressions from left to right, returning the last operand's result (which, in your case, isn't stored anywhere, and is perfectly valid).

参考：

• http://www.ecma-international.org/ecma-262/5.1/#sec-11.14


• https://developer.mozilla.org/zh-CN/docs/JavaScript/参考/操作员/ Comma_Operator

• http://www.ecma-international.org/ecma-262/5.1/#sec-11.14
• https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/Comma_Operator

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