为什么加括号可以防止错误? [英] Why does adding parentheses prevent an error?

问题描述

为什么在chrome控制台中编写 {}.key = 0 时会出现错误:

Why is it when I write {}.key = 0 in the chrome console I get an error:

> {}.key = 0
> Uncaught SyntaxError: Unexpected token .

但是当我将上面的表达式封装在括号(())中时，我没有收到错误:

But when I encapsulate the above expression in parentheses (( )) I get no error:

> ({}.key = 0)
> 0

这到底是怎么回事?我会以为在第一种情况下遇到的相同错误仍然适用于第二种情况?

What exactly is going on here? I would have thought the same error I got in the first scenario still applied to the second?

控制台输出图像:

推荐答案

{} 在JavaScript语法中已重载.它们用于(语句的)块和对象文字.规则是:如果在语句的开头出现 {，则将其解析为块；否则，将其解析为块.否则它是对象文字.

{ } are overloaded in JavaScript syntax. They're used for both blocks (of statements) and object literals. The rule is: If a { appears at the start of a statement, it is parsed as a block; otherwise it is an object literal.

在 {}.key 中， {出现在语句的开头.解析为

In {}.key the { appears at the start of the statement. It parses as

{
    // this is an empty block
}
.key  // syntax error here

在 {之前添加任何令牌(例如())可以将其解析为对象文字，例如， 42，{} .key = 0 也可以.

Adding any token before { (such as () makes it parse as an object literal. For example, 42, {}.key = 0 would also work.

这篇关于为什么加括号可以防止错误?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！