JavaScript语法:语法禁止索引对象文字? [英] JavaScript grammar: indexing object literals syntactically forbidden?

问题描述

在Chrome 63.0和Firefox 58.0中，似乎向对象文字添加索引会阻止将该对象解析为对象.

In Chrome 63.0 and Firefox 58.0, it appears that adding an index to an object literal prevents the object from being parsed as an object.

{"a":"b"}
17:37:32.246 {a: "b"}

{"a":"b"}["a"]
17:37:36.578 VM288:1 Uncaught SyntaxError: Unexpected token :

有人可以解释为什么解析器不按我的方式解析吗? (我认为规范的实现中肯定有一个错误……)似乎认为这不是在解析对象.

Can anybody explain why the parser doesn't parse this the way I do? (I think there must be a bug in my implementation of the spec...) It seems to think it's not parsing an object.

将对象括在方括号中会产生语法正确的表达式:

Surrounding the object in brackets results in syntactically correct expression:

({"a":"b"})["a"]
17:42:03.993 "b"

推荐答案

它与控制台将{}评估为 block 而不是对象文字有关. >.

It has to do with the console evaluating the {} as a block instead of an object literal.

此问题类似于在控制台中定义JavaScript对象" 和有关StackOverflow的问题.

This question is similar to "Defining a JavaScript object in console" and "Is the curly brackets object notation valid in any expression?" question on StackOverflow.

规范中的关键部分是:

ExpressionStatement:[lookahead∉{{，function}] Expression ;

ExpressionStatement: [lookahead ∉ {{, function}] Expression ;

注意: ExpressionStatement 不能以开头的卷曲开始 大括号，因为它可能与 Block 模棱两可.还有， ExpressionStatement 不能以function关键字开头，因为 可能会与 FunctionDeclaration 混淆.

NOTE: An ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

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