JavaScript grammar: indexing object literals is syntactically forbidden?

Question

In Chrome 63.0 and Firefox 58.0, it seems that adding an index to the object literal does not allow the object to be parsed.

{"a":"b"} 17:37:32.246 {a: "b"} {"a":"b"}["a"] 17:37:36.578 VM288:1 Uncaught SyntaxError: Unexpected token : 

Can someone explain why the parser doesn't parse this the way I do? (I think there should be a mistake in my specification implementation ...) It seems that this is not a parsing of the object.

An environment in parentheses results in a syntactically correct expression:

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

Answer 1

This is because the console evaluates {} as a block instead of an object literal .

This question is similar to "Defining a JavaScript object in the console" and "Is the curly brace of an object notation valid in any expression?" on stackoverflow.

The key part from the specification :

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

NOTE. ExpressionStatement cannot begin with opening curly because it can make it ambiguous with Block . In addition, ExpressionStatement cannot begin with the keyword function , because that can make it ambiguous with FunctionDeclaration.

Answer 2

{"a":"b"}["a"] - {} is defined as a body or block of braces - not an object. Let's see an example in an if , you can have something like this

 if(condition) { "a": "b" } 

So you get a message like "a" : "b" , which is not valid.

({"a":"b"})["a"] - () evaluates the expression inside it, so {} is defined as an object