为什么直接在Object文字上访问属性会抛出一个SyntaxError？

Question

When trying to access the property a of the object {}

{}.a

I get the error

SyntaxError: Unexpected token .

With parens all is fine:

({}).a

Why do I get an error in the fist place? Is there ambiguity?

Answer 1

The curly braces are interpreted as a block statement , not as an object literal. You cannot begin an expression statement with a left curly brace.

The specification states:

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 .

Source: http://es5.github.com/x12.html#x12.4

Answer 2

the {} are there to build the object. usually you first assign the new object to a variable.

var o = {
    a: "b"
};

console.log(o.a);

but this is also possible:

console.log({
    a: "b"
}.a);