为什么添加括号可以防止错误？

Question

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

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

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

> ({}.key = 0)
> 0

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

Image of console output :

Answer 1

{ } 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.

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

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

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