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:
{ } 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.