The statement:
{ a: 1 };
is not an object literal. It’s a block statement with one labeled expression in it. It’s valid.
This:
{ "a": 1 };
is a syntax error because it’s just not parseable. The quoted “a” starts an expression statement inside the block, but then the next token after the string is a colon, and there’s no expression form that looks like an expression followed by a colon.
Now:
var x = { "a": 1 };
works because the “{” is not interpreted as the start of a block statement. That statement starts with var, so it’s a variable declaration. Within the expression on the right side of the “=” token, the only thing that a “{” can mean is the start of an object literal. Similarly, note that:
({ "a": 1 });
is OK because the opening parenthesis makes the parser expect a nested subexpression, so again the “{” unambiguously means that it’s the start of an object literal.