Question

I’m playing with JSON objects in the Chrome console and I came across this unusual behaviour:

> {a:1}
1
> {"a":1}
SyntaxError: Unexpected token :
> b={a:1}
Object
> b={"a":1}
Object

Why does the first statement return 1 instead of an object, and why does the second statement work? I expected the first two statements to return the same output as the last two statements.

Answer 1

A JavaScript expression statement can not start with a { because it would cause an ambiguity for the interpreter, which could also see it as a statement block.

So this is considered a statement block with a statement label and a numeric literal instead of an object literal:

{a:1}

But this is considered a statement block with invalid syntax, since there’s no statement that can begin with "a":

{"a":1}

But these don’t start with a {. They start with the b = so the { is considered to begin the object literal.

b = {a:1}
b = {"a":1}

12.4 Expression Statement

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.

---

This means that all you need to do is start the expression statement with a different character to make it valid.

For example, you could wrap it in parentheses, and it will work:

({"a": 1})