Rules for unquoted JavaScript Object Literal Keys?

Solution 1:

From Unquoted property names / object keys in JavaScript, my write-up on the subject:

Quotes can only be omitted if the property name is a numeric literal or a valid identifier name.

[…]

Bracket notation can safely be used for all property names.

[…]

Dot notation can only be used when the property name is a valid identifier name.

-1 is not a numeric literal, it’s a unary - operator followed by a numeric literal (1).

I also made a tool that will tell you if any given property name can be used without quotes and/or with dot notation. Try it at mothereff.in/js-properties.

Solution 2:

Interesting question.

The thing is, there's no difference between typing

var d = {24: 'foo'};

and

var d = {"24": 'foo'};

You can verify this by doing:

var d = {24:'foo', "24":'bar'};

Notice that it only has one "24" property (and fails in strict mode).

So while this doesn't explain why you can't do -1 without quotes, hopefully it does explain that "-1" is just as good.

Interestingly, unquoted fractional numbers seem to work fine.

Edit: Felix Kling explains why it doesn't work in a comment on another answer. -1 isn't a numeric literal, it's an expression with a numeric literal and a unary - operator -- therefore it's not suitable as an object key.

Solution 3:

Its because -1 isn't a valid variable identifier.