Rules for unquoted JavaScript Object Literal Keys?

Question

In JavaScript, you can define an object like this:

var d = {1: 'test'};

and I can set a key with a negative number index like this:

d[-1] = 'test2';

but if I try to use a negative number in the literal initialization, I get an error:

var d = {1: 'test', -1: 'test2'};
Uncaught SyntaxError: Unexpected token -

Why is this? Why can't I use a literal negative number as a key to an object? Is there a workaround that allows me to initialize it as a literal. I know I could use strings instead, but I want to use integers.

Glutelin

use quotes around property name like '-1':'test2' – Chameleon

Answer 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.

Freedafreedman

I think you meant "unary - operator." – Rummage

Your tool claims that -1 and +1 can be used as unquoted property names, even though they don't actually work. – Honna

@StuartCook Thanks for the report! Fixed: mothereff.in/js-properties#-1 (You might need to refresh to update the appcache.) – Freedafreedman

@MathiasBynens, Are there any limits if the key is quoted? – Mcmorris

@Mcmorris As long as you use a valid string, there are no such limits at the language-level, no. – Freedafreedman

Answer 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.

Rummage

Answer 3

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

Monzon

But I can still assign it by key later, so is this a limitation of the literal syntax that keys have to be valid variable names? – Glutelin

Correct. its just a representation of hte object. – Monzon

@jterrace, @Daniel: Property names in object initializers can be string literals and numeric literals as well (not only identifier names). But apparently, -1 is not a numeric literal: es5.github.com/#x7.8.3 – Disposal

@FelixKling aha, you're right, it's a numeric literal with a unary operator prepended, of course! – Rummage

Answer 4

Change to

var d = { 1: 'test', '-1': 'test2'};

Identifiers, that are keywords or can not be a normal identifier for other reasons, can still be used if surrounded by quotes.

Fakery

1 cannot be used as identifier either... just saying ;) – Disposal

Answer 5

I'm not sure exactly why, but you have to quote the negative number in the object initializer:

var d = {1: 'test', '-1': 'test2'};

All object keys are treated as strings internally, so you can address this key using either:

d[-1] or d['-1']

Dynamite

Answer 6

When the key contains certain characters, you have to put it in quotes like this:

var d = {"1": 'test', "-1": 'test2'};

Here's a pretty good reference on what characters are allowed without the quoting and it has a specific section about object property declaration too: http://asenbozhilov.com/articles/identifiers-en.html#identifier_start.

Londalondon