14

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.

jterrace
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6 Answers6

18

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.

Screenshot

Mathias Bynens
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4

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.

Dagg Nabbit
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3

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

Daniel A. White
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  • 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? – jterrace Feb 20 '12 at 19:58
  • Correct. its just a representation of hte object. – Daniel A. White Feb 20 '12 at 19:59
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    @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: http://es5.github.com/#x7.8.3 – Felix Kling Feb 20 '12 at 20:04
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    @FelixKling aha, you're right, it's a numeric literal with a unary operator prepended, of course! – Dagg Nabbit Feb 20 '12 at 20:05
3

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.

Sirko
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1

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']
1

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.

jfriend00
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