Property names in JavaScript

Question

I have a question that has been really bugging me for quite a while and I cannot seem to find any resources that cover the topic. How can property names in JavaScript be string literals or numeric literals?

var obj = {
    "bar": "foobar",
    "foo": function() { return bar; }
}

This topic has bugged me ever since I learned about it a few years ago. I don't know where to get more in-depth information or what this is even called. I not confused on how this is set up as I know that a new object is being creating with members, bar & foo and then its assigned to the obj variable.

You cannot create a variable like var "bar" = "foobar"; because you'll get a syntax error. How is it valid for object literals? Any help on this would be very much appreciated.

Answer 1

In JavaScript, property names are String values - any String values. That's just how the language is specified.

The relevant production:

PropertyName :  
    IdentifierName
    StringLiteral
    NumericLiteral

If an identifier, or a numeric literal is supplied, it is converted to a string value (SV).

See: http://ecma-international.org/ecma-262/5.1/#sec-11.1.5

So, for example:

var obj = {
    foo: true, // the name of this property becomes 'foo'
    'bar': true, // the name of this property becomes 'bar'
    123: true // the name of this property becomes '123'
};

---

You can even use the empty string as a property name:

var obj = {
    '': 'foo'
};

obj[''] // 'foo'