CODEWITHSUNDEEP
javascriptobjectliteralsstring-literals
Codist
Codist

Reputation: 1208

Property names in JavaScript

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.

Upvotes: 0

Views: 1956

Answers (4)

Explosion Pills
Explosion Pills

Reputation: 191779

The object literal syntax you are using is just part of JavaScript's syntax. You can use numeric or string literal as a property name as well as any valid variable name as a property name. Note that invalid variable names must be wrapped in quotes, but can still be property names (numeric literals being an exception).

That is, you can have obj = {'"': value}, i.e. a quote, as a valid object property name. However, if you left off the apostrophes there it would be a syntax error.

The variable name syntax, e.g. {nameWithoutQuotes: "value"} is allowed, as far as I can tell, for convenience. It has no special meaning and is treated as if it were a string literal property name. It would look very odd to have " everywhere in an object literal definition, and it also makes sense when using similar accessor syntax. For example:

obj = {"with quotes": "q", withoutQuotes: "x"};
obj["with quotes"];
obj.withoutQuotes;

Note that the method of access with a property name that requires quotes also requires quotes whereas when quotes are not required access can be done without them.

As for why "obj" = "string" is not allowed, other than the fact that it is invalid syntax, that is because the "obj" literal does not create a reference in memory that can be assigned to. The obj = {} notation creates a reference that is stored in obj and memory is allocated for each of its properties as described by the literal syntax. You could make a similar statement about obj = "string";

It may also be worth nothing that the quotes cannot be omitted from a JSON string for property names. Many parsers will not allow it.

Upvotes: 2

Šime Vidas
Šime Vidas

Reputation: 185963

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'

Upvotes: 8

vittore
vittore

Reputation: 17589

In order to see that it is absolutely valid you just have to recall that objects are associative arrays , ie 

 foo.bar === foo['bar']

In this case keys of associative array are any strings.

Upvotes: 1

ktm5124
ktm5124

Reputation: 12123

JavaScript object literals are hashmap implementations: i.e., key-value pairs. The keys can be represented either in quotes or without quotes.

That said, if you want to access a property as a string, you use the syntax below:

obj[str]

But if you want to access a property by its name, you use

obj.name

Upvotes: 2

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