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.
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.
Share Improve this question asked Mar 12, 2013 at 3:17 CodistCodist 1,2082 gold badges12 silver badges29 bronze badges 8-
var bar = 'foobar';can't work in your case? – Raptor Commented Mar 12, 2013 at 3:19 - 2 That's the way the syntax works. – SLaks Commented Mar 12, 2013 at 3:20
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obj['property']syntax requires it. – Jared Farrish Commented Mar 12, 2013 at 3:21 - 1 Your example doesn't have a numeric literal, and both the given examples are valid property names. I personally don't understand what you're after. – Phix Commented Mar 12, 2013 at 3:21
- 1 I remend reading "JavaScript - the Good Parts". After a couple chapters you'll understand stuff like this pletely. – jahroy Commented Mar 12, 2013 at 3:36
4 Answers
Reset to default 8In 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/ecma-262/5.1/#sec-11.1.5
So, for example:
var obj = {
foo: true, // the name of this property bees 'foo'
'bar': true, // the name of this property bees 'bar'
123: true // the name of this property bees '123'
};
You can even use the empty string as a property name:
var obj = {
'': 'foo'
};
obj[''] // 'foo'
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
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.
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.