What is the difference between these two notations for Javascript objects/associative array?

Question

What is the difference between this

var person = {
    name: "Bob",
    age: "99"
};

and this?

var person = {
    "name": "Bob",
    "age": "99"
};

Or do they mean the same thing? If they do, what if I want the key to be an object? How would I specify the object as the key if name means "name"?

@ElmoVanKielmo that statement is both vague and misleading. The _string_ `"object"` is a perfectly valid property name. — Matt Ball, Jul 04 '13 at 19:51
exact duplicate of [What is the difference between object keys with quotes and without quotes?](http://stackoverflow.com/questions/4348478/what-is-the-difference-between-object-keys-with-quotes-and-without-quotes) — Bergi, Jul 04 '13 at 19:55
@MattBall, do I say _string_ `"object"` is not a valid property name? I don't even say `x = {object: "foo"};` is invalid! It is valid but the key `object` of `x` is converted implicitly to _string_ `"object"` and has nothing to do with `object` from global namespace. Misleading? Yes, using keys like `object` or `"object"` is misleading, confusing etc. And don't try to tell me that _string_ is an _object_ too. I know that. The main thing is that whatever the key is, it will be converted to _string_ if it isn't _string_ already. No way to have "object key" with custom methods and attrs. — ElmoVanKielmo, Jul 05 '13 at 07:17

score 4 · Accepted Answer · answered Jul 04 '13 at 19:48

4

There is no difference. Quotes are only necessary if you want to use a string as a property name, but that string is not a valid identifier. Further,

An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string. However, any property name that is not a valid JavaScript identifier (for example, a property name that has space or dash, or starts with a number) can only be accessed using the square bracket notation.

(source)

Object literal syntax is covered in-depth on MDN.

answered Jul 04 '13 at 19:48

Matt Ball

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Another good link I found with helpful explanations. [link](http://mathiasbynens.be/notes/javascript-properties) – Daniel Simpkins Jul 04 '13 at 19:52
Indeed. Mathias knows his stuff. – Matt Ball Jul 04 '13 at 19:53
and on most browsers, quotes are necessary if the property name is a keyword – Alnitak Jul 04 '13 at 19:54
@Alnitak keywords are a subset of strings which are not valid identifiers, but it can't hurt to be explicit. – Matt Ball Jul 04 '13 at 19:56
@MattBall, keywords are not a subset of strings which are not valid identifiers. First of all, `foo bar` is a string and is not a valid identifier and is not a keyword in any programming language I know. Furthermore, term _string_ in programming is a data type or/and class, in some languages equivalent of array of chars. `Keywords` are a subset of `words` or `tokens` - they are called `reserved words` too as they have special meaning in language grammar. But you can't even say that keyword can't be identifier - `this` is a keyword and an identifier as well in javascript. – ElmoVanKielmo Jul 05 '13 at 07:33
@ElmoVanKielmo I don't see how `foo bar` serves as a counterexample to my statement. By definition, [every keyword/reserved word in JavaScript](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Reserved_Words) is not a valid identifier. From that linked page: _"The following are keywords and may not be used as variables, functions, methods, or **object identifiers**"_ (emphasis added) – Matt Ball Jul 05 '13 at 15:37
@MattBall, I say you are using _string_ term in wrong context. – ElmoVanKielmo Jul 05 '13 at 15:41

score 0 · Answer 2 · answered Jul 04 '13 at 19:48

0

They are equivalent in this case, but the quoted version allows you to use keys that are not valid JS identifiers. For example, this does not work:

{ -test: 42 }

while this does:

{ "-test": 42 }

You cannot specify an object as the key no matter what.

answered Jul 04 '13 at 19:48

Jon

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@akonsu: `console.log(h)` to see what happened. Not what you thought. – Jon Jul 04 '13 at 19:49
2

You can specify _anything_ as a key, but it will be converted to a string first. – Matt Ball Jul 04 '13 at 19:49
it gives this: `Object {[object Object]: 1}` – akonsu Jul 04 '13 at 19:50
@MattBall: I'm not sure if being technically correct is more useful to a beginner than telling them "how it works". – Jon Jul 04 '13 at 19:51
1

@akonsu: `console.log("[object Object]")` will print `1`. As you see, the key is not really an object. – Jon Jul 04 '13 at 19:52
1

@MattBall: Personally I appreciate that. Beginners usually don't. ;-) – Jon Jul 04 '13 at 20:01

score 0 · Answer 3 · answered Jul 04 '13 at 20:36

They mean the same thing. Valid keys are identifiers, string literals, or numeric literals. See http://ecma-international.org/ecma-262/5.1/#sec-11.1.5

You can't yet use objects themselves as keys, but WeakMap objects as proposed for EcmaScript 6 will solve this. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap

What is the difference between these two notations for Javascript objects/associative array?

3 Answers3