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Please refer to the code below, when I "comment in" either of the commented out lines, it causes the error (in IE) of "':' expected". So then is my conclusion correct, that this inability to provide a reference to an object value, as an object key in a string literal; is this strictly an interpreter/parsing issue? Is this a candidate for an awful (or at least "bad") "part" of Javascript, in contrast to Crockford's "good parts"?

<script>
var keys = {'ONE': 'one'};

//causes error:
//var obj1 = {keys.ONE: 'value1'};
//var obj1 = {keys['ONE']: 'value1'};

//works
var obj1 = {};
obj1[keys.ONE] = 'value1';

//also works
var key_one = keys.ONE;
var obj2 = {key_one: 'value1'};
</script>
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7
  • 1
    BTW var key_one = keys.ONE; var obj2 = {key_one: 'value1'}; does not work as you expect it does. obj2 s property will be key_one and not the the value of key.ONE thus not one. Commented May 20, 2010 at 11:29
  • 1
    Note that although the last example works, it creates an object with the property key_one, not the property keys.One. The variable key_one is irrelevant to the literal object in the next line. Commented May 20, 2010 at 11:30
  • got it....that could be confusing....it should either disallow all "unquoted" strings, or de-reference the references, whatever they may be Commented May 20, 2010 at 11:35
  • 1
    They are already disallowed in JSON specs. But javascript objects can have methods defined as {somevar: 1, methodname:function(){ /*stuff*/ } } and I suppose somebody didn't like putting his methodname in quotes ;) Commented May 20, 2010 at 11:44
  • @George: Alternatively they could have disallowed quoted names in object literals, like some other languages... Commented May 20, 2010 at 11:47

4 Answers 4

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15

The limitation of the literal object syntax is that the names has to be literal. As the names can be specified as an identifer as well as a string, it's not possible to use a variable instead.

This will create an object with a property n, not a property answer:

var n = 'answer';
var o = { n: 42 };
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1 Comment

Numeric literals are valid property names as well. Also, in ES5, the names can be specified as an identifier name (i.e., not just identifiers, but also reserved words). See stackoverflow.com/a/9568622/96656.
6

You cannot use variables as keys when defining object with {}

Therefore they are being interpreted as string names and can consist only of characters avaliable for variable names

the

objectname[anythingThatReturnsValue]='value1'; is the way to go.

ALSO

You can generate a string and parse it

var s='{"'+keys.ONE+'": "value1"}';
var obj=JSON.parse(s);
//or
s='var obj2='+s;
eval(s);

Both above methods are bad practices for creating objects in JavaScript and I don't recommend them.

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7 Comments

You CAN use variables as keys when defining objects with {}, see the second "also works" block.
@George: No, you can't. The code runs, but it does not use the variable in the literal object.
@George: He means its a syntax error and there isn't anything you can do about it :)
@George - to be clear: var obj2 = {key_one: 'value1'}; is the same as var obj2 = {"key_one": 'value1'}; - key_one will be the name of the field.
@George: Clarifying Guffa's reply: The key_one in in the second "also works" example is the name of the key; if you examine the object, it will have a property called "key_one", not "one" as it would if the value of key_one were being used when you created the literal.
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2

Think about it: if it were to work the way you want it would totally introduce a language ambiguity.

var obj = {something:"red"}
var obj = {"something":"red"}

The two statements are equivalent in JavaScript, because bareword keys are "autoquoted." So if something means the literal string "something", how it could also refer to the variable "something". It can't. So if you want to use variables they have to go in square bracket notation instead of key : value notation.

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1

You can try:

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

obj[val] = 'bar'; 

obj.foo >>> 'bar';
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