CODEWITHSUNDEEP
javascriptobjectparsingobject-literal
Dexygen
Dexygen

Reputation: 12561

Is this Javascript object literal key restriction strictly due to parsing?

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>

Upvotes: 10

Views: 5545

Answers (4)

Aamir Afridi
Aamir Afridi

Reputation: 6411

You can try:

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

obj[val] = 'bar'; 

obj.foo >>> 'bar';

Upvotes: 1

naugtur
naugtur

Reputation: 16915

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.

Upvotes: 6

jpsimons
jpsimons

Reputation: 28100

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.

Upvotes: 2

Guffa
Guffa

Reputation: 700372

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 };

Upvotes: 15

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