Strange javascript notation - variable declaration with colon?

Question

I've been looking through some library javascript source code and I've found a statement that I totally don't understand. This library is based on dojo and uses its implementation of "class inheritance". Here is a simplified version of code that is a subject:

dojo.declare("myCustomClass", {
    constructor:function(){
        // what does this statement mean?
        isContentShowing : false;

        //here some code that uses isContentShowing in callbacks
        dojo.connect(this, "fakeEvent", this, function(){
            if(this.isContentShowing){
            //do some stuff
            }
            //do more stuff
        });
    }
});

So the question is what does the isContentShowing: false; inside function body mean?

@ChrisMorgan That's not. I think pst explained it quite well. Everybody forgets about labels nowadays... — MaxArt
– MaxArt, Commented Jun 4, 2012 at 7:43
@MaxArt: hmm... I thought JavaScript didn't support them. I know about them (and thought about them before adding the comment!), but thought they weren't in JavaScript. I stand corrected. — Chris Morgan
– Chris Morgan, Commented Jun 4, 2012 at 7:53

score 7 · Accepted Answer · 2012-06-04 08:14:00Z

7

It's not a variable. It is a label:

Provides a statement with an identifier that you can refer to using a break or continue statement.

In this case, it is useless: it is not used as a label, it does not perform an assignment, and the result of evaluating false is discarded.

I suspect a = and this are desired as this.isContentShowing = false makes more sense given the conditional below. Perhaps the original author never ran into/realized this bug due to this.isContentShowing evaluating to undefined (and thus still being falsey, like, well, false) later on.

Or, as Chris suggests in a comment, the intent might have been:

dojo.declare("myCustomClass", {
  isContentShowing : false, // but , and not ;
  constructor:...
});

edited Jun 4, 2012 at 8:14

answered Jun 4, 2012 at 7:31

user166390

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5 Comments

MaxArt Over a year ago

Extremely confusing. Why putting false; right after that? I'm guessing it's some kind of bug...

user166390 Over a year ago

@MaxArt It is confusing - I believe that it is indeed incorrect code.

nnnnnn Over a year ago

@MaxArt - yes it's a bug, though as pst explained it's not a syntax error. The original coder most likely intended this.isContentShowing=false;.

Chris Morgan Over a year ago

Or perhaps isContentShowing: false, outside the function (inside the object literal).

user166390 Over a year ago

@ChrisMorgan That could very well have been the intent, although as the ; would need to be a , in the object-literal... but in any case, as good an explanation for the [misguided] appearance as mine :-)

Someth Victory · Accepted Answer · 2012-06-04 07:37:33Z

-1

This is called object literal in Javascript. It contains key value pair, e.g:

var car = {
    color: 'blue',
    price: 10000
}

so when you call car['color'] or car.color it will return blue. In the my example, color is the key and value is 'blue'. That's called key value pair.

Anyway, in the your example this.isContentShowing will return false by default.

answered Jun 4, 2012 at 7:37

Someth Victory

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1 Comment

nnnnnn Over a year ago

In your answer you've shown an object literal. In the question the code is defining a function, not an object literal. In context it's fairly clear that the original coder meant this.isContentShowing = false;. In the question this.isContentShowing will return undefined, not false (though in the context of an if condition undefined will act the same as false).

peller · Accepted Answer · 2012-06-04 16:14:38Z

-1

So it looks like a misplaced statement (interpreted instead as a label, as explained by @pst) That property should likely have been a peer of constructor: All properties on the object passed to dojo.declare are put on the Object prototype. Because an undefined value is falsey, in the end, the behavior is pretty much the same. Declaring falsey properties on the prototype is of limited value, unless someone is actually iterating through properties to check for existence, as dojo.parse does.

answered Jun 4, 2012 at 16:14

peller

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