Dynamic Property of JavaScript object? - Stack Overflow

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I am wondering if this is possible in JavaScript, I want to have an Object which could contain dynamic properties.

Give an example:

function MyObject()
{
}

var myobj = new MyObject();
myobj.property1 = "something";

alert(myobj.property1); // something
alert(myobj.property2); // this should never report error, instead the property should be evaluated to null, as it has never been set. 

Is there any way to intercept property calls in JavaScript so I can proactively set a no-value property as null?

Thanks.

I am wondering if this is possible in JavaScript, I want to have an Object which could contain dynamic properties.

Give an example:

function MyObject()
{
}

var myobj = new MyObject();
myobj.property1 = "something";

alert(myobj.property1); // something
alert(myobj.property2); // this should never report error, instead the property should be evaluated to null, as it has never been set. 

Is there any way to intercept property calls in JavaScript so I can proactively set a no-value property as null?

Thanks.

• javascript
• dynamic
• prototype
• properties

Share Follow asked Nov 18, 2009 at 0:17 NomineeNominee 4111 silver badge22 bronze badges 2

• 3 Why do you explicitly want it to be "null"? Doesn't "undefined" suffice? – Asbjørn Ulsberg Commented Nov 18, 2009 at 0:30
• I thought my application doesn't take undefined value, I just tried, undefined does the same job, so I guess this can be closed now. Thanks everyone. – Nominee Commented Nov 18, 2009 at 0:50

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8 Answers 8

Sorted by: Reset to default 4

This is about as close as you can get to achieving your goal.

Code:

var obj = {};
alert("prop: " + obj.prop);
obj.prop = "something";
alert("prop: " + obj.prop);
delete obj.prop;
alert("prop: " + obj.prop);

Behavior:

Alert: "prop: undefined"
Alert: "prop: something"
Alert: "prop: undefined"

'Proxy' can do that

var myobj = new Object();
var handler = {
    get:function (obj, name, proxyed){
        if(obj[name] !== undefined) // if obj[name] exist
            return obj[name];       // then return obj[name]
        return null; // if obj[name] is not exist then return null;
    }
};
var obj = new Proxy(myobj, handler);
obj.property1 = "something";

alert(myobj.property1); // something
alert(myobj.property2); // undefined

alert(obj.property1); // something
alert(obj.property2); // null

Yes, but only in version 2.0 and higher. The exact syntax is still TBD but it's looking like it'll be get * () {...} for object literals at least.

Nope. JavaScript is not Smalltalk.

There is no way to intercept direct property accesses in JavaScript. When a property is retrieved that hasn't been set than the result will be undefined. Although null and undefined are usually considered to be the same thing they are in fact different entities.

In JavaScript undefined means no value and null means a value of null. In some cases you can mix undefined and null. For example, when using the == operator they are equivalent ((null == undefined) === true). Using the non-coercing operator, ===, they are different ((null === undefined) === false).

You can use this to your advantage. While most people will claim that you should use the non-coercing equality operator (===) it's mostly safe to put null and undefined in the same bucket, in less of course you actually care about the difference between the two. Where it gets tricky is that undefined is a property of the global object and can therefore be assigned a new value.

If someone were to say undefined = 'donkey' then null == undefined would start to return false. In practice this is almost never a problem since most people aren't foolish enough to reassign the value of undefined.

So, in a roundabout sort of way, you don't need to trap property accesses to return null for properties that have not been set so long as you pare the result against null using ==.

No unless you are manipulating an object controlled by an NPAPI plugin in which case you could implement the intended behavior.

In other words, through an NPAPI plugin, you could implement the behavior you are looking for.

Check out javascript prototypes. I think that will give you at least some of what you are looking for. Just google up "javascript prototype".

In your example the second alert will not generate an error. It will just alert undefined. Accessing properties of properties will generate an error:

myobj.property2.property3

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