Functions

czambran · July 7th, 2006, 10:57 AM

On page 89 there is the following code:

Code:

var oCar = new Object;
oCar.color = âredâ;
oCar.showColor = function () {
alert(this.color); //outputs âredâ
};

I was wondering if there is any difference between using

Code:

oCar.showColor = function () {
alert(this.color); //outputs âredâ
};

AND

Code:

oCar.showColor = new Function ('alert(this.color)');

Thanks

Christian

nzakas · July 7th, 2006, 12:39 PM

They two are roughly equivalent, but using new Function() causes the JavaScript interpreter to invoke another JavaScript interpreter to interpret the text you have passed in. Generally, the second method is frowned upon for this reason.

Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/

czambran · July 7th, 2006, 02:08 PM

And you can create closures using both methods, right?

Thanks

Christian

AGS · July 7th, 2006, 02:09 PM

I would say that the main difference is that the function created using Function constructor contains only global object in its scope chain. It can be illustrated by following example:

Code:

        function myFunc() {

            var myVar = "Test";

            var oCar = { color: "Red" };

            oCar.showColor = function () {
                alert(this.color);
                alert(typeof myVar);
            };

            oCar.showColor1 = new Function ('alert(this.color); alert(typeof myVar); ');           

            return oCar;        
        }

        var car = myFunc();

        car.showColor();
        car.showColor1();

Am I wrong, Nicholas?

Best regards,
Alexei

czambran · July 7th, 2006, 02:17 PM

In other words, you can't create closures using the Function constructor, right?

Christian

nzakas · July 7th, 2006, 02:17 PM

Yes, but it's harder to create using a string because you can't lay out your code in a logical way.

Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/

czambran · July 7th, 2006, 02:29 PM

Nicholas,

Why then, on AGS example, the second function can not access myVar?

Thanks a lot guys for all your help,

Christian

Christian

AGS · July 7th, 2006, 02:37 PM

It's your turn, Nicholas ;). The whole point of closures is in scope chain. And there is no one with the exception of global object.

Alexei

czambran · July 11th, 2006, 04:23 PM

Nicholas,

Is there a reason why on AGS' example the function defined with the Function Constructor cannot access the variable 'myVar'?

Christian

nzakas · July 11th, 2006, 05:09 PM

Basically, creating a function creates a new scope. When you do it the old fashioned way, you can include objects from outside of that scope, so Alexei's showColor() function can access myVar. The showColor1() function, because it is created using a string, doesn't have that notion of a containing scope from which variables can be accessed. Within that code, myVar hasn't been defined yet, and since there is no other scope for it to check, it is considered undefined by the interpreter.

Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/