Rob Eberhardt

I just asked a guru of advanced Object-Oriented Javascript, Douglas Crockford the following question.  For posterity, and for other possible takers, I’m posting it here too.

Also, for the record, this has nothing to do with IE’s proprietary — but wonderful — setExpression method or CSS expression capabilities.  Those dynamic properties only apply to DOM objects, not custom Javascript objects.

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Do you know of a way to define dynamic object properties in Javascript?

For example, VBScript lets us define classes such as this:

CLASS myClass

    PUBLIC phrase

    PUBLIC PROPERTY GET firstword    ‘get first word from phrase

        firstword = left(phrase, instr(phrase, ” “))

    END PROPERTY

    PUBLIC PROPERTY LET firstword(v) ‘set phrase’s new first word

        phrase = v & ” ” & phrase

    END PROPERTY

END CLASS

This demonstrates two important features:
1. the firstword property can return dynamic results (depending on the current value of the phrase property). 
2. setting the firstword property can run other code (which dynamically prepends to the phrase property).

We can fully accomplish #1 (Property Get) in Javascript, by reassigning the toString method/property to a function like so:

function myConstructor(){

    var self = this;    //preserve object’s context

    this.phrase = ”;

    this.firstword = function(v){

        if(v){self.phrase = v + ‘ ‘ + self.phrase};    //LET

        return self.phrase.substring(0, self.phrase.indexOf(‘ ‘)); //GET

    }

    this.firstword.toString = this.firstword;

}

…But I’ve found no way to achieve #2 (Property Let or Set) in Javascript.  I can set firstword as a method, but setting the property value overwrites the method definition (and all subsequent GETs return that static value).

I’ve extensively searched, but found no answer (at least not before Javascript 2.0, which doesn’t yet exist).  Any ideas from the experts?

Update: Douglas Crockford responded that he doesn’t care for getter/setter properties, since “it allows simple assignment to have side effects, which I think works against reliability.”  (I take that as a “No, it’s not possible.”)
    My current need is that I’m patterning my custom object after part of the DOM, whose objects certainly do have getter/setters.  I agree with Douglas that getter/setter side effects can be dangerous (esp. in the hands of a poor coder, as with any powerful code construct), but I think the use of methods -vs- getter/setter properties should be in the realm of “best practice” rather than a language limitation. 

I’ve run into this often: the need to have a VBScript constant containing a line-break character.  Today I finally got a definitive answer on whether it’s possible (for the impatient, the answer is NO).

If you’re interested in history…

Of course this is right out:

CONST blah = “hello

there”

…It’s just bad syntax.  The closing string quote has to be on the same line as the opening one.

The normal approach I’ve tried was this:

CONST blah = “hello ” & vbCRLF & ” there”

..But the ampersand (concatenation operator) automatically makes it an expression to the VBScript compiler, and therefore it assumes “not constant.” This is of course despite the fact that both parts are known at the time of compilation (which is the main criterion for a constant — value is known at compile time).  Anyway, the ampersand is right out.

Now in JScript/Javascript/ECMAScript, you can do this:

var blah = “hello \r\n there”

…The \r\n switches define the line-break character, they go inside the string, and they are only interpreted when it’s read. 

Unfortunately, there’s no similar switch in VBScript.  While HTML does honor ASCII codes like 
 or 
, and web browsers honor hexadecimal codes in URLs, VBScript does neither.  So these also don’t work:

CONST blah =“hello 

 there”

CONST blah = “hello %0A%0D there”

CONST blah = “hello 0x0A0x0D there”

So I was reading a blog entry by Eric Lippert (who I believe wrote most of VBScript) today, and he mentioned Constant Folding, and proceeded to outline the mechanics behind this VBScript constant problem.  It rung a bell, I asked about line-breaks in VBScript constants in his comments, and Eric responded:
“Sorry, you are correct — there’s no way to do that. Sub-optimal, I know. “
…Straight from the dev’s mouth.

SO, if you need a line-break in a VBScript constant, just use a variable instead:

DIM blah

blah = “hello ” & vbCRLF & ” there”

…Yes, there may be some slight performance penalty compared to a constant, but it is script after all — racing performance ain’t the point.