resetting the state of an observable to undefined/zero/null

[bert bruynooghe]

Suppose I have a variable var test= ko.observable(1);
How do I change the state so that test() returns undefined?

test(undefined) doesn't seem to do the job, as this interpreted as a
read call...

Kind regards,
Bert Bruynooghe.

[Stacey Thornton]

What about just

test = ko.observable(); just like when you first initialize it?

[Ian Drake]

Not that I have a better answer, but that wouldn't work, or at least won't notify subscribers that "test" has changed.

[Ian Drake]

You can't use null instead?  

test(null);

[Stacey Thornton]

Fair enough. Can you not just set it to null?

test(null)

[Koviko]

Undefined is a lack of definition. To define something as undefined is counter-intuitive.

What are you trying to accomplish?

[bert bruynooghe]

Basically, I have a list of items, and I have an observable that keeps
track of the selected item. If nothing is selected (value undefined),
some items are not displayed on the page, aso.
And I must have the possibility to revert to a status of no item
selected.

Btw, I had the same problem in 1.2.1 when trying test(0), in a case
where I wanted to maintain the index. Not sure if this last situation
is still a problem in 1.3 ...

[bert bruynooghe]

Exactly the same problem. I guess someone implemented a check with
param==undefined iso. typeof(param) == "undefined", but I can't
exactly locate the code...

On 14 dec, 18:23, Stacey Thornton <stacey.cielia.l...@gmail.com>
wrote:

> Fair enough. Can you not just set it to null?
>
> test(null)

[Michael Best]

Here's a fiddle showing that setting an observable to undefined works:
http://jsfiddle.net/mbest/JgRTU/

-- Michael

[Michael Best]

BTW, you didn't specify which version of Knockout you're using. My
fiddle uses the latest version (2.0.0rc).

[bert bruynooghe]

You're right, it just works. I am sorry for the misinformation I
posted here; I guess I need some more sleep...

[Matthew Schinckel]

I'd personally think null was appropriate in that context.

However, I can think of a good use case for undefined. When serialising an object to JSON, if you set the value of a key to undefined, it will not appear in the serialisation.

[Mark Hahn]

>   if you set the value of a key to undefined, it will not appear in the serialisation. 

I use this feature all the time.  Please don't do anything to remove it.

[Matthew Schinckel]

So do I: that was my point. If I have a value that is 'unknown', then it should be null. If I have a value that I don't want to appear in my serialisation, then it is undefined.

:)

[Matthew Schinckel]

Oh, and this is a JSON.stringify thing too, not just knockout converting to json.

[bsin...@gmail.com]

While this is totally correct, this is one caveat...

 

There is a difference between explicitly setting a field to undefined and clearing out an associated input box by deleting the characters.

Type "France", see the value appear in the viewModel.... if you delete every character, the observable becomes a zero length string... still sent to the server in the JSON.stringify

See here (with viewmodel displayed for convenience): http://jsfiddle.net/bsingin64/JgRTU/7/

The user can never just clear out a field and make it go back to the observable's original value.  At least, not in a text box.  (if you use the optionsCaption in a select box, the value of (undefined) gets assigned when the user clicks the "Select..." node)

 

I implemented a quick nullable observable extension which allows any observable to be nullable... http://jsfiddle.net/bsingin64/sh23a/

When the characters inside a text box are all deleted, the field is set to whatever is desired (either undefined or null, or really, whatever you like (0, -1, 'N/A', whatever you need)....  it's really just a "what to do when the field is empty"

 

... I'm using it to handle serialization of numeric values. An empty box that is expected to deliver a number should never deliver an empty string... it should instead deliver null.

 

BTW: numeric observable extension:  http://knockoutjs.com/documentation/extenders.html

[sheikh...@gmail.com]

You can use it 

if (test() == undefined ){

test() = null

//statment 

}

else

{

//statment 

}