Default function parameter

[Börni]

Hi,

Is there a way to provide a default for a function parameter.
i tried
function func (type, message, obj = "message")
but it doesn't seem to work.
It seems a bit ugly to make an if/else statement right at the begining
of a function, just to imitate this behavior.

[Ulrik Skovenborg]

Re: Default function parameter

Börni wrote:[color=blue]
> Is there a way to provide a default for a function parameter.
> i tried
> function func (type, message, obj = "message")
> but it doesn't seem to work.[/color]

I usually use this way to make default parameters:
function func (type, message, obj) {
obj = (typeof obj == "undefined" ) ? "message" : obj;

or just
obj = (!obj) ? "message" : obj;
but I think IE5 (or whatever) has a problem with that.

[Michael Winter]

Re: Default function parameter

On Thu, 23 Dec 2004 20:20:31 +0000, Börni <b.reiter@onlin ehome.de> wrote:
[color=blue]
> Is there a way to provide a default for a function parameter.[/color]

Typically, this is done with:

function myFunction(arg) {arg = arg || ???;
/* ... */
}

where ??? is your default value.

The logical OR operator is a bit different in ECMAScript when compared to
other languages. Normally,

opr1 || opr2

would evaluate to true or false, depending on whether either opr1 or opr2
is true. In ECMAScript, the operands are tested for true- or "falseness" ,
but it's their values that are actually returned. Consider:

false || 2

The literal, false, will obviously evaluate to false so the result of the
expression is 2. With:

'hello' || 'world'

the non-empty string, 'hello', will evaluate to true so the result of the
expression is 'hello'.

If code called the function above with an argument that evaluated to false
(null, undefined, false, 0, or ''), or no argument at all, the second
operand would be assigned to arg. Of course, if one of those values is a
legal argument that shouldn't be changed, this particular approach isn't
suitable. In that case, you can use Ulrik's first suggestion.

[snip]

Hope that helps,
Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.

[Douglas Crockford]

Re: Default function parameter

> Is there a way to provide a default for a function parameter.[color=blue]
> i tried
> function func (type, message, obj = "message")
> but it doesn't seem to work.[/color]

Of course it doesn't work. You can't make up your own syntax.

You can use the default operator.

a || b

If a is falsy (null, undefined, 0, or empty string) then the value is a.
Otherwise it is b. Missing parameters get the value undefined, which is
falsy.

http://www.crockford.com/javascript/survey.html

[Ivo]

Re: Default function parameter

"Michael Winter" said[color=blue]
> Börni asked[color=green]
>> Is there a way to provide a default for a function parameter[/color]
>
> Typically, this is done with:
>
> function myFunction(arg) {arg = arg || ???;
> /* ... */
> }
>
> where ??? is your default value.
>[/color]

Probably the shortest syntax is:

function myFunction(arg) { arg |= ???;

When no arg is provided, it will be of type 'undefined'. To catch the
occasion when it may actually be false, null, an empty string or zero, you
need to include a typeof test.

HTH
--
Ivo

http://www.ariel.shakespearians.com/

[Michael Winter]

Re: Default function parameter

On Fri, 24 Dec 2004 01:54:05 +0100, Ivo <no@thank.you > wrote:

[snip]
[color=blue]
> Probably the shortest syntax is:
>
> function myFunction(arg) { arg |= ???;[/color]

It might be the shortest syntax, but it's not very appropriate. That
operation is performed in terms of 32-bit integers only. Moreover,
consider when the operand is another number other than zero. If arg is
zero, the default will be set to the operand as expected, but if arg is
non-zero it will be assigned the value of (arg | operand), which will
mutate the argument in what is probably an undesirable way.

Unfortunately, the production

arg ||= def;

doesn't exist.

[snip]

Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.

[Börni]

Re: Default function parameter

[color=blue][color=green]
>> function myFunction(arg) {arg = arg || ???;
>> /* ... */
>> }[/color][/color]

I took the above approach as i can be sure that i never will pass an
argument that contains 0, undefined...
( it gets passed the id of an html element )

[Ivo]

Re: Default function parameter

"Ivo" wrote[color=blue]
> "Michael Winter" said[color=green]
> > Börni asked[color=darkred]
> >> Is there a way to provide a default for a function parameter[/color]
> >
> > Typically, this is done with:
> >
> > function myFunction(arg) {arg = arg || ???;
> > /* ... */
> > }
> >
> > where ??? is your default value.
> >[/color]
>
> Probably the shortest syntax is:
>
> function myFunction(arg) { arg |= ???;
>
> When no arg is provided, it will be of type 'undefined'. To catch the
> occasion when it may actually be false, null, an empty string or zero, you
> need to include a typeof test.[/color]

Slightly related, to remember the arg used last time the function was
called, but without creating a global variable, use "this". In the following
example, the function "myFunc" gets its argument "p" from:
1. where ever it is called from, or -if called without argument-
2. a prompt box, with a default value of:
1. the argument used last time myFunc ran, or -if this is the first time-
2. 'myArgument'.

function myFunc(p) {
p= p || window.prompt( 'Enter your argument:', this.last ||
'myArgument' );
if(p) {
this.last = p;
/* ... */
}
}

// this will invoke prompt box:
someElement.onc lick = myFunc;

// this will use 'someargument' :
someotherElemen t.onclick = function() { myFunc( 'someArgument' ); };

Other functions may feature their own "this.last" history without
interference. It works for me, although I don't know whether "this" is the
calling element or the function scope itself...
--
Ivo

[Michael Winter]

Re: Default function parameter

On Fri, 24 Dec 2004 13:58:39 +0100, Ivo <no@thank.you > wrote:

[snip]
[color=blue]
> // this will invoke prompt box:
> someElement.onc lick = myFunc;
>
> // this will use 'someargument' :
> someotherElemen t.onclick = function() { myFunc( 'someArgument' ); };
>
> Other functions may feature their own "this.last" history without
> interference. It works for me, although I don't know whether "this" is
> the calling element or the function scope itself...[/color]

It depends how myFunc is called. With the first approach where myFunc is,
and will be called as, a method of someElement, this will refer to that
element. With the second where myFunc is called directly, this will refer
to the global object.

Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.