Rails - Jul 2006 - Does Javascript let you default input params like Ruby does?

Can you have a Javascript function with a param that''s defaulted if it 
isn''t present?

function X(a, b = false, c = ''dog'') {
}

X(5) calls X with a=5, b=false, c=''dog''
X(4, true) calls X with a=4, b=true, c=''dog''
X(3, true, ''cat'') calls X with a=3, b=true,
c=''cat''

?

-- 
Posted via http://www.ruby-forum.com/.

nope.

On 7/13/06, Wes Gamble <weyus@att.net> wrote:
>
> Can you have a Javascript function with a param that''s defaulted
if it
> isn''t present?
>
> function X(a, b = false, c = ''dog'') {
> }
>
> X(5) calls X with a=5, b=false, c=''dog''
> X(4, true) calls X with a=4, b=true, c=''dog''
> X(3, true, ''cat'') calls X with a=3, b=true,
c=''cat''
>
> ?
>
> --
> Posted via http://www.ruby-forum.com/.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
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>


-- 
Mark Van Holstyn
mvette13@gmail.com
http://lotswholetime.com
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robert bradford wrote:
> I hope I am wrong, but I believe I remember hearing that this cannot be 
> done like it is in RoR, PHP, etc. I remember seeing something like this:
> 
> <script type="text/javascript">
>   function hello(a) {
>     if (typeof a==''undefined'') a = "world";
//Do this for each param you
> would like to default
> 
>     alert(''Hello, '' + a);
>   }
> </script>
> 
> <%= button_to_function "Greet the world", "hello()"
%>
> <%= button_to_function "General Greeting",
"hello(''there'')" %>
> 
> This is kind of a lame work around, but it works. Hopefully there is a 
> better solution.
I probably shouldn''t have asked - I found out that you _can_ do this
but
not defaulting in the function definition.

You have to interrogate the built-in Javascript "arguments" array to
see
if there are extra arguments and then default them based on your needs, 
etc.

There is no explicit contract between the caller and the callee - I can 
pass as many arguments as I want to a Javascript function and the number 
of arguments that it receives can be less.

In this way, Javascript is even more dynamic than Ruby in it''s calling 
semantics since Ruby will throw a run time error if you call a function 
with the wrong number of parameters - as I''m sure all of us Railists 
know :)!

Wes

-- 
Posted via http://www.ruby-forum.com/.

I hope I am wrong, but I believe I remember hearing that this cannot be 
done like it is in RoR, PHP, etc. I remember seeing something like this:

<script type="text/javascript">
  function hello(a) {
    if (typeof a==''undefined'') a = "world"; //Do
this for each param you
would like to default

    alert(''Hello, '' + a);
  }
</script>

<%= button_to_function "Greet the world", "hello()" %>
<%= button_to_function "General Greeting",
"hello(''there'')" %>

This is kind of a lame work around, but it works. Hopefully there is a 
better solution.

-- 
Posted via http://www.ruby-forum.com/.

On Thu, Jul 13, 2006, Wes Gamble wrote:
> Can you have a Javascript function with a param that''s defaulted
if it
> isn''t present?
> 
> function X(a, b = false, c = ''dog'') {
> }
> 
> X(5) calls X with a=5, b=false, c=''dog''
> X(4, true) calls X with a=4, b=true, c=''dog''
> X(3, true, ''cat'') calls X with a=3, b=true,
c=''cat''
The syntax is not so clean, but I ran across this the other day which
looks like what you want:

http://earthcode.com/blog/2006/01/optional_args.html

Ben

Wes Gamble wrote:
> Can you have a Javascript function with a param that''s defaulted
if it
> isn''t present?
> 
> function X(a, b = false, c = ''dog'') {
> }
> 
> X(5) calls X with a=5, b=false, c=''dog''
> X(4, true) calls X with a=4, b=true, c=''dog''
> X(3, true, ''cat'') calls X with a=3, b=true,
c=''cat''
> 
> ?
There is no language support for it but there are ways to emulate it 
since Javascript does not require you to send all arguments. Eg.

function X(a, b, c) {
	if( arguments.length < 3 ) c = ''dog''
	if( arguments.length == 1 ) b = false

	//... implementation
}

Some people will use the short-circuited "or" operator as follows:

function Y(m, n, o) {
	n = n || ''foo''
	o = o || ''bar''

	//... implementation
}

This can be dangerous if a valid value for "n" or "o" can
evaluate to
false. In Ruby only "nil" and "false" evaluate to false but
in
Javascript the follow all evaluate to false:

undefined
null
0
-0
"" (empty string)
NaN

An empty string and 0 are often valid values. Prototype has this fault 
in it''s implementation of the $F() function for <select> input
elements.

Another entirely different option is to use object literals to emulate 
optional named arguments. So:

function M(name, options) {
	defaults = {
		foo: ''bar'',
		baz: ''cat''
	}
	options.prototype = defaults
	//... implementation using "options" to access
	//... optional arguments
}

You would call this like:

M(''some_val'', {
	foo: ''dog'',
	baz: ''car''
});

If you remove an option it would get the default specified. So:

M(''another_val'', { foo: ''dog'' });

Eric