Comments on: Default Arguments in JavaScript Functions

By: fatbrain

fatbrain — Thu, 25 Oct 2007 16:58:01 +0000

Thanks for input e-voc. What if we consider:

Function.prototype.defaults = function()
{
var _f = this;
var _a = Array(_f.length-arguments.length).concat(
Array.prototype.slice.apply(arguments));
return function()
{
return _f.apply(this, Array.prototype.slice.apply(arguments).concat(
_a.slice(arguments.length, _a.length)));
}
}

Or did I get it wrong? Anyhow, give it a testspin and get back to me.

Damn I’m hungry, need food for brain. (I only change the “this” argument in the apply on the inner function)

Cheers // fatbrain

By: e-voc

e-voc — Mon, 17 Sep 2007 15:34:02 +0000

For normal functions, one would pass null or window as execution context

var bar = function (who) { return who; }.defaults(null, ‘me’);
bar();

Regards
e-voc

By: e-voc

e-voc — Mon, 17 Sep 2007 15:30:43 +0000

sorry, the last on was buggy.
here’s the working one:

Function.prototype.defaults = function(o) {
var _f = this;
var args = Array.prototype.slice.call(arguments, 1);
return function() {
var a = Array.prototype.slice.call(arguments);
return _f.apply(o, a.concat(args.slice(a.length)));
}
}

function Foo() { this.bla = 1; }
Foo.prototype.foo = function(a, b) {
return this.bla + ‘:’ + a + ‘/’ + b;
};

var f = new Foo;
f.foo = f.foo.defaults(f, 42, ‘default_b’);

f.foo(10, 20); // returns “1:10/20″
f.foo(10); // returns “1:10/default_b”
f.foo(); // returns “1:42/default_b”

By: e-voc

e-voc — Mon, 17 Sep 2007 15:07:20 +0000

Your way won’t work with objects.

This way will: http://phpfi.com/263389

Regards
e-voc

By: colossalus

colossalus — Fri, 13 Jul 2007 07:04:51 +0000

I have a hybrid idea that uses your method with associative arrays:
http://www.adventsun.com/code/JS_Function_Defaults.htm

Thanks for pointing me in the right direction! :)

By: fatbrain

fatbrain — Fri, 06 Jul 2007 10:16:28 +0000

Thanks for all your lovely comments, very much appreciated :-)

By: Stefan

Stefan — Mon, 02 Jul 2007 11:07:33 +0000

Nice idea and elegant implementation, although I think I am not going to use it because:
1. it is less readable than the “if typeof” version or the shorter “= .. || ..”, because the defaults are seperated (lines of code wise) far from the argument list.
2. if you add another argument to the function you have to update the default list, which might lead to hard to find processing errors. Maybe one could work around by requiring that the default call always get’s the same number of arguments as the function itself expects, causing an error if it doesn’t.

By: Scott

Scott — Fri, 29 Jun 2007 17:56:20 +0000

I’ve read hundreds of articles containing js snippets for accomplishing various things, most of which are either no-brainers or completely irrational.

This one is not. Well done.

By: Binny V A

Binny V A — Fri, 29 Jun 2007 12:19:12 +0000

In Javascript, I use 3 different methods to get this effect…
Optional Function Arguments in JavaScript

Your method is the fourth - Thanks!

By: Ariel

Ariel — Mon, 07 May 2007 19:37:28 +0000

I added a “minor than” symbol in the previous post and the post is not showing properly, I’ll repost it without the symbol.

fatbrain, I loved your code, I was trying to code something like this but I couldn’t, so I searched and found yours, I modified it a bit so that it won’t throw an error if the defaults array is larger than the arguments one. Also will update parameters that are submitted as null or undefined.
All this changes are not much relevant.. the only problem I found is that you were passing _f as scope of the Function.apply.. instead I think it should be receiving “this” to behave as expected. here’s my version.

Function.prototype.defaults = function(){
var f = this;
var def = Array.prototype.slice.apply(arguments);
function update(a){
var na = Array.prototype.slice.apply(a);
for(var i=0;i