Undefined variable as function argument javascript

Question

I've looked a fair bit, so pardon me if this has already been answered.

I'm also curious as to what the actual term is called; Is it "Ambiguous" for the type of arguments I am handling?

Anyways, the problem is that I want to be able to call a function like this:

prompt(_.define(variable, "DEFAULT VALUE"));

Basically, so that variables can have default values.

However, every time I try to do this, I get this error:

Timestamp: 6/11/2012 1:27:38 PM
Error: ReferenceError: thisvarisnotset is not defined
Source File: http://localhost/js/framework.js?theme=login
Line: 12

Here is the source code:

function _() {
return this;
};

(function(__) {
__.defined = function(vrb, def) {
    return typeof vrb === "undefined" ? ((typeof def === "undefined") ? null : def) : vrb;
    };
})(_());


prompt(_.defined(thisvarisnotset, "This should work?"), "Can you see this input?");

Not sure why it's doing this? I've called undefined variables as arguments in functions before and it worked just fine.

var variable = variable || 'default'; would be better, since you don't know if variable is initialised yet. Otherwise, you will be initialising variable in the global scope. — Cameron Martin
– Cameron Martin, Commented Jun 11, 2012 at 17:56
Thanks, that makes things a little bit more simple than what I was previously doing, the (var || 'default'). I didn't know that you can not pass undefined variables that way, I figured that the function would automatically assign it as undefined if it was. Thank you! — John Som
– John Som, Commented Jun 11, 2012 at 18:08

machineghost · Accepted Answer · 2012-06-11 17:45:22Z

1

A completely undeclared variable can't be passed in JS; you can only pass declared variables or undeclared properties of other variables.

In other words:

var a; // you can do _.defined(a)
var a = undefined; // you can do _.defined(a)
a.b; // you can do _.defined(a.b), even though we never defined b

answered Jun 11, 2012 at 17:45

machineghost

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9 Comments

machineghost Over a year ago

To fix your code, one option is to do this all the time "window.thisvarisnotset"; since window is the global scope, this should work. Of course, it bears mentioning that you're almost trying to create a new variant of JS here; good luck to anyone else who has to work with your code.

Cameron Martin Over a year ago

If thisvarisnotset if not declared elsewhere, it will refer to window.thisvarisnotset. This doesn't solve the problem though.

machineghost Over a year ago

Oh, good point. Yeah, really you're trying to change JS, and that can't be done. The much more JS-ish solution to this, as ddlshack mentioned, is to just do "thisvarisnotset || defaultValue" everywhere, instead of even trying to define a function like the one you're trying to make.

machineghost Over a year ago

(Well, and even MORE JS-ish solution would be to restructure your code so that it doesn't rely on ambiguously declared variables in the first place ...)

machineghost Over a year ago

(... the foo || default pattern is useful, and you'll find it used in major libraries like jQuery, but you should only need it in a few specific situations; not so much that you feel like you need to make a function. If you do feel like you need it a lot, there's probably a better way to do what you're trying to accomplish.)

|

Cameron Martin · Accepted Answer · 2012-06-11 17:51:57Z

1

Basically, so that variables can have default values.

Why don't you just initialize the variable with a default value?

Or, just initialise the variable before calling defined.

var variable; // Note that this will not overwrite the variable if it is already set.

Or, even better.

var variable = variable || 'default';

answered Jun 11, 2012 at 17:51

Cameron Martin

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