Question: Question
--------

What is a good way of assigning a default value to an optional parameter?

Background
----------

I'm playing around with *optional* parameters in JavaScript, and the idea of assigning a default value if a parameter is not specified. My method in question accepts two parameters, the latter of which I deem to be optional, and if unspecified should default to `false`. My method looks something like this...

```
// selects the item, appropriately updating any siblings
// item to select [, toggle on / off]
this.selectItem = function(item, toggle)
{
    toggle = toggle && typeof toggle === 'boolean';
    if (toggle)
    {
       // ...
    }
}

```

Testing
-------

After running a few tests on [this jsFiddle](http://jsfiddle.net/nLAAt/2/), using the following principal for default value assigning:

```
function checkParamIsBoolean(param)
{
    param = param && typeof param === 'boolean';
}

checkParamIsBoolean('me'); // boolean
checkParamIsBoolean([]); // boolean
checkParamIsBoolean(true); // boolean
checkParamIsBoolean(false); // boolean
checkParamIsBoolean(1 == 1); // boolean
checkParamIsBoolean(1); // boolean
checkParamIsBoolean(null); // object
checkParamIsBoolean(undefined); // undefined
​

```

As you can, the results vary, and aren't desired.

Expected
--------

`null` = `false`  

`undefined` = `false`

Actual
------

`null` = `object`  

`undefined` = `undefined`

Summary
-------

Are there any alternative approaches to assigning a default value to an optional parameter if it's unspecified; would it be better to use `_toggle` as the parameter and then assign the value to `var toggle` within the method?

Answer: 
Better solution is to use named arguments wraped in a object. It keeps your code clean and prevents errors in case of complex functions (with many default and non-default arguments). Otherwise you need to remember about the order of arguments and assign default values based on the types of arguments present (that's what you're trying to do).

That method is a primary way of passing params [jQuery and jQuery's plugins](http://jquery-howto.blogspot.de/2009/01/how-to-set-default-settings-in-your.html). 

```
function test(options) { 

  // set up default options 
  var defaults = { 
    param1:      'test', 
    param2:      100
  }; 

  // combine options with default values
  var options = $.extend({}, defaults, options); // If you're not using jQuery you need different function here

  alert(options.param1)
  alert(options.param2)

}

test({'param2': 200}) // Call the function with just the second param

```
You can use this construction for optional parameters in your code:

```
//...
optionalParam = optionalParam || 'some default value';

```

In your concrete exaple it would be something like this:

```
this.selectItem = function(item, toggle)
{
    toggle = toggle || false;
   // ...
}

```

But in your case you could use toogle (optional parameter) directly in if statement (since you only want to check for its existence. Or optionally you could enforce boolean value like this `toogle = !!toogle` (double negation).