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: 
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).
It can easilly be done with [ArgueJS](https://github.com/zvictor/ArgueJs):

```
this.selectItem = function()
{
  arguments = __({item: undefined, toggle: [Boolean, false})
  if (arguments.toggle)
  {
     // ...
  }
}

```

---

You can also check the type of `item` by changing `undefined` by the desired type.