Optional parameters in JavaScript

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, 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 1

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.

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

Answer 2

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).

Answer 3

A very simple way to check if toggle is undefined, and if it is then set a default value:

if (toggle === undefined) toggle = "<your default value>";

Note that this does not change toggle if it is specified but of another type than what you expected.

Answer 4

Try it the other way around:

param = typeof param === 'boolean' && param;

This will make sure all are a boolean, see this fiddle

Answer 5

Why do you check the parameter to be boolean, if you expect an other result? The usual way would be checking for typeof toggle != "undefined", but in your case

toggle = Boolean(toggle);

should do the task. You can also use the shortcut !!toggle, or use it directly in the if-clause:

if (toggle) { // will evaluate to "false" for undefined
    ...

BTW, no need for a _toggle parameter; you can just reassign to toggle.

Answer 6

You can replace

param = param && typeof param === 'boolean';

with

param = !!(param && typeof param === 'boolean');

This will convert the result of the statement to the boolean negate, and then negate it back again.

Answer 7

If you consider null(as well as no args passed), undefined and ""(empty string) as "not an argument has passed to my function" use this statement:

toggle = toggle&&toggle||default_value

Now if you pass null, undefined or "" to your function as toggle arg, this line fills it with default_value.

Answer 8

Take a look at jQuery extend and how they handle optional parameters.

Answer 9

It can easilly be done with 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.