JavaScript: The Definitive Guide, 6th Edition

Function Arguments and Parameters

JavaScript function definitions do not specify an expected type for the function parameters, and function invocations do not do any type checking on the argument values you pass. In fact, JavaScript function invocations do not even check the number of arguments being passed. The subsections that follow describe what happens when a function is invoked with fewer arguments than declared parameters or with more arguments than declared parameters. They also demonstrate how you can explicitly test the type of function arguments if you need to ensure that a function is not invoked with inappropriate arguments.

Optional Parameters

When a function is invoked with fewer arguments than declared parameters, the additional parameters are set to the undefined value. It is often useful to write functions so that some arguments are optional and may be omitted when the function is invoked. To do this, you must be able to assign a reasonable default value to parameters that are omitted. Here is an example:

// Append the names of the enumerable properties of object o to the
// array a, and return a.  If a is omitted, create and return a new array.
function getPropertyNames(o, /* optional */ a) {
    if (a === undefined) a = [];  // If undefined, use a new array
    for(var property in o) a.push(property);
    return a;
}

// This function can be invoked with 1 or 2 arguments:
var a = getPropertyNames(o);  // Get o's properties into a new array
getPropertyNames(p,a);        // append p's properties ...