myFunction.length in ECMAScript 262

The number of arguments declared with the function.
This property is read-only.

Property Type

This property is informative only, since calls to functions may choose to not pass parameters which are declared (in which case the parameters have a value of undefined inside the function), or may choose to pass more parameters than are declared (which may be accessed through the arguments array inside the function).

function AddUpValues(){
    var total=0;
    for (var i=0,len=arguments.length;i<len;i++) total += arguments[ i ]*1;
    return total;

var sum = AddUpValues(1,2,3,4,5,6,7,8,9);
var len = AddUpValues.length;
// ** sum is 45
// ** len is 0

function SayHi(firstName,lastName){
    return "Hello "+firstName+" "+lastName;
var msg = SayHi();
var len = SayHi.length;
// ** msg is "Hello undefined undefined";
// ** len is 2