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arguments object

The arguments object

Within a function’s code, a special object called

arguments

gives the developer access to the function’s

arguments without specifically naming them. For example, in the

sayHi()

function, the first argument

is given the name

message

. The same value can also be accessed by referencing

arguments[0]

, which

asks for the value of the first argument (the first argument is in position 0, the second is in position 1,

and so on.). Therefore, the function can be rewritten without naming the argument explicitly:

function sayHi() {

if (arguments[0] == “bye”) {

return;

}

alert(arguments[0]);

}

The

arguments

object can also be used to check the number of arguments passed into the function by

referencing the

arguments.length

property. The following example outputs the number of arguments

each time the function is called:

function howManyArgs() {

alert(arguments.length);

}

howManyArgs(“string”, 45); //outputs “2”

howManyArgs(); //outputs “0”

howManyArgs(12); //outputs “1”

This snippet shows alerts displaying

“2”

“0”

, and

“1”

(in that order). In this way, the

arguments

object

puts the responsibility on the developer to check the arguments that are passed into a function.

Unlike other programming languages, ECMAScript functions don’t validate the number of arguments

passed against the number of arguments defined by the function; any developer-defined function accepts

any number of arguments (up to 255, according to Netscape’s documentation) without causing an

error. Any missing arguments are passed in as

undefined

; any excess arguments are ignored.

By using the arguments object to determine the number of arguments passed into the function, it is pos-

sible to simulate the overloading of functions:

function doAdd() {

if(arguments.length == 1) {

alert(arguments[0] + 10);

} else if (arguments.length == 2) {

alert(arguments[0] + arguments[1]);

}

doAdd(10); //outputs “20”

doAdd(30, 20); //outputs “50”

Chapter 2

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