Passing Values to Functions | Real World XML (2nd Edition)

The values you pass to functions are called arguments. When you pass data in arguments to a function, the code in the function has access to those values. When you create a function, you specify which arguments are to be passed to the function in an argument list.

Here's an example. In this case, I'll create a function named adder that will add two values and return their sum. Here's how I start creating adder :

 function adder()  {     .     .     . }

This time, we're going to pass arguments to the function. So, we list the arguments that we'll pass by giving them names in the argument list, which is enclosed in the parentheses following the function name . Here, I'll call the two arguments passed to addervalue1 and value2 :

 function adder(value1, value2)  {     .     .     . }

Calling by Value

By default in JavaScript, what's really passed to functions are not the actual arguments themselves , but copies of those arguments, provided that those arguments evaluate to primitive (nonobject) values. This process is named calling by value. On the other hand, in JavaScript, objects are passed by reference, which means that what's actually passed is not the object itself, or even a copy of the object, but the location of the object in memory.

Now you're free to refer to the passed values by the names you've given them in the argument list. To return the sum of value1 and value2 , all I have to do is to add those values and use the return statement, like this:

 function adder(value1, value2)  {  return(value1 + value2)  }

To make use of this function, you pass values to it in parentheses like this, where I'm finding the sum of the values 47 and 99 :

Listing ch06_11.html

 <HTML>     <HEAD>         <TITLE>             Passing Arguments to Functions in JavaScript         </TITLE>     </HEAD>     <BODY>         <CENTER>             <H1>                 Passing Arguments to Functions in JavaScript             </H1>         </CENTER>         <SCRIPT LANGUAGE = "JavaScript">  document.writeln("47 + 99 = " +  adder(47, 99))   function adder(value1, value2)   {   return(value1 + value2)   }  </SCRIPT>     </BODY> </HTML>

That's all it takes; now we're passing arguments to the adder function. You can see the results in Figure 6-11. As you see there, everything is working perfectly : The sum of 47 and 99 is displayed as it should be. (You might also note that even though the value returned from the adder function is a number, JavaScript is smart enough to treat that number as a text string when it's time to print it with document.writeln .)

Figure 6-11. Passing arguments to functions in JavaScript.

graphics/06fig11.gif

Using a Variable Number of Arguments

In case you're interested, you can actually call a function with fewer arguments than those that appear in its formal argument list (that is, the number of arguments that you've defined it with). There's no problem unless you try to access arguments by name that had no value passed to them. In addition, you can pass more arguments than a function's formal argument list specifies. If you do that, you can access the additional arguments from the arguments array (and we'll see more about arrays in a few pages). For example, the first argument passed to adder can also be referenced inside the adder function as adder.arguments[0] , the next as adder.arguments[1] , and so on.