A JavaScript function to add two numbers:

	function add()
	{
	document.write( 5+5 );
	}
	add();
      

And functions to perform various other arithmetical operations:

	function minus()
	{

	document.write("<p>" + (6-4) );

	}
	function times()
	{

	document.write("<p>" + 6*4 );

	}
	add();
	minus();
	times();
      

Most mathematical functions do not display their results in an HTML document: they only calculate and return intermediate results for use by other functions.

For example, the code below uses the Math.pow( value, power ) function to calculate the area of a circle. Also the Math.round() function is used to create roundedArea.

	// calculate circle ara
	var radius = 10;
	var circleArea = 3.1415 * Math.pow( radius, 2 );
	// round to 2 decimal places
	var roundedArea = Math.round( circleArea * 100) / 100;
	document.write( "<p>> Area of circle with radius " + radius + "
	has area of " + circleArea );
	document.write( "<p> rounded area is " + roundedArea );
      

Creating a user-defined function that returns a value is straightforward: at some point in the function's execution a return statements needs to be executed. Typically, functions returning a value require arguments that will be used in the calculation of that value.

For example we might wish to define a function addVAT( total ) which adds 14% to a total, and returns this new value. The specification for our function might appear as follows:

	function addVAT( total )
	{
	return (total * 1.175);
	}
	var goodsTotal -=50;
	writeln(" total before tax = " + goodsTotal );
	var newTotal = addVAT(goodsTotal);
	writeln("<p> total with tax added " + newTotal );
      

As can be seen, to make a function return a value we include a return statement after having done the appropriate calculation.

Arguments are named like variables (in fact they can be though of as a kind of local variable). So these named arguments can be referred to in the processing and calculation of the function.

The function below displays the current date and time:

	//function name
	function today()
	//begin function statements
	{
	//declare variable dayAndTime
	// and initialise to become a new Date object
	var dayAndTime = new Date()
	//write date to browser page
	document.write("The date is: " + dayAndTime)
	//end function statements
	}
	//invoke function
	today()
      

The above code includes many comments explaining how it functions. Without these comments the function and its invocation would look as follows:

	function today()
	{
	//declare variable dayAndTime
	// and initialise to become a new Date object
	var dayAndTime = new Date()
	//write date to browser page
	document.write("The date is: " + dayAndTime)
	}
	today()
      

The browser output is as follows:

This function has been defined with:

	function today() 
	{
	...
	} 
      

The first statement declares a variable called dayAndTime initialised with a Date object. Date was discussed in previous units, and allows access to the current date and time.

At this point, the function now has the date and time stored, but has not yet displayed it. The second statement does this:

    document.write("The date is: " + dayAndTime)

In the second statement the document object refers to the primary Web browser window. The method write allows content to be written to the browser.

The output is determined in this particular instance by:

    ("The date is: " + dayAndTime)

The value of variable dayAndTime is converted to text and concatenated (added) to the String "The date is: ".

This new String is sent to the write statement and displayed in the browser window.