JavaScript Functions
A function is a reusable code-block that will be executed by an event, or
when the function is called.
Examples
Function
How to call a function.
Function with arguments
How to pass a variable to a function, and use the variable in the function.
Function with arguments 2
How to pass variables to a function, and use these variables in the function.
Function that returns a value
How to let the function return a value.
A function with arguments, that returns a value
How to let the function find the
product of 2 arguments and return the result.
JavaScript Functions
To keep the browser from executing a script as soon as the page is loaded,
you can write your script as a function.
A function contains some code that will be executed only by an event or by a
call to that function.
You
may call a function from anywhere within the page (or even from other pages if
the function is embedded in an external .js file).
Functions are defined at the beginning of a page, in the <head> section.
Example
<html>
<head>
<script type="text/javascript">
function displaymessage()
{
alert("Hello World!")
}
</script>
</head>
<body>
<form>
<input type="button" value="Click me!"
onclick="displaymessage()" >
</form>
</body>
</html>
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If the line: alert("Hello world!!"), in the example above had not been written
within a function, it would have been executed as soon as the line was loaded.
Now, the script is not executed before the user hits the button. We have added
an onClick event to the button that will execute the function displaymessage()
when the button is clicked.You will learn more about JavaScript events in the
JS Events chapter.
How to Define a Function
The syntax for creating a function is:
function functionname(var1,var2,...,varX)
{
some code
}
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var1, var2, etc are variables or values passed into the function. The { and
the } defines the start and end of the function.
Note: A function with no parameters must include the parentheses ()
after the function name:
function functionname()
{
some code
}
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Note: Do not forget about the importance of capitals in JavaScript!
The word function must be written in lowercase letters, otherwise a JavaScript
error occurs! Also note that you must call a function with the exact same
capitals as in the function name.
The return Statement
The return statement is used to specify the value that is returned from the
function.
So, functions that are going to return a value must use the return
statement.
Example
The function below should return the product of two numbers (a and b):
function prod(a,b)
{
x=a*b
return x
}
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When you call the function above, you must pass along two parameters:
The returned value from the prod() function is 6, and it will be stored in the variable
called product.
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