1	Functions Chapter 7 of Reed Abstraction and User-Defined Functions
2	Abstraction abstraction is the process of ignoring minutiae and focusing on the big picture in modern life, we are constantly confronted with complexity we don't necessarily know how it works, but we know how to use it e.g., how does a TV work? a car? a computer?
3	User-defined Functions JavaScript’s predefined functions represent a collection of useful, general-purpose abstractions the programmer can add additional abstractions via user-defined functions once defined, a user-defined function can be used the same way as a predefined function e.g., consider converting a temperature from Fahrenheit to Celsius tempInCelsius = (5/9) * (tempInFahr - 32);
4	Dissecting the Function the 1^st line specifies that we are defining a function named FahrToCelsius that takes one input the variable name in parentheses is known as a parameter when the function is called, the provided input is assigned to the parameter e.g., for the call FahrToCelsius(32), the value 32 would be assigned to tempInFahr for the calculation
5	General Function Form in general, the form of a user-defined function is as follows: function names follow the same rules as variables: consist of letters, digits, and underscores and must start with a letter note that there may be: more than one parameter, separated by commas no parameters at all, in which case the function name is followed by () more than one statement inside the curly braces ,separated by semicolons
6	Programmer defined function define the function in the HEAD so that it’ll be available when it’s called. function is not executed till it is called. once the function is defined in the HEAD, it can be called just like a predefined function. to include a user-defined function in a page, the simplest way is to include its definition in SCRIPT tags in the HEAD so it will be ready for use.
7	Temperature Conversion Page
8	Function without Return
9	Function without Return here is a function that displays a verse of the song "Old MacDonald had a Farm" the function has two parameters for the animal name and sound by calling the function with different inputs, you can display different verses how much work is it to rearrange the verses? add a new verse? change all the verses?
10	Old MacDonald each call to the OldMacVerse function displays the verse with the specified animal & sound to rearrange the verses, simply rearrange the calls in the BODY to add a new verse, simply add a new call in the BODY to make a change to all verses, (e.g., change indentation) simply make the change once in the function definition
11	Multiple Inputs if a function has more than one input, parameters in the function definition are separated by commas input values in the function call are separated by commas values are matched to parameters by order 1^st input value in the function call is assigned to the 1^st parameter in the function 2^nd input value in the function call is assigned to the 2^nd parameter in the function . . .
12	Designing Functions functions do not add any computational power to the language a function definition simply encapsulates other statements still, the capacity to define and use functions is key to solving complex problems, as well as to developing reusable code encapsulating repetitive tasks can shorten and simplify code functions provide units of computational abstraction – user can ignore details functions are self-contained, so can easily be reused in different applications
13	Parameters parameters play an important role in functions they facilitate the creation of generalized computations the function defines a formula, but certain values within the formula can differ each time the function is called
14	Dynamic Images just as you can use user-initiated events to change the contents of text boxes, you can also dynamically modify images <img name="photo" src="happy.gif" alt="Happy Face" /> causes the image stored in the file happy.gif to appear in the page you can change the image by reassigning its SRC attribute in a script. similar to the way that text boxes have their VALUE attribute reassigned absolute name of an image: document.images.IMAGE_NAME.src document.images.photo.src = "sad.gif"; replaces happy.gif with sad.gif note: images do not need to be embedded in forms
15	Dynamic Image Example initially the image is set to happy.gif when a button is clicked, the ChangeImage function is called to change the image the new image is specified by the parameter to the function
16	Dynamic Image Example