2.1 Game 2302-0310 javascript  (Page 3/11)

Purpose of document.write

Although this is a simplification, for purposes of the modules in this collection, it will suffice to say that the purpose of the document.write command is to cause the items in its argument list to be displayed in the browser window. You can place one or more items in the argumentlist. If there is more than one item in the argument list, the items must be separated by a comma as shown in Listing 1 .

Displaying strings

With some exceptions, items that appear in the argument list surrounded by matching quotation marks (strings) will be displayed in the browser window.

The exceptions include the second item in the above list . This item is a special command to the browser, commonly known as a break tag. The occurrence of a break tag tells the browser to go down to the next line before displaying any additional material.

Such browser commands usually begin with a left angle bracket as shown in Listing 1 . Because of this, it is usually wise to avoid displaying strings that begin with left angle brackets unless you know for sure that your string won'tbe interpreted as a command to the browser.

Displaying material other than strings

I will show you how to display material other than strings in a later section titled The string concatenation operator . Before getting into that, however, I will discuss several other topics.

Structured programming

When a student enrolls in the Object-Oriented Programming course that I teach at Austin Community College, I expect that student to have knowledge of something calledstructured programming, which is generally defined as including the following topics:

1. Functions with parameter passing
2. Sequence
3. Selection (if-else)
4. Repetition (for, while, and do-while loops)

I also expect them to know how to use variables and how to use operators (add, subtract, multiply, divide, etc.).

You will need to have an introductory knowledge of these topics to understand the JavaScript scripts that I will use to explain physics concepts in latermodules. I will briefly explain these topics in this module and will discuss them further in later modules where they are used.

Functions

Functions, or procedures as they are called in some languages, provide a fundamental building block for virtually every programming language. The purposeof a function is to encapsulate the ability to perform a task into a single set of code and to be able to execute that code from a variety ofdifferent locations within the script. This can eliminate the requirement to repeat the same code over and over when the same task is required at multiplepoints in the script.

The surface area of a sphere

For example, if you write a script that frequently needs to calculate the surface area of a sphere, you can encapsulate those calculations in a function.Then, whenever your script needs to know the surface area of a sphere, it can simply call the function and provide the radius of the sphere as a parameter.The function will perform the calculation and return the answer to be used by the script at that point.