Lab 06 - The Smart Money


You’ve been offered a “starter loan” of $36,000. This is a fantastic opportunity, but what are you going to do with that money? This week's lab will help you explore three options: bank savings, casino games, and stocks. We will, of course, be doing this all with JavaScript!


Part of your grade for this lab will depend on how well you make use of JavaScript functions.


You must create a folder on your web drive called "Lab06" (without the quotes) and store your work in that directory. This lab does NOT draw on the previous labs, so there is no need to copy your old web pages into this directory.

This week you will make a single new web page, money.html, such that:

  1. Your page has a prelude declaring it as an XHTML document, and it roughly follows XHTML rules.  Validation is encouraged but not required. 
    (Hint: place HTML comments around your JavaScript, as the book does, so that the validator won’t complain about your JavaScript).
  2. Your page is reasonably appealing to look at (e.g. using styles, colors, etc). Your page should be tasteful and easy to read.  No need to spend a lot of time on this, just make it reasonable.
  3. When finished, you will have a page that looks something like this when it is first loaded:

    You do not have to make it look exactly like this (colors etc.) but the general layout should be similar.
  4. The page will have a single button that, when clicked, performs all of the calculations described below. It is strongly recommended that you first make the HTML for the left column only, and get the JavaScript working for just this column (“investing”), then later handle the “craps” and “stocks” columns.  When complete, here is one sample output after clicking the “Calculate Results” button:
  5. You must have separate functions for the savings, craps, and stocks computations (described below). You should also define other functions as appropriate – avoid repeating code where possible! At least one of your functions must return some value, and at least one function must have one or more arguments (you will likely have more than this).

6.      You must use at least one JavaScript array when performing your calculations.

  1. As always, you must provide comments for your code!  Give a brief description of each function, and (aside from very short functions) the major sections inside each function. Remember that Java Script comments start with // or are enclosed in /*  */.

  2. The leftmost column shows what would happen if you put your money (called “principal” in finance) in a savings account. The user will provide the annual interest rate earned on the money (initially $36,000) in an input field. You will have four additional input fields below the interest rate which, when the calculate button is hit, will be populated with the value of your investment after 10, 20, 30, and 40 years, respectively. The formula for computing the value of your investment over time is:

Value = Principal * e(rate * time in years)

(Hint: There are some handy functions in the Math object that will help you – see textbook section 11.3)

Your page should generate an error (popup message) if the user does not specify valid interest rate: a number greater than zero and less than or equal to 0.2 (20%). Additionally, your page should not perform any calculations (e.g., do nothing for the investing, craps, or stocks columns) until the user enters a legal value. Instead, do nothing and just wait for them to enter a correct value and click the calculate button again.

  1. The middle column represents what would happen if you played craps with your money. The user will provide the amount bet on each hand in an input field, and you simulate playing a total of 1,000 games. You will have four additional input fields below the bet amount which, when the calculate button is hit, will be populated with the amount of money you have after 10 games, after 90 more games if you keep playing (a total of 100 games), after 400 more games if you keep playing (total 500 games), and after 500 more games if you keep playing (total 1000 games). Label these fields appropriately! If at any time the amount of money you have becomes zero (or less!), it should remain zero for the rest of the games (i.e. it's impossible to go into debt).

    Do not simulate the whole craps game here (but see the extra credit). The odds of winning each individual craps game is 49.3%. If the player wins, their cash increases by the bet amount. If they lose, the bet amount is subtracted from their cash. You will need a random number generator to play the odds. Hint: JavaScript function Math.random() should be useful – see textbook section 9.5.

    Your page should generate an error if the user does not specify a valid bet (a number greater than zero and less than or equal to $36,000). Additionally, your page should not perform any calculations (e.g., do nothing for the investing, craps, or stocks columns) until the user enters a legal bet value.
  2. The last section represents what happens if you invested all your money in stocks.  The user can choose low or high risk.  For low risk, you normally earn 7% (simple interest – no need to use the compounding equation above) on your money each year, though there is a 5% chance that a stock market crash instead wipes out half of your money.  For high risk, you normally earn 10% on your money, with a 15% chance that a crash instead wipes out half of your money.

    Use radio buttons to select low or high risk. If a particular button has the ID of “lowrisk” then document.getElementById("lowrisk").checked returns a value (true or false) indicating checked or not.

    The default should be that neither low or high risk is checked in the form. Display an error message if the user has not picked one when they hit the calculate button.  Again, your page should not perform any calculations (e.g., do nothing for the investing, craps, or stocks columns) until the user has made a selection for this.


1.      You do NOT want to create all of the HTML for the table, input boxes etc. using JavaScript.  Instead almost all of this will be done with XHTML in the <body> section.  Then you will use JavaScript to interact with the XHTML you create. 

2.      The example on the slides and in section 9.6 is very helpful for understanding a.) how to use JavaScript to read/write the value of an XHTML input box b.) how to use JavaScript to make something happen when you click on a button.  Note though that the book example is simulating individual craps games, which is different from what you are doing.

3.      Using Mozilla Firefox is highly recommended for testing (use the Error Console).  However, your final result should run on Firefox and IE.

4.      You should not use document.writeln() inside your functions that are called by ‘onclick’.  Doing so erases your whole page and starts over with just the new content that you just output.  Instead if you need to output something to the user (or to debug) either use window.alert() or change the value of some input box.

5.      Avoid using variables with the same name as the ‘name’ of some element in your form – in some browsers, this can cause difficult behavior.

6.      Don’t use names that start with a number, like “10years” – this goes both for regular variable names as well as the “name” and “id” of your text boxes, which are sometimes treated like variables. All names and ids in XHTML as well as variable names in JavaScript must start with a letter.

7.      If you want to use an external script file, we suggest the following syntax (modified from the lecture).  Otherwise, the browser may ignore later <script> tag if you try to include JavaScript inline in your document.  Use this:
      <script type = "text/javascript" src="calc.js"  >  </script> 
Instead of using:
      <script type = "text/javascript" src="calc.js"  />

8.      When you use JavaScript to read a value from a form, the value you get is a “string.”  You may need to convert it to a number before using it.


Your web page must be constructed using Notepad or a similar text-only editor. The use of programs such as Microsoft Word, Microsoft Frontpage, DreamWeaver, ColdFusion, Mozilla Composer,. etc. will be considered an honor offense.

Extra Credit

For a nominal amount of extra credit do some/all of the following:

1.      Make a new table that shows dynamically how your money changes over time.  For instance, have three boxes showing the amount of money you have under each of three scenarios and update these periodically (perhaps twice a second) to show how much money you have left after a certain amount of time.  To do this you will have to make some assumption about how many craps game you do per year.

2.      Write a simulator for some casino game.  Not craps, because the book does that, but perhaps for poker or blackjack.  Show the cards that were played in some appropriate way. Have a button that causes one game to be played, or else have different buttons that lets the user choose how to play (e.g., hit or stand?  what to bet? double down?).  Update your total money appropriately when a game/hand is over.


  1. Your page should contain all of the elements described in the requirements section above.
  2. Add a link from your default.htm page to Lab06/money.html
  3. All of your files should be in a folder called "Lab06" (without the quotes) on your web drive. Your instructor will assume that your web page is viewable at where XXXXXX is your alpha number. You may want to check that this URL is viewable and that everything works correctly from a computer where somebody else is logged in. If you've goofed and linked to a file on your X drive, this will help you catch it!
  4. Turn in the following hardcopy at the beginning of class on the due date, stapled together in the following order (coversheet on top):
    1. A completed assignment coversheet.  Your comments will help us improve the course.
    2. A printout of the source to your Lab06/money.html file.
    3. If you use any external script files, include a printout of those too.