Form Follows Function: HTML Form Validation with JavaScript

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Our neighbor once asked my wife how her husband could be traveling all over the country teaching Java. He figures he, a nonprogrammer, taught himself JavaScript in a couple of weekends, so why would programmers travel somewhere to learn Java or JavaScript (or whatever they call it)? Well, my neighbor—or anyone else for that matter—is not going to be developing an accounts payable system in JavaScript soon, but a good programmer can develop an A/P system in Java.

The point is that JavaScript is not Java. JavaScript is a small interpreted scripting language that runs in the context of a browser. JavaScript’s most common use is to jazz up user interfaces and perform simple validation of user input. Java, as with RPG and COBOL, is a complete, compiled programming language that professional programmers use to build entire applications.

Originally, JavaScript was called LiveScript. When Netscape, the LiveScript creators , first introduced its new scripting language, no one cared; but, when Netscape’s marketing department renamed LiveScript to JavaScript, Netscape’s stock jumped $20 in one day.

Valifying Data (or Validate and Verify)

You can do a lot of cool things with JavaScript to liven up your Web pages. Personally, I leave that stuff to Web developers. For me, a business programmer, all I want JavaScript to do is validate HTML input fields on the browser. The trick to developing Web applications that perform well is to use the Internet as little as possible. A Web application that passes data from an HTML input form to a server-side application program is inefficient if the server-side application responds with “invalid date” or “quantity must be entered.” This is true if it’s an AS/400 RPG CGI program, cross-platform Java servlet, or a Microsoft Active Server Page (ASP).

Simple editing of HTML input fields can be done on the client-side of the application by the browser using JavaScript, and this article shows you how easy it is.

Figure 1 (page 79) shows a simple address input form. The user entered his address but keyed the wrong state abbreviation. The pop-up window at the top of the form

informs the user of the inappropriate value. Figure 2 shows the HTML file that contains the HTML input form in Figure 1, as well as the JavaScript that edits that form. (For more on HTML forms, see “In the HTML Driver’s Seat” by Teresa Pelkie, MC, December 1999.)

On Submit

The form statement includes an option called onSubmit. Qualifying the onSubmit option with the return keyword and the name of a JavaScript function creates a link between an HTML input form and a JavaScript edit function. The keyword this, by the way, simply means the validate function is passed the form data from the current form. If an AS/400 systems programmer had designed JavaScript, he would have probably used *CURRENT instead.

The code for the validate function in Figure 2 immediately follows the HTML form. Notice that the validate function is embedded to the HTML document. That means that, as with standard HTML, the browser is responsible for parsing the JavaScript code. The JavaScript function is wrapped by a set of script tags. The opening tag identifies the language as JavaScript. Many browsers support several scripting languages. You might, for instance, use VBScript (although that would limit your clients to those with Internet Explorer [IE]). Since both Netscape Navigator and Microsoft Internet Explorer support JavaScript, I recommend you stick with JavaScript. If you think your client base may not be running Netscape 2.x or IE 3.x or higher, you should add the comment line that follows the opening tag:

Those two lines effectively hide the JavaScript code from browsers that don’t support JavaScript.

Mandatory Code

The validate routine begins by defining and initializing a set of variables. Notice that the parameter to the function, called form, is a handle to all the input values from the HTML form. The function retrieves values from input fields by qualifying the field name with the form parameter name and then asking for its value. For instance, to set the value of the city variable, I used the following statement:

var city =;

The validate function then checks to see if the mandatory fill fields contain data. The exclamation point is a NOT operator so !firstName is a Boolean condition that evaluates to true when no value is entered for the HTML form’s first name field (but firstName != “” would work just as well). The bodies of the mandatory fill conditional statements all position the cursor to the form field and then append an error statement to the errorMessage string. After all mandatory fill fields are checked, the following block of code pops up an error window if the errorMessage contains any data:

if (errorMessage != “”) {

errorMessage = “Required field: “ + errorMessage;


return false;


The alert function popped up the error window shown in Figure 1. The return statement forces the validate function to exit, and, when it returns false, it tells the browser not to send the invalid HTML input data to the server for processing.

Numbers and Arrays

The code for validating the state code illustrates how you can use arrays in JavaScript, although it would be better if the form contained a select list for state. To declare an array, you initialize its value with a comma-separated list. The states array is all uppercase, so the validate function uses JavaScript’s toUpperCase function to convert the user-entered state code to uppercase. The indexOf function, which is automatically available to any array and is comparable to RPG’s lookup opcode, is then executed on the states array passing the uppercase value of the state code. When the indexOf function returns negative one, you know the user entered an invalid state.

The value of a form variable can be modified with a JavaScript method as shown in the following statement to force uppercase state code.

form.state.value = upState;

The ZIP code is validated by checking two steps: First by making sure that it is a number using the isNaN function; and second, that the number is either five or nine digits long.

Birth dates are sensitive pieces of information, so the date routine validates only if year, month, or day is entered. I use the parseInt function, which ignores non-numeric characters, to convert the string value entered by the user into an integer before I perform some simple date edits. You may want to strengthen this routine further to handle things like leap year and the number of days in a particular month.

Last Minute Tips

Let me leave you with a few last minute tips. First, to debug your JavaScript, make liberal use of alerts. Second, take advantage of modular-programming techniques by putting your JavaScript routines into separate files then including them in your HTML files with the src option of the SCRIPT tag:

Third, test your JavaScript code on both Netscape and IE because scripts may behave differently on the various browsers. Both Netscape and IE have tempting extensions to the base JavaScript language, which was standardized last year by the European Computer Manufacturers Association (ECMA) as ECMAScript, that do not work on competitors’ browsers. For more information on JavaScript, visit the Web sites listed in the References and Related Materials list.


• “e-RPG: Building AS/400 Web Applications with RPG” by Bradley V. Stone, Midrange Computing (

•’s JavaScript Tip Archive: html

•’s Validate page: html/validation.html

• Microsoft Windows Script Technologies Web site:

• Sun Microsystems/Netscape Communications Alliance Java Script Sample Code Web site: javascript.html

Form_Follows_Function-_HTML_Form_Validation...204-00.png 400x308

Figure 1: Basic editing of dates, codes, and mandatory fill fields should be performed on the client not on the server.

Form Validation with JavaScript

onSubmit="return validate(this)">

Name and Address:






Birthdate (MM/DD/YY):  



Figure 2: JavaScript is embedded into HTML. It is parsed and interpreted by the Web browser not the Web server.

Don Denoncourt

Don Denoncourt is a freelance consultant. He can be reached at

MC Press books written by Don Denoncourt available now on the MC Press Bookstore.

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