Controlling Data Entry Using Form Fields
Creating 'Encoded' Name & Value Pairs
Passing data from one form to another
Addressing Form Field Validation with Regular Expressions and JavaScript 1.2
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Published on: Sunday 29th April 2001 By: Paul Wilton
Forms provide us with a way of grouping HTML interaction elements with a common purpose together. For example, a form may contain elements that enable the input of a user's data for registering on a web site. Another form may contain elements that enable the user to ask for a car insurance quote. It's possible to have a number of separate forms in a single page. Pages containing multiple forms need not worry us until we are submitting information to a web server - then we need to be aware that only the information from one of the forms on a page can be submitted to the server at once.
To create a form, we use the <FORM> and </FORM> tags to declare where it starts and where it ends. The <FORM> tag has a number of attributes, such as the ACTION attribute, which determines where the form is submitted to, the METHOD attribute, which determines how the information is submitted, and the TARGET attribute, which determines the frame to which the response to the form is loaded.
Generally speaking, for client-side scripting where we have no intention of submitting information to a server, these attributes are not necessary. When in a later chapter we look at programming server pages, then these properties will come into play. For now the only attribute we need to set in the <FORM> tag is the NAME attribute, so that we can reference the form.
So, to create a blank form, the tags required would look something like:
<FORM NAME="myForm"> </FORM>
You won't be surprised to hear that these tags create a Form object, which we can use to access the form. We access this object in two ways.
Firstly, we can access the object directly using its name, here document.myForm.
Alternatively we can access the object through the document object's forms[] array property. Remember in the last chapter we talked about the document object's images[] array, and how we could manipulate it like any other array. Exactly the same applies to the forms[] array, except that instead of each element in the array holding an IMG object, it now hold a Form object. For example, if our Form was the first Form in the page, we would reference it using document.forms[0].
Many of the attributes of the <FORM> tag can be accessed as properties of the Form object. In particular, the name property of the Form object mirrors the NAME attribute of the <FORM> tag. Try It Out - The forms Array Let's have a look at an example that uses the forms array. Here we have a page with three forms on it. Using the forms[] array we access each Form object in turn and show the value of its name property in a message box.
<HTML>
<HEAD>
<SCRIPT LANGUAGE=JavaScript>
function window_onload()
{
var numberForms = document.forms.length;
var formIndex;
for (formIndex = 0; formIndex < numberForms; formIndex++)
{
alert(document.forms[formIndex].name);
}
}
</SCRIPT>
</HEAD>
<BODY LANGUAGE=JavaScript onload="window_onload()">
<FORM NAME="form1">
<P>This is inside form1</P>
</FORM>
<FORM NAME="form2">
<P>This is inside form2</P>
</FORM>
<FORM NAME="form3">
<P>This is inside form3</P>
</FORM>
</BODY>
</HTML>Save this as ch6_examp1.htm. When you load it into your browser, you should see three alert boxes, each of which shows a name of a form.
Within the body of the page we define three forms. Each form is given a name, and contains a paragraph of text.
Within the definition of the <BODY> tag, the window_onload() function is connected to the window object's onload event handler.
<BODY LANGUAGE=JavaScript onload="return window_onload()">
This means that when the page is loaded, our window_onload() function will be called.
The window_onload() function is defined in a script block in the head of the page. Within this function we loop through the forms[] array. Just like any other JavaScript array, the forms[] array has a length property, which we can use to determine how many times we need to loop. Actually, as we know how many forms there are, we could just write the number in. However, here I'm demonstrating the length property, since it is then easier to add to the array without having to change the function. Generalizing your code like this is a good practice to get into. The function starts by getting the number of Form objects within the forms array and stores it in variable numberForms.
function window_onload()
{
var numberForms = document.forms.length;Next we define a variable, formIndex, to be used in our for loop. After this comes the for loop itself.
var formIndex;
for (formIndex = 0; formIndex < numberForms; formIndex++)
{
alert(document.forms[formIndex].name);
}Remember that since the indices for arrays start at zero, our loop needs to go from an index of 0 to an index of numberForms - 1. We do this by initializing the formIndex variable to zero, and setting the condition of the for loop to formIndex < numberForms.
Within the for loop's code, we pass the index of the form we want (that is, formIndex) to document.forms[], which gives us the Form object at that array index in the forms array. To access the Form object's name property, we put a dot at the end and the name of the property, name.
Controlling Data Entry Using Form Fields
Creating 'Encoded' Name & Value Pairs
Passing data from one form to another
Addressing Form Field Validation with Regular Expressions and JavaScript 1.2