Validating Form Input in JavaScript

This time we'll make a form that collects information about the
visitor at your site. You must have filled-in copious
registration forms or survey forms where you had to enter your
name, your email, your address, etc. Sometimes users,
intentionally or unintentionally, enter wrong information that
can either spoil your database scheme or give you lots of useless
data and hence, waste your precious server space.

To avoid such problems, as much as it can be managed, we
programmatically try to make sure, that data is entered in an
orderly fashion, and no unusable fields are entered. Checking
individual fields of the form does this.

We'll see a form here with three fields: Name, Phone and Email.
In this form, no field should be left blank, there should be no
numbers in the Name field [1,2,3,4,.], and in the Email field, no
email should be without the "@" sign. We can carry out more
complex validations, but at the moment, these three should
suffice.

/// Remove the extra dots while testing. They have been just
inserted so that some email programs don't freak out at the
presence of a JavaScript in the email.

<..script language="JavaScript1.2">
function CheckName(HoldName)
{
NoNumThere='true';
for(i=0; i {
for(j=0; j<10; j++)
{
if(HoldName.charAt(i)==j.toString())
{
NoNumThere='false';
break;
}
}
if(NoNumThere=='false')
{
break;
}
}
return NoNumThere;
}
function CheckMail(HoldMail)
{
IsValid='true';
if(HoldMail.indexOf("@")<=0)
{
IsValid='false';
}
return IsValid;
}
function checkfields()
{
var AllFilled='true';
for(i=0; i<3; i++)
{
if(visitor.elements[i].value.length==0)
{
alert("The field [" + visitor.elements[i].name + "] can not be
left blank.");
AllFilled='false';
visitor.elements[i].focus;
return;
}
}
if(AllFilled=='true')
{
var NameValid=true;
var EmailValid=true;
NameValid=CheckName(visitor.vname.value);
EmailValid=CheckMail(visitor.vemail.value);
if(NameValid=='false')
{
alert("Sorry, your name can not contain numbers.");
visitor.vname.focus;
}
if(EmailValid=='false')
{
alert("Sorry, this does not seem like a valid email
address.");
}
}
if(NameValid=='true' & EmailValid=='true')
{
alert("RIGHTO!!!");
}
}

Enter your name:



Enter you phone:



Enter your email:




onclick="checkfields();">

Copy and paste the code as it is, and save the entire content as
a new HTML page. Then load it on to your browser. Unless you see
the result, it'll be difficult to follow the script if you do not
have prior programming background. The first condition is, none
of the fields can be submitted blank. Click on the submit button
without entering anything and observe the reaction.

Here, we are making ample use of the recently learnt for(){.}
loop. Then we have used function too, to carry out certain
validations. Our main function, checkfields(), is associated with
the OnClick attribute of the "Submit" button, that is, when you
click on the "Submit" button, this function gets triggered.

Some new terms in today's script are: true, false, charAt(),
toString(), break, indexOf(), string.length, and
orm.elements[ ].

A quick explanation to make things easier:

If at 10:30 pm, I say, "It is night", then

var fact='true'

and if I say at 10:30 pm that "It's afternoon", then

var fact='false'

Which explains the use of true and false, which are also called
Boolean operators, which means, a Boolean variable can either be
true or false, but NEVER both.

Until we learn about arrays, every character in a string has an
index position. For instance, if we have

var city="Delhi"

then city.charAt(0)="D", city.charAt(1)="e",
city.charAt(2)="l"...city.charAt(4)="i".

toString(), converts another data type to a string data-type. For
example,

var num1=31
var num2=21
var char1=num1.toString()
var char2=num2.toString()

So,

num1+num2=52
and
char1+char2=3221

In the second case, instead of being added, the variables are
being concatenated, which indicates that they are strings, not
numbers. We'll see its application later.

break, true to its name, breaks something. In this case, it
breaks the loop in which it occurs, and takes the execution of
the program to the line immediately after the loop, without
meeting the condition required to complete the loop.

indexOf() tells us about the position of a particular character
in a string. Look its use in the following code:

var city="Delhi"

Referring to this code, city.indexOf("e") should give us a value
1 and city.indexOf("h") should give us a value 3.
city.indexOf("z") should give us a value less than zero,
indicating that it does not belong to the given string.

String.length gives us the length of the string, for instance, if
city="Delhi", then city.length would give us 5.

Again, elements[ ] is an array, and we haven't dealt with them
yet, so we leave the rest of the explanation to the next section.