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How can I add an animation on my HTML form when submit input is clicked and the input is empty on Javascript?

Basically, I'm coding a Javascript code that validate if a form is empty, so if one input is empty, it add an animation from Animate.css library. And if two inputs are empty, both will make the shake animation, if the whole form is empty, it will shake.
I've tried a global function with conditions that add a class and it doesn't work.
This is my form:
<form action="" id="form">
<label for="name">Name</label>
<input
type="text"
placeholder="Name"
id="name"
minlength="3"
required
/>
<br />
<label for="email">Email</label>
<input type="email" placeholder="Emai" id="email" required />
<br />
<label for="subject">Subject</label>
<input
type="text"
placeholder="Subject"
id="subject"
minlength="3"
required
/>
<br />
<label for="message">Message</label>
<textarea
name="message"
id="message"
minlength="5"
placeholder="Message"
required
style="resize: none; height: 200px"
></textarea>
<br />
<button type="submit" class="paper-btn" id="submit">
Send message
</button>
</form>
Javascript:
(function () {
var form = document.getElementById("form"),
name = form.name,
email = form.email,
subject = form.subject;
message = form.message;
function validateName(e) {
if (name.value == "" || name.value == null) {
form.classList.add("animate__animated");
name.classList.add("animate__shakeX");
e.preventDefaul();
} else {
console.log("error");
}
}
function validateEmail(e) {
if (email.value == "" || email.value == null) {
email.classList.add("animate__animated");
email.classList.add("animate__shakeX");
e.preventDefaul();
}
}
function validateSubject(e) {
if (subject.value == "" || subject.value == null) {
subject.classList.add("animate__animated");
subject.classList.add("animate__shakeX");
e.preventDefaul();
}
}
function validateMessage(e) {
if (subject.value == "" || subject.value == null) {
message.classList.add("animate__animated");
message.classList.add("animate__shakeX");
e.preventDefaul();
}
}
function validateForm(e) {
validateName(e);
validateEmail(e);
validateSubject(e);
validateMessage(e);
}
form.addEventListener("submit", validateForm);
});

I think you just need to invoke the function. ValidateForm isn't being called
(function() {
var name = document.getElementById("name");
if (name.value == "" || name.value == null) {
name.classList.add("animate__animated");
name.classList.add("animate__shakeX");
}
})();

How to pass a value to a function and cont execute

I have a form and I'm validating the fields "onblur". what I trying to do is that when the user clicks submit make that any field is empty.
What I was trying to do is to pass the value to a function and run that function when the user click "submit" but I'm having a problem in doing that.
can somebody point me in the right direction on how to fix my problem.
HTML:
<form method="post" name="registerForms" >
<div class="form-group">
<label for="nusernames">Username: <span id="nusernamesErr" class="error">* </span></label>
<input type="text" class="form-control" id="nusernames" name="nusernames" onblur="validateForm('nusernames')">
</div>
<div class="form-group">
<label for="nemail">Email: <span id="nemailErr" class="error">* </span></label>
<input type="email" class="form-control" id="nemail" name="nemail" onblur="validateForm('nemail')">
</div>
<input type="submit" class="btn btn-default" value="Submit" id="registerButton">
</form>
JS:
function validateForm(id)
{
var value = document.getElementById(id).value;
var ok = true;
if(value === "" || value == null)
{
document.getElementById(id+'Err').innerHTML = "* <img src='images/unchecked.gif'> Field is required";
ok = false
yesNo(ok);
}
else
{
document.getElementById(id+'Err').innerHTML = "* ";
}
}
var button = document.getElementById('#registerButton');
button.onclick = function yesNo(ok)
{
alert("There's something wrong with your information!")
if(ok == false)
{
alert("There's something wrong with your information!")
return false;
}
}

If you want to attach the validation on the click event for your submit button I would suggest you to repeat the validation for each input field like you do on blur event.
Moreover, I would suggest you to save the ok value as an attribute of each input field. Set those attributes at dom ready to false and change it to true/false in validateForm function.
When submitting it's a good idea to run your valodator function and test for false fields.
You can use addEventListener in order to register a event handler, querySelectorAll for selecting elements.
The snippet:
function validateForm(id) {
var value = document.getElementById(id).value;
if (value === "" || value == null) {
document.getElementById(id+'Err').innerHTML = "* <img src='images/unchecked.gif'> Field is required";
document.getElementById(id).setAttribute('yesNo', 'false');
} else {
document.getElementById(id+'Err').innerHTML = "* ";
document.getElementById(id).setAttribute('yesNo', 'true');
}
}
document.addEventListener('DOMContentLoaded', function(e) {
document.querySelectorAll('form[name="registerForms"] input:not([type="submit"])').forEach(function(ele, idx) {
ele.setAttribute('yesNo', 'false');
});
document.getElementById('registerButton').addEventListener('click', function(e) {
var ok = true;
document.querySelectorAll('form[name="registerForms"] input:not([type="submit"])').forEach(function(ele, idx) {
validateForm(ele.id);
if (ele.getAttribute('yesNo') == 'false') {
ok = false;
}
});
if (ok == false) {
console.log("There's something wrong with your information!")
e.preventDefault();
}
});
});
<form method="post" name="registerForms" action="http://www.google.com">
<div class="form-group">
<label for="nusernames">Username: <span id="nusernamesErr" class="error">* </span></label>
<input type="text" class="form-control" id="nusernames" name="nusernames" onblur="validateForm('nusernames')">
</div>
<div class="form-group">
<label for="nemail">Email: <span id="nemailErr" class="error">* </span></label>
<input type="email" class="form-control" id="nemail" name="nemail" onblur="validateForm('nemail')">
</div>
<input type="submit" class="btn btn-default" value="Submit" id="registerButton">
</form>

You were trying to define var button with this
var button = document.getElementById('#registerButton');
but it needs to be this with regular javascript
var button = document.getElementById('registerButton');
That seemed to solve the problem

Creating a dynamic form with input buttons

I've tried many different methods, and even tried searching on SO. No answer was what I was looking for.
What I want is to have two input buttons that do some things in pure javascript.
Button one: Have it say "Add" when the page loads. When clicked, the value changes to "Cancel." Also, when it's clicked, have it display a form with three fields. When it's clicked again, have the form disappear. One named 'name', the second named 'location', the third named 'type'. I want the user to be able to submit these three things and have them be stored in the code.
Button two: Take the user input from the form and each time the user clicks, it displays all three information values, but have the button act as random generator. Let's say the code has 5 separate entries, I want them to be randomly selected and displayed when the button is clicked.
Like I said, I tried to make this work, but couldn't quite get over the top of where I wanted to go with it. If you want to see my original code, just ask, but I doubt it will be of any assistance.
Thanks in advance.
EDIT: Added the code.
function GetValue() {
var myarray = [];
var random = myarray[Math.floor(Math.random() * myarray.length)];
document.getElementById("message").innerHTML = random;
}
var testObject = {
'name': BWW,
'location': "Sesame Street",
'type': Bar
};
localStorage.setItem('testObject', JSON.stringify(testObject));
var retrievedObject = localStorage.getItem('testObject');
function change() {
var elem = document.getElementById("btnAdd1");
if (elem.value == "Add Spot") {
elem.value = "Cancel";
} else elem.value = "Add Spot";
}
window.onload = function() {
var button = document.getElementById('btnAdd1');
button.onclick = function show() {
var div = document.getElementById('order');
if (div.style.display !== 'none') {
div.style.display = 'none';
} else {
div.style.display = 'block';
}
};
};
<section>
<input type="button" id="btnChoose" value="Random Spot" onclick="GetValue();" />
<p id="message"></p>
<input type="button" id="btnAdd1" value="Add Spot" onclick="change();" />
<div class="form"></div>
<form id="order" style="display:none;">
<input type="text" name="name" placeholder="Name of Resturant" required="required" autocomplete="on" />
<input type="text" name="type" placeholder="Type of Food" required="required" autocomplete="off" />
<input type="text" name="location" placeholder="Location" required="required" autocomplete="off" />
<input type="submit" value="Add Spot" />
</form>
</div>
</section>
The randomizer works, and so does the appear/hide form. Only thing is storing the input and switching the input value.

Here's one way to do this. Each form submission is stored as an object in an array. The random button randomly selects an item from the array and displays it below.
HTML:
<section>
<input type="button" id="btnChoose" value="Random Spot" />
<p id="message"></p>
<input type="button" id="btnAdd1" value="Add Spot" />
<div class="form">
<form id="order" style="display:none;">
<input id="orderName" type="text" name="name" placeholder="Name of Resturant" required="required" autocomplete="on" />
<input id="orderType" type="text" name="type" placeholder="Type of Food" required="required" autocomplete="off" />
<input id="orderLocation" type="text" name="location" placeholder="Location" required="required" autocomplete="off" />
<input type="submit" value="Add Spot" />
</form>
</div>
</section>
<div id="randomName"></div>
<div id="randomLocation"></div>
<div id="randomType"></div>
JS:
var formData = [];
var formSubmission = function(name, location, type) {
this.name = name;
this.location = location;
this.type = type;
}
var spotName = document.getElementById("orderName"),
spotLocation = document.getElementById("orderLocation"),
spotType = document.getElementById("orderType");
var addClick = function() {
if (this.value === 'Add Spot') {
this.value = "Cancel";
document.getElementById('order').style.display = 'block';
}
else {
this.value = 'Add Spot';
document.getElementById('order').style.display = 'none';
}
}
document.getElementById("btnAdd1").onclick = addClick;
document.getElementById('order').onsubmit = function(e) {
e.preventDefault();
var submission = new formSubmission(spotName.value, spotLocation.value, spotType.value);
formData.push(submission);
submission = '';
document.getElementById('btnAdd1').value = 'Add Spot';
document.getElementById('order').style.display = 'none';
this.reset();
}
var randomValue;
document.getElementById('btnChoose').onclick = function() {
randomValue = formData[Math.floor(Math.random()*formData.length)];
document.getElementById('randomName').innerHTML = randomValue.name;
document.getElementById('randomLocation').innerHTML = randomValue.location;
document.getElementById('randomType').innerHTML = randomValue.type;
}

I was working on something since you first posted, and here is my take on it:
HTML:
<section>
<p id="message">
<div id="name"></div>
<div id="location"></div>
<div id="type"></div>
</p>
<input type="button" id="btnAdd" value="Add" onclick="doAdd(this);" />
<input type="button" id="btnShow" value="Show" onclick="doShow(this);" />
<div class="form">
<script id="myRowTemplate" type="text/template">
<input type="text" name="name" placeholder="Name of Resturant" required="required" autocomplete="on" onchange="onChanged(this, {{i}})" />
<input type="text" name="type" placeholder="Type of Food" required="required" autocomplete="off" onchange="onChanged(this, {{i}})" />
<input type="text" name="location" placeholder="Location" required="required" autocomplete="off" onchange="onChanged(this, {{i}})" />
</script>
<form id="order" style="display:none;">
<div id="formItems">
</div>
<input type="button" value="Add Spot" onclick="addSpot()" />
</form>
</div>
</section>
JS:
function GetValue() {
if (enteredItems.length) {
var entry = enteredItems[Math.floor(Math.random() * enteredItems.length)];
document.getElementById("name").innerHTML = entry.name;
document.getElementById("location").innerHTML = entry.location;
document.getElementById("type").innerHTML = entry.type;
}
}
function doAdd(elem) {
switch (elem.value) {
case "Add":
document.getElementById('order').style.display = "";
elem.value = "Cancel";
break;
case "Cancel":
document.getElementById('order').style.display = "none";
elem.value = "Add";
break;
}
}
function doShow(elem) {
GetValue();
}
function addSpot(index) { // (note: here, index is only for loading for the first time)
if (index == undefined) index = enteredItems.length;
var newRowDiv = document.createElement("div");
newRowDiv.innerHTML = document.getElementById("myRowTemplate").innerHTML.replace(/{{i}}/g, index); // (this updates the template with the entry in the array it belongs)
if (enteredItems[index] == undefined)
enteredItems[index] = { name: "", location: "", type: "" }; // (create new entry)
else {debugger;
newRowDiv.children[0].value = enteredItems[index].name;
newRowDiv.children[1].value = enteredItems[index].location;
newRowDiv.children[2].value = enteredItems[index].type;
}
document.getElementById("formItems").appendChild(newRowDiv);
}
function onChanged(elem, index) {
enteredItems[index][elem.name] = elem.value;
localStorage.setItem('enteredItems', JSON.stringify(enteredItems)); // (save each time
}
// update the UI with any saved items
var enteredItems = [];
window.addEventListener("load", function() {
var retrievedObject = localStorage.getItem('enteredItems');
if (retrievedObject)
enteredItems = retrievedObject = JSON.parse(retrievedObject);
for (var i = 0; i < enteredItems.length; ++i)
addSpot(i);
});
https://jsfiddle.net/k1vp8dqn/
It took me a bit longer because I noticed you were trying to save the items, so I whipped up something that you can play with to suit your needs.

Form Validation using JavaScript?

I'm trying to use form validation using JavaScript, however I don't seem to get any response, not even an alert even though it's there.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Example Form</title>
<script type="text/javascript">
function CheckForBlank() {
if(document.getElementById('name').value="") {
alert("enter something valid");
return false;
}
}
</script>
</head>
<body>
<form method="post" action="2013.php" onsubmit="return CheckForBlank();">
Name: <input type="text" name="name" id="name"/>
Age: <input type="text" name="age" id="age"/>
Email: <input type="text" name="email" id="email"/>
<p><input type="submit" value="Submit" /></p>
</form>
</body>
</html>

use === or == for condition checking in javascript.
if(document.getElementById('name').value === ""){
alert("enter something valid");
return false;
}

You have to use == for comparison.= is used for assigment
if(document.getElementById('name').value == ""){
alert("enter something valid");
return false;
}

Working Demo
Here Your issue is regarding if condition only! You must use == OR === in JavaScript for comparison.
Below is corrected script!
function CheckForBlank() {
if(document.getElementById('name').value=="") {
alert("enter something valid");
return false;
}
}
If you remove Or avoid return false, form will postback Even if Validation fails! So, return false means, exiting from function after if is must, and which is missed out in another answer!!

You are using = that is assignment operator, use == comparison operator that's work fine
<head>
<title>Example Form</title>
<script type="text/javascript">
function CheckForBlank() {
if(document.getElementById('name').value=="") {
alert("enter something valid");
return false;
}
}
</script>
</head>
<body>
<form method="post" onsubmit="return CheckForBlank();">
Name: <input type="text" name="name" id="name"/>
Age: <input type="text" name="age" id="age"/>
Email: <input type="text" name="email" id="email"/>
<p><input type="submit" value="Submit" /></p>
</form>
</body>

I can't believe I've never realised this until now, although if you attach your Javascript to the form submission event, instead of the button submit event; the normal browser verification works (ie. input[type="email], required="required", etc.).
Works in Firefox & Chrome.
// jQuery example attaching to a form with the ID form
$(document).on("submit", "#form", function(e) {
e.preventDefault();
console.log ("Submitted! Now serialise your form and AJAX submit here...");
})

I have done a better way to do form validation using bootstrap. You can take a look at my codepen http://codepen.io/abhilashn/pen/bgpGRw
var g_UnFocusElementStyle = "";
var g_FocusBackColor = "#FFC";
var g_reEmail = /^[\w\.=-]+\#[\w\.-]+.[a-z]{2,4}$/;
var g_reCell = /^\d{10}$/;
var g_invalidFields = 0;
function initFormElements(sValidElems) {
var inputElems = document.getElementsByTagName('textarea');
for(var i = 0; i < inputElems.length; i++) {
com_abhi.EVENTS.addEventHandler(inputElems[i], 'focus', highlightFormElement, false);
com_abhi.EVENTS.addEventHandler(inputElems[i], 'blur', unHightlightFormElement, false);
}
/* Add the code for the input elements */
inputElems = document.getElementsByTagName('input');
for(var i = 0; i < inputElems.length; i++) {
if(sValidElems.indexOf(inputElems[i].getAttribute('type') != -1)) {
com_abhi.EVENTS.addEventHandler(inputElems[i], 'focus', highlightFormElement, false);
com_abhi.EVENTS.addEventHandler(inputElems[i], 'blur', unHightlightFormElement, false);
}
}
/* submit handler */
com_abhi.EVENTS.addEventHandler(document.getElementById('form1'), 'submit' , validateAllfields, false);
/* Add the default focus handler */
document.getElementsByTagName('input')[0].focus();
/* Add the event handlers for validation */
com_abhi.EVENTS.addEventHandler(document.forms[0].firstName, 'blur', validateFirstName, false);
com_abhi.EVENTS.addEventHandler(document.forms[0].email, 'blur', validateEmailAddress, false);
com_abhi.EVENTS.addEventHandler(document.forms[0].address, 'blur', validateAddress, false);
com_abhi.EVENTS.addEventHandler(document.forms[0].cellPhone, 'blur', validateCellPhone, false);
}
function highlightFormElement(evt) {
var elem = com_abhi.EVENTS.getEventTarget(evt);
if(elem != null) {
elem.style.backgroundColor = g_FocusBackColor;
}
}
function unHightlightFormElement(evt) {
var elem = com_abhi.EVENTS.getEventTarget(evt);
if(elem != null) {
elem.style.backgroundColor = "";
}
}
function validateAddress() {
var formField = document.getElementById('address');
var ok = (formField.value != null && formField.value.length != 0);
var grpEle = document.getElementById('grpAddress');
if(grpEle != null) {
if(ok) {
grpEle.className = "form-group has-success has-feedback";
document.getElementById('addressIcon').className = "glyphicon glyphicon-ok form-control-feedback";
document.getElementById('addressErrorMsg').innerHTML = "";
}
else {
grpEle.className = "form-group has-error has-feedback";
document.getElementById('addressIcon').className = "glyphicon glyphicon-remove form-control-feedback";
document.getElementById('addressErrorMsg').innerHTML = "Please enter your address";
}
return ok;
}
}
function validateFirstName() {
var formField = document.getElementById('firstName');
var ok = (formField.value != null && formField.value.length != 0);
var grpEle = document.getElementById('grpfirstName');
if(grpEle != null) {
if(ok) {
grpEle.className = "form-group has-success has-feedback";
document.getElementById('firstNameIcon').className = "glyphicon glyphicon-ok form-control-feedback";
document.getElementById('firstNameErrorMsg').innerHTML = "";
}
else {
grpEle.className = "form-group has-error has-feedback";
document.getElementById('firstNameIcon').className = "glyphicon glyphicon-remove form-control-feedback";
document.getElementById('firstNameErrorMsg').innerHTML = "Please enter your first name";
}
return ok;
}
}
function validateEmailAddress() {
var formField = document.getElementById('email');
var ok = (formField.value.length != 0 && g_reEmail.test(formField.value));
var grpEle = document.getElementById('grpEmail');
if(grpEle != null) {
if(ok) {
grpEle.className = "form-group has-success has-feedback";
document.getElementById('EmailIcon').className = "glyphicon glyphicon-ok form-control-feedback";
document.getElementById('emailErrorMsg').innerHTML = "";
}
else {
grpEle.className = "form-group has-error has-feedback";
document.getElementById('EmailIcon').className = "glyphicon glyphicon-remove form-control-feedback";
document.getElementById('emailErrorMsg').innerHTML = "Please enter your valid email id";
}
}
return ok;
}
function validateCellPhone() {
var formField = document.getElementById('cellPhone');
var ok = (formField.value.length != 0 && g_reCell.test(formField.value));
var grpEle = document.getElementById('grpCellPhone');
if(grpEle != null) {
if(ok) {
grpEle.className = "form-group has-success has-feedback";
document.getElementById('cellPhoneIcon').className = "glyphicon glyphicon-ok form-control-feedback";
document.getElementById('cellPhoneErrorMsg').innerHTML = "";
}
else {
grpEle.className = "form-group has-error has-feedback";
document.getElementById('cellPhoneIcon').className = "glyphicon glyphicon-remove form-control-feedback";
document.getElementById('cellPhoneErrorMsg').innerHTML = "Please enter your valid mobile number";
}
}
return ok;
}
function validateAllfields(e) {
/* Need to do it this way to make sure all the functions execute */
var bOK = validateFirstName();
bOK &= validateEmailAddress();
bOK &= validateCellPhone();
bOK &= validateAddress();
if(!bOK) {
alert("The fields that are marked bold and red are required. Please supply valid\n values for these fields before sending.");
com_abhi.EVENTS.preventDefault(e);
}
}
com_abhi.EVENTS.addEventHandler(window, "load", function() { initFormElements("text"); }, false);
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="container">
<div class="row">
<h1 class="text-center">Interactive form validation using bootstrap</h1>
<form id="form1" action="" method="post" name="form1" class="form-horizontal" role="form" style="margin:10px 0 10px 0">
<div id="grpfirstName" class="form-group">
<label for="firstName" class="col-sm-2 control-label"><span class="text-danger">* </span>First Name</label>
<div class="col-sm-10">
<input type="text" class="form-control" id="firstName" placeholder="Enter first name">
<span id="firstNameIcon" class=""></span>
<div id="firstNameErrorMsg" class="text-danger"></div>
</div>
</div>
<div class="form-group">
<label for="lastName" class="col-sm-2 control-label">Last Name</label>
<div class="col-sm-10">
<input type="text" class="form-control" id="lastName" placeholder="Enter last name">
</div>
</div>
<div id="grpEmail" class="form-group">
<label for="lastName" class="col-sm-2 control-label"><span class="text-danger">* </span>Email </label>
<div class="col-sm-10">
<input type="email" class="form-control" id="email" placeholder="Enter email">
<span id="EmailIcon" class=""></span>
<div id="emailErrorMsg" class="text-danger"></div>
</div>
</div>
<div id="grpCellPhone" class="form-group">
<label for="lastName" class="col-sm-2 control-label"><span class="text-danger">* </span>Cell Phone </label>
<div class="col-sm-10">
<input type="text" class="form-control" id="cellPhone" placeholder="Enter Mobile number">
<span id="cellPhoneIcon" class=""></span>
<div id="cellPhoneErrorMsg" class="text-danger"></div>
</div>
</div>
<div class="form-group" id="grpAddress">
<label for="address" class="col-sm-2 control-label"><span class="text-danger">* </span>Address </label>
<div class="col-sm-10">
<textarea id="address" class="form-control"></textarea>
<span id="addressIcon" class=""></span>
<div id="addressErrorMsg" class="text-danger"></div>
</div>
</div>
<div class="form-group">
<div class="col-sm-offset-2 col-sm-10">
<button type="submit" class="btn btn-success">Save</button>
</div>
</div>
</form>
</div> <!-- End of row -->
</div> <!-- End of container -->
Please check my codepen to better understand code.

Validation With Java Script and printing

I have a form in HTML and that if the fields are left blank, the Javascript will print inside the fields error. Please can some one give me a piece of code that will validate the form and then will print Error on top of the form if its left blank and not inside the fields of the form?
My Form:
<form id="contact" onsubmit="checkContactForm(); return false;" onreset="resetForm();">
<p>Fill in the form below to send me a message!</p>
<div id="errormessage"></div>
<p>
<label for=""> </label>
<input type="text" name="" id="" onfocus="" />
<p>
<label for="name">Name:</label>
<input type="text" name="name" id="name" onfocus="resetField(this);" />
</p>
<p>
<label for="email">E-mail address:</label>
<input type="text" name="email" id="email" onfocus="resetField(this);" />
</p>
<p>
<label for="message">Your Message:</label>
<textarea name="message" id="message" rows="5" cols="25" onfocus="resetField(this);"></textarea>
</p>
<p>
<button type="submit">Send Message</button>
<button type="reset">Reset Form</button>
</p>
My Javascript:
var requiredFields = ["name", "email", "message"];
function checkContactForm() {
var myForm = document.forms[0];
for (i in requiredFields) {
fieldName = requiredFields[i];
if (!myForm[fieldName].value || myForm[fieldName].value == "Error") {
myForm[fieldName].style.color = "#f66";
myForm[fieldName].value = "";
var emptyFields = true;
}
}
if (!emptyFields) { myForm.submit(); }
}
function resetField(myField) {
if (myField.value == "Error") {
myField.style.color = "#000";
myField.value = "";
}
}
function resetForm(myForm) {
var myForm = document.forms[0];
for (i in requiredFields) {
fieldName = requiredFields[i];
myForm[fieldName].style.color = "#000";
}
}

Since HTML5 there is a form-validation API (http://www.w3schools.com/js/js_form_validation.asp)
Here you can find a pretty good "tutorial": http://www.smashingmagazine.com/2009/07/07/web-form-validation-best-practices-and-tutorials/

If I've understand you want the error will be printed in the #errormessage div, right?
If so you can simply do something like this:
function addError(str){
document.getElementById("errormessage").innnerHTML = document.getElementById("errormessage").innerHTML + str + "<br>";
}
function checkContactForm() {
var myForm = document.forms[0];
for (i in requiredFields) {
fieldName = requiredFields[i];
if (!myForm[fieldName].value || myForm[fieldName].value == "Error") {
myForm[fieldName].style.color = "#f66";
myForm[fieldName].value = "";
var emptyFields = true;
addError(fiedlName+" is empty!");
}
}
if (!emptyFields) { myForm.submit(); }
}