We Keep Coding

Why's my form action still being triggered when validating the form despite using e.preventDefault()?

I'd like to display a message above the name field if the user submits a name with a length greater than 20. This means the form will not get submitted - in other words, the form's action won't be triggered.
I've tried almost every suggestion I could find to prevent the form action from being triggered upon form validation but nothing seems to be working.
I've hit a wall with this and can't figure out what I'm doing wrong. How can rectify this?
html:
<form method="POST" id="form" action="/post.php">
<span class="nameError"></span>
<input type="text" class="name" name="name" placeholder="Name" required/>
<input class="button" type="submit" value="Submit"/>
</form>
Here's my jquery:
let name = $('.name');
let nameError= $('.nameError');
$(document).ready(function() {
$('input[type=submit]').on('click', function(e) {
if (name.length > 20) {
e.preventDefault();
nameError.val("Too many characters!");
return false;
}
});
});

I have modified the logic for validation. Basically we need to capture the submit event for the form and use the correct jquery methods to retreive data based upon the selectors.
$(document).ready(function() {
$("#form").submit(function( event ) {
let name = $('.name').val();
let nameError= $('.nameError');
if (name.length > 20) {
nameError.text("Too many characters!");
event.preventDefault();
}
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script>
<form method="POST" id="form" action="/post.php">
<input type="text" class="name" name="name" placeholder="Name" required/>
<label class="nameError"></label> <br/>
<input class="button" type="submit" value="Submit"/>
</form>

How to check if textbox is empty and display a popup message if it is using jquery?

I'm trying to check if the textbox is empty for my form. However, whenever I try to hit submit instead of an alert box message, telling me Firstname is empty I get "Please fill out filled".
('#submit').click(function() {
if ($('#firstname').val() == '') {
alert('Firstname is empty');
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form id="elem" autocomplete="on">
First Name:
<br>
<input type="text" name="firstname" id="firstname" required placeholder="Enter the first name" pattern="[A-Za-z\-]+" maxlength="25"><br>
<input type="submit" id="submit" value="Submit" />
</form>

Firstly I'm assuming that the missing $ is just a typo in the question, as you state that you see the validation message appear.
The reason you're seeing the 'Please fill out this field' notification is because you've used the required attribute on the field. If you want to validate the form manually then remove that attribute. You will also need to hook to the submit event of the form, not the click of the button and prevent the form submission if the validation fails, something like this:
$('#elem').submit(function(e) {
if ($('#firstname').val().trim() == '') {
e.preventDefault();
alert('Firstname is empty');
}
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form id="elem" autocomplete="on">
First Name:
<br>
<input type="text" name="firstname" id="firstname" placeholder="Enter the first name" pattern="[A-Za-z\-]+" maxlength="25"><br>
<input type="submit" id="submit" value="Submit" />
</form>
Personally I'd suggest you use the required attribute as it saves all of the above needless JS code - unless you need more complex logic than just checking all required fields have been given values.

Because you have the required property set.It is giving you Please fill out field validation as the error message.It is the validation that HTML5 is performing.

For this please make one function like :
function Checktext()
{
if ($('#firstname').val() == '') {
alert('Firstname is empty');
return false;
}
else
{
return true;
}
}
now call this function on submit button click like :
<input type="submit" id="submit" value="Submit" onclick="return check();" />

Passing value from one field to another

I want to pass the value 9112232453 of one textfield to another.
I know I need Javascript for this but I don't know how to do it.
HTML
<form method = "post" action="">
<input type="checkbox" name="phone" value="9112232453" onclick='some_func();' >
<input type="text" name="phone" value="" id="phone">
<input type="submit" name="Go">
</form>
Then later, I want to use the value in my php.

You could use a JS. function to take param (this.value) like:
<script>
var some_func = function (val){
var input = document.getElementById("phone");
input.value = val;
}
</script>
<form method = "post" action="">
<input type="checkbox" name="phone" value="9112232453" onclick='some_func(this.value);' >
<input type="text" name="phone" value="" id="phone">
<input type="submit" name="Go">
</form>

The best way is to not obtrude the HTML code with Javascript event handlers.
So, you can add a DOMContentLoaded event listener to the document, and as soon as DOM is loaded:
You add a change event listener to the input[type=checkbox], and then:
1.1. If the checkbox is checked, then you change the input#phone's value to its value
1.2. If not, then you empty the input#phone's value.
document.addEventListener('DOMContentLoaded', function() {
document.getElementById('cbphone').addEventListener('change', function(e) {
var phone = document.getElementById('phone');
if (this.checked) {
phone.value = this.value;
// you can even enable/disable the input#phone field, if you want to, e.g:
// phone.disabled = false;
}
else {
phone.value = '';
// phone.disabled = true;
}
});
});
<form method="post" action="">
<input type="checkbox" name="cbphone" id="cbphone" value="9112232453">
<input type="text" name="phone" id="phone">
<input type="submit" name="Go" value="Go">
</form>

before submit form use validation and check whether the field value is filled up or not. if yes get value of the field.
if(document.getElementBy("fieldIdfirst").value!="")
{
document.getElementBy("fieldIdSecond").value=document.getElementElementById("fieldIdfirst");
}
Thanks it..

Try this: http://jsfiddle.net/yhuxy4e1/
HTML:
<form method = "post" action="">
<input type="checkbox" name="phone" value="9112232453" onclick='some_func();' id="chk_phone">
<input type="text" name="phone" value="" id="txt_phone">
<input type="submit" name="Go">
</form>
JavaScript:
some_func = function() {
var checkBox = document.getElementById('chk_phone');
var textBox = document.getElementById('txt_phone');
textBox.value = checkBox.value;
}

JavaScript custom validation not working

Here is my code :
<script type="text/javascript">
function submitform() {
if(document.getElementById('name').value=='') {
alert('Please enter a name');
return false;
}
}
</script>
<form action="mail.php" method="post" onsubmit="submitform();">
<input type="text" id="name" name="name" placeholder="name">
<input type="submit" value="submit">
</form>
as expected, the form when submitted should call the submitform function, and if the name field is blank, it should return false and give an alert.
But, it just goes through.
Any explainations?

You need to call the function with return, so that the false value prevents default action (form submission)
<form action="mail.php" method="post" onsubmit="return submitform();">
<input type="text" id="name" name="name" placeholder="name">
<input type="submit" value="submit">
</form>

You need to stop a little.
You can use onSubmit, but it's best to delete your input submit and put a button.
Then on button click you can do what you want and eventually submit the form
Form:
<form action="mail.php" method="post" id="mailForm">
<input type="text" id="name" name="name" placeholder="name">
<button id="submitMailForm">Submit</button>
JS:
$( document ).on( "click", "#submitMailForm", function(e) {
//My control Here
//If all ok
$("#mailForm").submit();
});

You can use jquery instead of javascript for this kind of validation is will be very easy to implement.
<form action="mail.php" method="post">
<input type="text" id="name" name="name" placeholder="name">
<input type="submit" value="submit" id="submit">
</form>
<script>
$(document).ready(function(){
$("#submit").click(fucntion(e){
if($("#name").val() == ""){
alert("Name is empty");
e.preventDefault();
}
});
});
</script>
And dont forget to add jquery library before the script tag.

You need to change your onSubmit attribute as follows
onsubmit="return submitform();"
So your html look like this
<form action="mail.php" method="post" onsubmit="return submitform();">
<input type="text" id="name" name="name" placeholder="name">
<input type="submit" value="submit">
</form>

To cancel submission, the listener needs to return true or false. Also, if the function validates the fields, far better to name it for what it does rather than when it does it so call it something like "validateForm".
Also, giving a control a name of "name" masks the form's own name property. While that doesn't matter here, in general it's not a good idea to give any form control a name that is the same as a standard property of a form (e.g. "submit" or "reset").
So you might end up with something like:
<script>
function validateForm(form) {
if (form.personName.value == '') {
alert('Please enter a name');
return false;
}
}
</script>
<form ... onsubmit="return validateForm(this);">
<input type="text" name="personName">
<input type="submit">
</form>

<script type="text/javascript">
function submitform(event) {
if(document.getElementById('name').value=='') {
alert('Please enter a name');
event.preventDefault();
return false;
}
}
</script>
<form action="mail.php" method="post" onsubmit="submitform(event);">
<input type="text" id="name" name="name" placeholder="name">
<input type="submit" value="submit">
</form>
You need to prevent default of submit. In JS return false does not stop the propagation of the "submit" function (with frameworks can be different).
I suggest you to read:
event.preventDefault() vs. return falseenter link description here

just try this script
function submitform() {
var x = document.forms["fname"].value;
x = x.trim(); // Remove white spaces
if (x==null || x=="") {
alert("First name must be filled out");
return false;
}
}

Why my form doesn't validate correctly?

I'm trying to validate a form, but doesn't work :\ , When I submit the form goes to mail.php even if the required fields are missing, but I set onsubmit to validate() so it should check, but doesn't work. What's the problem with my code? I can't find it.
HTML:
<form action="mail.php" onsubmit="return validate()" method="post" class="contact-form" id="contactForm">
<div id="errors"></div>
<label for="author">Name:</label><br/><br/>
<input type="text" name="author" id="message" /><br/><br/>
<label for="author">Message:</label><br/><br/>
<textarea name="message" id="message"></textarea><br/><br/>
<input type="submit" class="button" value="Send Message"/>
</form>
Javascript:
<script type="text/javascript">
function error(message){
return "<p class=\"error\">"+message+"</p>";
}
function validate(){
var form = document.getElementById("contactForm");
var author = document.getElementById("author");
var message = document.getElementById("messsage");
var errors = document.getElementById("errors");
alert(author.value);
if(message.value == '' || author.value == ''){
errors.innerHTML = error("Please fill in all fields.");
return false;
} else {
return true;
}
}
</script>

id=author on your first input element.
Also check out jQuery it will save you time in the long run

You have two elements with the id message and none with author.
The Markup Validator would have picked this up for you.

var message = document.getElementById("messsage");
message has an extra "s".
<input type="text" name="author" id="message" />
You need to change "message" to "author"

This is wrong:
<input type="text" name="author" id="message" />
Need to set name and id to the same values (you're using id="message" for the next field, so there's a clash.
Also both your label tags have for="author"; the second one is wrong.
I guess your problem here is too much copy+paste. ;)