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Hide/locking a <form> button until form is completely filled with validation

I make my JSON script with a form and a button, but when I demo it on someone, they can press the submit button whenever they like. Here is my code:
<form action="action_page.php" method="GET">
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"<br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"<br><br>
</form>
All I need now is validation and how to lock or hide the button. I tried using <script> with function and getElementById but I simply do not know how to lock it. Here is the message that will be inputted when it is locked and the form isn't completed:
You have not finished the information input for the survey. Please
input "Your First Name" and "Your Last Name" to enter the survey.
When they finish, I will input a loading icon until fully loaded.
Code Language(s)
I use fiddles to make my code, so I use:
HTML
JSON
Pinch of jQuery (loading icon)
Answer Expectations
In my answers, I need:
Recommended Code Language
Code for note purposes
Crossed out attributes
Attributes before coding explanations
EDIT: I just noticed that I can use the disabled boolean attribute here, so all I need is validation to disable the boolean attribute so they can press the button.

Hello I recommend JavaScript but it is not secure at all. However its the most effective in this case. Here is the code: (Note that i changed the html too)
<form action="action_page.php" method="get" >
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname" onkeyup="inputEntries()"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname" onkeyup="inputEntries()"><br><br>
<button type="submit" name="submit" id="submit_btn" disabled="true">submit</button>
</form>
<script>
var fname_var = document.getElementById("fname");
var lname_var = document.getElementById("lname");
var submit_btn_var = document.getElementById("submit_btn");
function inputEntries(){
if(lname_var.value != "" && fname_var.value != "")
submit_btn_var.disabled = false;
else
submit_btn_var.disabled = true;
}
</script>
What it does is that if both input fields are empty the button is not clickable but if both are it can be clicked. hope it helped!

You can use either pure javascript or jQuery to validate.
In the below code you would see we are first binding a change event to the input field and getting its values. If both values are present, the disabled attribute of the submit button is removed. Else the opposite.
<p id="msg">You have not finished the information input for the survey. Please input "*Your First Name*" and "*Your
Last Name*" to enter the survey.</p>
<form action="action_page.php" method="GET">
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"<br><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname"<br><br><br>
<button type="submit" id="btn-submit">Submit</button>
</form>
$(document).ready(function(){
('#btn-submit').prop('disabled', true);
$("input[type=text]").change(function(){
if ($('#fname').val() && $('#lname').val()) {
$('#btn-submit').prop('disabled', false);
$('#msg').hide();
} else {
$('#btn-submit').prop('disabled', true);
$('#msg').show();
}
});
});
Stackblitz - https://stackblitz.com/edit/jquery-kgzhuo?file=index.html

Hiding button under condition in form in javascript

This is my first post so don't judge me if i do something wrong because i'm still learning but i have a question i'm making a form and i have a problem. When i left the button without any extra stuff when i tested the form i got as many entries as many times i clicked the button so i used a code snippet from here in JS and made the button so it disappears, problem is when i don't fill the form correctly it doesn't send the entry, so the question is how can i add the condition in form so the button only disappears when every field is written correctly? I would add the whole html but it's houndreds of lines of code now since it's pretty much where i test things here is the form code though.
My other thought was to maybe try to edit the php code, but to be honest i have no knowledge of that because my friend made that.
I also searched google and here for some tips but couldn't find any, thanks for help in advance.
$("#hideme").click(function(){
$(this).hide();
});
<form method="post" action="thankyou.php" style="overflow:hidden" class="accident-form4">
<input type="hidden" name="lang" value="pl">
<div class="form-group required">
<label for="name">Imię i nazwisko<span></span></label>
<input type="text" class="form-control inputToUpper" name="firstname" placeholder="Andrzej Kowalski" required="required" pattern=".*\S+.*" title="Wpisz swoje imię">
</div>
<div class="form-group required">
<label for="email">Email<span></span></label>
<input type="email" class="form-control" required="required" name="email" placeholder="np. andrzej.kowalski#gmail.com" title="Wpisz poprawny adres email!" pattern="[a-z0-9._%+-]+#[a-z0-9.-]+\.[a-z]{2,4}$">
</div>
<div class="form-group">
<label for="tel">Numer telefonu<span></span></label>
<input type="text" name="tel" placeholder="079 1234 5678" class="form-control" required="required" size="12" value="">
</div>
<label for="hour">Preferowana godzina kontaktu</label>
<select class="form-control" name="hour" placeholder="" title="" />
<option>Dowolna</option>
<option>Rano</option>
<option>Po południu</option>
<option>Wieczorem</option>
</select>
<input type="submit" name="lead" class="send" id="hideme" value="ROZPOCZNIJ CLAIM →">
</form>

you can simply check the value of the required fields before hiding the button.
$("#hideme").click(function(){
if (document.getElementsByName('firstname')[0].value != '' &&
document.getElementsByName('email')[0].value != '' &&
document.getElementsByName('tel')[0].value != '') {
$(this).hide();
}
});
or use your RegExp in Javascript again.
$("#hideme").click(function(){
if (document.getElementsByName('firstname')[0].value.match (/.*\S+.*/) &&
document.getElementsByName('email')[0].value.match (/[a-z0-9._%+-]+#[a-z0-9.-]+\.[a-z]{2,4}$/) &&
document.getElementsByName('tel')[0].value.match (/[0-9]+/)) {
$(this).hide();
}
});
that should to the trick (but i didn't test it).

addEventListener with a form element

I'm new with Javascript and I'm learning by myself. I have a problem with a form on my page. I just want to test with a simple javascript code that I can manipulate my input "type=submit" by adding a function to it to console.log a string when the form is submitted (I watched it on a video and I wanted to do it by myself).
Here is my code:
(function() {
"use strict";
document.getElementById('enviar').addEventListener('submit', enviar);
function enviar(event) {
event.preventDefault();
console.log("you submitted the form");
}
})();
and this is my HTML code:
<form id="escribenos" method="post">
<label for="name">Nombre y Apellido</label>
<input type="text" name="name" id="name" pattern="[A-Za-z]+\s+[A-Za-z]+" required value=""/>
<label for="email">Correo electrónico</label>
<input type="email" name="email" id="email" required />
<label for="asunto">Asunto</label>
<input type="text" name="asunto" id="asunto" />
<label for="comentario">Mensaje</label>
<textarea maxlength="250" rows="5" name="comentario" id="comentario" style="resize:none;"></textarea>
<input id="enviar" value="enviar" type="submit" ></input>
</form>
The problem that I'm having here is that is not working at all.. my ID element is selected properly but I don't know what is wrong with my code. I created a variable and console my ID selected to see if I was getting the right element from the DOM and I'm getting in the console the right input element. please if someone knows why is not working.
plus: On my text input field I have a regular expression but I'm not getting the output I want.. the goal is that the user has to write at least two names (First and Last), so when they write just one it will be incorrect.. the problem that I'm having with this regular expression if when someone writes more than two names (First, Middle and Last) I DON'T want to make that an incorrect answer because technically is correct. So I need to make a better regular expression to get that result (when the user writes two or more names, not just two) but I don't know too much about Regular Expressions.

You are getting the element with the id enviar which is the submit button element. You need to be querying based on the form's element id which is escribenos. Try running the snippet below and you can see that it has the expected outcome.
(function() {
"use strict";
document.getElementById('escribenos').addEventListener('submit', enviar);
function enviar(event) {
event.preventDefault();
console.log("you submitted the form");
}
})();
<form id="escribenos" method="post">
<label for="name">Nombre y Apellido</label>
<input type="text" name="name" id="name" pattern="[A-Za-z]+\s+[A-Za-z]+" required value=""/>
<label for="email">Correo electrónico</label>
<input type="email" name="email" id="email" required />
<label for="asunto">Asunto</label>
<input type="text" name="asunto" id="asunto" />
<label for="comentario">Mensaje</label>
<textarea maxlength="250" rows="5" name="comentario" id="comentario" style="resize:none;"></textarea>
<input id="enviar" value="enviar" type="submit" ></input>
</form>

jQuery.validationEngine v2.6.1 Only Validates Sometimes?

The code appears to be hooked up (like this)
jQuery("#contactForm").validationEngine();
because it will validate and raise an error bubble if:
you tab out of required field without any input
you type at least one character into a field that requires more and then click the submit button
But it will not validate and raise an error bubble if you do nothing at all except click the submit button. In that case, it just submits. Once you click in the field or enter anything at all, it seems to work.
What can I be looking for that I've mis-configured?
The HTML:
<form class = "contactform" id = "contactForm">
<fieldset>
<div class="contactform-name contactform-field">
<label class="contactform-label" for="contactform-name">Name:
<br>
</label>
<input class="validate[required,minSize[8]] contactform-input" type="text" id="contactform-name" name="name" />
</div>
<div class="contactform-email contactform-field">
<label class="contactform-label" for="contactform-email">Email Address:<br></label>
<input value class="validate[required,custom[email]] contactform-input" type="email" id="contactform-email" name="contactform-email" />
</div>
<div class="contactform-text contactform-field">
<label class="contactform-label" for="contactform-text">Message:
<br>
</label>
<textarea class="validate[required,minSize[12]]contactform-input" name="text" id="contactform-text" > </textarea>
</div>
<input class="contactform-button" type="submit" name="submit" value="Send" />
</fieldset>
</form>
The JavaScript (it's running in Meteor):
Template.Contact.rendered = function () {
jQuery("#contactForm").validationEngine();
}

I've never used this engine, but from the docs I found that 'attach' will attach the validator to form.submit. Can it be as simple as that?
https://github.com/posabsolute/jQuery-Validation-Engine#attach
EDIT:
You can also do stuff to the submit-event (if the tip above won't help).
Something like this (not tested, but should put you in the correct path):
Template.templateName.events({
'submit': function(event) {
// Prevent the submit with preventDefault()
event.preventDefault();
// Do something to check the submit etc.
}
});

Jquery Cookbook Modal button

I've been at this for two days and can't seem to get it. Basically, I'm using the JQuery Cookbook modal from scratch. My problem is the form html page loads fine but the code will not recognize my submit button. Here's the relevant parts of the code:
Separate HTML:
<div id="contact">
<form action="" id="register_form" method="post">
<p>First Name <br />
<input type="text" id="firstname" name="firstname" /></p>
<p>Last Name <br />
<input type="text" id="lastname" name="lastname" /></p>
<p>Username: <span class="micro">Must be a valid email address</span></span><br />
<input type="text" id="username" name="username" /></p>
<p><input type="submit" value="Register" id="register" /></p>
</form>
</div>
Here's the relevant parts of the modal code:
// Insert modal at end of </body>.
$('body').append('<div id="modal_wrapper"><!--[if IE 6]><iframe id="modal_iframe" frameborder="0"></iframe><![endif]--><div id="modal_overlay"></div><div id="modal_window"><div id="modal_bar"><strong>Modal window</strong>Close</div><div id="modal_content"><div id="contact"><form><p><input id="firstname" /></p><p><input id="register" /></p></form></div></div></div>');
$('#modal_content').load('mediaKitF.html#contact'.replace('#', ' #'), '', showModal);
$("input[type=text]").focus(function(){
// Select field contents
this.select();
});
$('input #firstname').focus();
$('#register').click(function () {
alert("hello there");
});

$('#modal_content').load() is an asynchronous method, which means that you are trying to attach your click event to the $('#register') element before receiving the new content. You need to either use $('#register').live('click', function() {}) or move the code attaching the click handler into your showModal function.