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Hide/Show button + not valid HTML

First Question: On W3 HTML Validator I get the following error:
Attribute value not allowed on element input at this point.
But, in my code, I am using the 'value' to change images so how could I fix this?
Second question: I need the image to remain the same even when I refresh. I am new to Javascript, but I know I need to utilise cookies or local storage in some way. Can someone help with this?
$(document).ready(function() {
$("#button").on("click", function() {
$("#collapse").slideToggle("slow");
if ($(this).val() == "Hide") {
$(this).val("Show");
$(this).attr("src","https://www.gravatar.com/avatar/2e83e2d35f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1");
} else {
$(this).val("Hide");
$(this).attr("src","https://www.gravatar.com/avatar/2e83e2d00f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1");
}
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input type="image" value="Hide" id="button" src="https://www.gravatar.com/avatar/2e83e2d00f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1"></button>
<div id="collapse">
Hello
</div>

If you just want to make the HTML valid, the simplest tweak would be to use a class or data attribute instead.
$(document).ready(function() {
$("#button").on("click", function() {
$("#collapse").slideToggle("slow");
if ($(this).data('status') == "Hide") {
$(this).data('status', "Show");
$(this).attr("src","https://www.gravatar.com/avatar/2e83e2d35f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1");
} else {
$(this).data('status', "Hide");
$(this).attr("src","https://www.gravatar.com/avatar/2e83e2d00f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1");
}
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input type="image" data-status="Hide" id="button" src="https://www.gravatar.com/avatar/2e83e2d00f0b889da7d5905c7bf574c2?s=32&d=identicon&r=PG&f=1"></button>
<div id="collapse">
Hello
</div>
To persist the value, retrieve the status on pageload and run the handler
const initialStatus = localStorage.imageStatus || 'Hide';
$('#button').data('status', initialStatus);
$('#button').click();
and set it after a change
localStorage.imageStatus = $('#button').data('status');

How can i change the value of checkbox upon change

I would like to change the value of the checkbox, but failed with the following code.
$("input:checkbox").on("change", function () {
if (this.checked == true) {
this.val("1");
}
else {
this.val("0");
}
});
I not sure why it is no responds without the code, which it should've the "checked" when I'm calling this element, but no. So i will need to add a value field manually.
<input class="c-switch-input" type="checkbox" name="pk_{{$d['id']}}" value="{{$d['status']}}">

You are mixing jQuery and DOM incorrectly
EITHER
this.value = "1";
OR
$(this).val("1")
But use a ternary:
$("input:checkbox").on("change", function () {
this.value = this.checked ? "1" : "0";
console.log(this.value)
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input type="checkbox" />

How to call javascript function only when checkbox is checked

i have tried this:
function stickyheaddsadaer() {
$("#page-header-inner").addClass("sticky");
}
<input type="checkbox" name="TT_sticky_header" id="TT_sticky_header_function"
value="{TT_sticky_header}" onclick="stickyheaddsadaer()"/>
so when i click checkbox, it just nothing happens....
but when i try this:
function stickyheaddsadaer() {
alert("I am an alert box!");
}
then works...
can you please help me ? I need to activate javasript function by checkbox
and when the js function is actived it add class to the div
Thank you

Better to use onchange event and check inside function if checked or not
function stickyheaddsadaer(obj) {
if($(obj).is(":checked")){
alert("Yes checked"); //when checked
$("#page-header-inner").addClass("sticky");
}else{
alert("Not checked"); //when not checked
}
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="checkbox" name="TT_sticky_header" id="TT_sticky_header_function" value="{TT_sticky_header}" onchange="stickyheaddsadaer(this)"/>

HTML
<input type="checkbox" id="switch" onclick="change()">
JS
function change() {
var decider = document.getElementById('switch');
if(decider.checked){
alert('check');
} else {
alert('unchecked');
}
}

function checkFluency(){
var checkbox = document.getElementById('fluency');
if (checkbox.checked != false) {
alert("Checkbox checked")
}else{
alert("Not Checked")
}
}
<input type="checkbox" onclick="checkFluency()" id="fluency" />

button is not being enabled if checkbox is on

I want to enable button only if checkbox is on. What I am doing wrong here ? Thanks in advance..
index.html
<p><input id="agree" type="checkbox" /> I agree</p>
<input id="continue" value="continue" type="button" disabled="disabled" />
custom.js
$( document ).ready(function () {
$('#agree').change(function () {
var state = $(this).attr('value');
if (state == 'on') {
$('#continue').removeAttr('disabled')
} else if (state == '') {
$('#continue').attr('disabled','disabled');
}
});
});

You could simplify it to the following:
Example Here
$('#agree').on('change', function () {
$('#continue').attr('disabled', !this.checked);
});
$('#agree').on('change', function () {
$('#continue').attr('disabled', !this.checked);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<p><input id="agree" type="checkbox" />I agree</p>
<input id="continue" value="continue" type="button" disabled="disabled" />
The reason your code wasn't working was because you were using .attr(). Since there is no value attribute, you needed to use .prop(). This still wouldn't work though because the value will always return on. You need to get the checked property accessing this.checked or .prop('checked') - working example using your original snippet.
$('#agree').on('change', function () {
if (this.checked) {
$('#continue').removeAttr('disabled')
} else {
$('#continue').attr('disabled', 'disabled');
}
});

Try this:
$( document ).ready(function() {
$('#agree').change(function() {
if(this.checked) {
$('#continue').removeAttr('disabled')
} else {
$('#continue').attr('disabled','disabled');
}
});
});

If you want to check if input has checked :
state = $(this).prop( "checked" );
this returns boolean value (true if checked or false if unchecked).

Change variable value when checkbox is checked/unchecked

I was trying to change the value of an variable according to the status of an checkbox
here is my code sample
<script type="text/javascript">
if(document.getElementByType('checkbox').checked)
{
var a="checked";}
else{
var a="not checked";}
document.getElementById('result').innerHTML ='result '+a;
</script>
<input type="checkbox" value="1"/>Checkbox<br/>
<br/>
<span id="result"></span>
Can you please tell me whats the problem with this code.

Try this:
if (document.querySelector('input[type=checkbox]').checked) {
Demo here
Code suggestion:
<input type="checkbox" />Checkbox<br/>
<span id="result"></span>
<script type="text/javascript">
window.onload = function () {
var input = document.querySelector('input[type=checkbox]');
function check() {
var a = input.checked ? "checked" : "not checked";
document.getElementById('result').innerHTML = 'result ' + a;
}
input.onchange = check;
check();
}
</script>
In your post you have the javascript before the HTML, in this case the HTML should be first so the javascript can "find it". OR use, like in my example a window.onload function, to run the code after the page loaded.

$('#myForm').on('change', 'input[type=checkbox]', function() {
this.checked ? this.value = 'apple' : this.value = 'pineapple';
});

try something like this
<script type="text/javascript">
function update_value(chk_bx){
if(chk_bx.checked)
{
var a="checked";}
else{
var a="not checked";
}
document.getElementById('result').innerHTML ='result '+a;
}
</script>
<input type="checkbox" value="1" onchange="update_value(this);"/>Checkbox<br/>
<span id="result"></span>

Too complicated. Inline code makes it cool.
<input type="checkbox" onclick="yourBooleanVariable=!yourBooleanVariable;">

For those who tried the previous options and still have a problem for any reason, you may go this way using the .prop() jquery function:
$(document.body).on('change','input[type=checkbox]',function(){
if ($(this).prop('checked') == 1){
alert('checked');
}else{
alert('unchecked');
}

This code will run only once and check initial checkbox state. You have to add event listener for onchange event.
window.onload = function() {
document.getElementByType('checkbox').onchange = function() {
if(document.getElementByType('checkbox').checked) {
var a="checked";
} else {
var a="not checked";
}
document.getElementById('result').innerHTML ='result '+a;
}
}