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Line across any device and in the centre, using canvas or html,css

I'm making an app using JavaScript and JQuery, which will tell the user if there device is straight or not, basically like a spirit level. I want to draw a line a straight line across the middle of the screen and i want this to be responsive no matter the size of the device. This will be used on mobiles and tablets. I used a canvas to the draw a line and so far i'm not sure if this is the right way to approach this?
if anyone could give me any advice i would really appreciate it. Below is my canvas line so far. And I've included some rough drawing of what i mean.
const c = document.getElementById("LineCanvas");
const drw = c.getContext("2d");
drw.beginPath();
drw.moveTo(10,45);
drw.lineTo(180,47);
drw.lineWidth = 5;
drw.strokeStyle = '#006400';
drw.stroke();
If the phone is aligned straight the line will be green else red

to draw the line you can use a pseudo element from HTML or body or any specific tag that you want to use in a specific page or click , then update rotation via transform:rotate() ; or rotate3D()
example ( without javascript, rotate values will have to be taken from your device via your app ):
let level = document.querySelector("#level");
document.querySelector("#spirit").onclick = function() {
level.classList.toggle('show');
}
#level {
width: 100vw;
height: 100vh;
position: fixed;
top: 0;
left: 0;
display: none;
pointer-events: none;
}
#level.show {
display: block;
}
#level::before {
content: '';
position: absolute;
width: 200vmax;
margin: 0 -50vmax;
border-top: 1px solid;
box-shadow: 0 0 1px 5px #bee;
top: 50%;
transform: rotate(0deg);
}
#level.show~#spirit::before {
content: 'Hide';
}
#level:not(.show)~#spirit::before {
content: 'Show';
}
/* animation to fake phone device moving */
#level::before {
animation: rt 10s infinite;
}
#keyframes rt {
20% {
transform: rotate3d(1, -1, 1, -0.25turn);
}
40% {
transform: rotate3d(1, 1, 1, 0.5turn);
}
60% {
transform: rotate3d(1, -1, 1, -0.75turn);
}
80% {
transform: rotate3d(1, 1, -1, -0.5turn);
}
}
<div id="level">
<!-- to show on a single page or via js on user request -->
</div>
<button id="spirit" type=button> that spirit level</button>

While drawing a line with canvas can work you might find it more straightforward to draw it with a simple div element. When you sense a slope you can change its color to red and back to green if it's level.
Of course you will have to do some calculations to decide what angle you want the line to be - but I guess that is the whole point of your webapp to show people how far off they are.
When you know the angle you want the line to be call slope(n) where n is the number of degrees. I've also put in a simple button so the user can choose whether to show the line or not but I expect you'll have your own code for that.
On any page where you want the user to be able to show the line put this in the head:
<style>
* {
margin: 0;
padding: 0;
}
.linecontainer {
position: absolute;
top: 0;
left: 0;
width: 100vw;
height: 100vh;
overflow: hidden;
z-index: 99999;
}
#line {
width: 200vmax;
height: 5px;
position: relative;
top: 50%;
left: calc(50vw - 100vmax);
transform: rotate(45deg);
background-color:red;
}
.hideline {
display: none;
}
#showbtn {
font-size: 20px;
background-color: blue;
color: white;
height: 2em;
width: auto;
padding: 2px;
}
</style>
and put this in the main body of the page:
<div class="linecontainer">
<div id="showbtn" onclick="document.getElementById('line').classList.toggle('hideline');">
Click me to show/hide the line
</div>
<div id="line"></div>
</div>
<script>
function slope(deg) {
let line = document.getElementById('line');
line.style.backgroundColor = ( deg%180 == 0 ) ? 'green' : 'red';
line.style.transform = 'rotate(' + deg + 'deg)';
}
</script>
Here's a snippet where you can show the line at different angles.
function slope(deg) {
let line = document.getElementById('line');
line.style.backgroundColor = ( deg%180 == 0 ) ? 'green' : 'red';
line.style.transform = 'rotate(' + deg + 'deg)';
}
* {
margin: 0;
padding: 0;
}
.linecontainer {
position: absolute;
top: 0;
left: 0;
width: 100vw;
height: 100vh;
overflow: hidden;
z-index: 99999;
}
#line {
width: 200vmax;
height: 5px;
position: relative;
top: 50%;
left: calc(50vw - 100vmax);
transform: rotate(45deg);
background-color:red;
}
.hideline {
display: none;
}
#showbtn {
font-size: 20px;
background-color: blue;
color: white;
height: 2em;
width: auto;
padding: 2px;
}
<div class="linecontainer">
<div id="showbtn" onclick="document.getElementById('line').classList.toggle('hideline');">
Click me to show/hide the line
</div>
<div id="line"></div>
</div>
<!-- this is just for the demo -->
<div style="background-#eeeeee;font-size: 20px;position:fixed;z-index:100000;bottom:0;left:0;">How many degrees do you want me to rotate? <input style="font-size:20px;"value="45" onchange="slope(this.value);"/></div>

Changing position of absolute element without container overflow

Maybe an obvious question but how do I make an element with a absolute position not overflow its container when moving it's position right? I know I could change it to relative position or move it 99% but for my project that won't due. I tried using margins, padding, object-fit, all with no success. Thanks for any help
var green = document.getElementById('green');
function myFunct() {
green.style.right = '100%';
}
h1 {
position: relative;
width: 80%;
height: 100px;
margin: auto;
background-color: red;
}
#green {
position: absolute;
right: 0px;
height: 100px;
background-color: green;
width: 20px;
}
<h1>
<div id = 'green'></div>
</h1>
<button onclick="myFunct()">FindHighScore</button>

Use CSS calc()
var green = document.getElementById("green");
function myFunct() {
green.style.right = "calc(100% - 20px)";
}
Or, apply left: 0 and right: auto (reset)
var green = document.getElementById("green");
function myFunct() {
green.style.left = "0";
green.style.right = "auto";
}
A <div> should not be in a <h1> tag by the way.

You can set overflow to hidden at parent container.
<h1> permitted content is Phrasing content
var green = document.getElementById('green');
function myFunct() {
green.style.right = '100%';
}
div:not(#green) {
position: relative;
width: 80%;
height: 100px;
margin: auto;
background-color: red;
overflow: hidden;
}
#green {
position: absolute;
right: 0px;
height: 100px;
background-color: green;
width: 20px;
}
<div>
<div id='green'></div>
</div>
<button onclick="myFunct()">FindHighScore</button>

How do I loop multiple scripts in javascript?

I have some code here and I want the squares to go blue, green, indigo then go back to the beginning. Each time I click the button the colour will change. The code here goes blue, green, indigo and then changes between indigo and black while I want it to go to blue again. Is there a way of restarting the entire code again?
<--code for animated squares!-->
<!DOCTYPE html>
<html>
<style>
#container {
width: 400px;
height: 400px;
position: relative;
background: Black;
}
div#animate {
width: 50px;
height: 50px;
position: absolute;
left: 175px;
top: 0px;
background-color: Blue;
}
</style>
<style>
#containertwo {
width: 400px;
height: 400px;
position: relative;
background: Black;
}
div#animatetwo {
width: 50px;
height: 50px;
position: absolute;
left: 175px;
top: 175px;
background-color: Black;
}
</style>
<body>
<style>
#containerthree {
width: 400px;
height: 400px;
position: relative;
background: Black;
}
div#animatethree {
width: 50px;
height: 50px;
position: absolute;
left: 175px;
top: 350px;
background-color: Black;
}
</style>
<body>
<p>
<button onClick="button_click();button_clicktwo();button_clickthree()">Change Colour</button>
</p>
<div id ="container">
<div id ="animate"></div>
<div id ="animatetwo"></div>
<div id ="animatethree"></div>
</div>
<div id="box" onClick="button_click(j)();"></div>
<script>
var colors = ["Black","Black","Blue"];
function button_click() {
var box = document.getElementById("animate");
var background_color = box.style.backgroundColor;
var i = colors.indexOf(background_color);
if (i === colors.length-1) {
i = -1;
}
animate.style.backgroundColor = colors[i+1];
}
</script>
<div id="box" onClick="button_clicktwo();"></div>
<script>
var colorstwo = ["Green","Black","Black",];
function button_clicktwo() {
var box = document.getElementById("animatetwo");
var background_color = box.style.backgroundColor;
var i = colorstwo.indexOf(background_color);
if (i === colorstwo.length-1) {
i = -1;
}
animatetwo.style.backgroundColor = colorstwo[i+1];
}
</script>
<div id="box" onClick="button_clickthree();"></div>
<script>
var colorsthree = ["Black","Indigo","Black"];
function button_clickthree() {
var box = document.getElementById("animatethree");
var background_color = box.style.backgroundColor;
var i = colorsthree.indexOf(background_color);
if (i === colorstwo.length-1) {
i = -1;
}
animatethree.style.backgroundColor = colorsthree[i+1];
}
</script>

There's quite a bit of invalid and repetitive code, so for simplicity, I just reworked the entire thing. I'll note some of the problems below.
Since there were so many changes made to the HTML, CSS and JS, I won't list them all, but will leave it to you to observe the differences.
// Gather the colors and elements, and set a shared `i` to `0`
var colors = ["Blue", "Green", "Indigo"];
var elems = document.querySelectorAll(".animate");
var i = 0;
// Have a single function that makes the current element black and the next
// one a different color
function button_click() {
elems[i].style.backgroundColor = "Black";
if (++i === colors.length) {
i = 0
}
elems[i].style.backgroundColor = colors[i];
}
#container {
width: 400px;
height: 400px;
position: relative;
background: Black;
}
.animate {
width: 50px;
height: 50px;
left: 175px;
position: absolute;
}
.animate:nth-child(1) {
top: 0px;
background-color: Blue;
}
.animate:nth-child(2) {
top: 175px;
}
.animate:nth-child(3) {
top: 350px;
}
<p>
<button onClick="button_click();">Change Colour</button>
</p>
<div id="container">
<div class="animate"></div>
<div class="animate"></div>
<div class="animate"></div>
</div>
Note that there are other ways to do this too, like having each color set in the CSS, and then setting the visibility to hidden or visible with JavaScript.
Some of the general problems were:
the same ID attribute used more than once
an extra <body> tag
lots of repeating CSS, JS and HTML that was able to be greatly reduced

Using jQuery to subtract 1 from a current css value on several divs

Does anyone know how I would use jQuery to subtract 1 from a current css value of several divs with the same class on a click? I have 5 divs with the same class, each have their own z-index setting. I want to subtract 1 from their current value when I click on one div. My goal is to bring one div forward on click setting z-index to 100. I then want to subtract 1 from the others' current value (whatever that is) on that same click to send them back.
$('.classname').mousedown(function() {
$(this).css("z-index", "100");
$(this).siblings().css("z-index", - 1);
});
});
This code just sets all the siblings z-index to -1.
Thanks for any help. I'm also open to suggestions on a better way to do this.

You can give the css() method a function to perform some logic on the current value. Try this:
$('.classname').mousedown(function() {
$(this)
.css("z-index", "100")
.siblings().css("z-index", function(i, val) {
return parseInt(val, 10) - 1;
});
});

So basically, you want to do this?
var divs = document.getElementsByClassName('classname');
function setZIndex(div) {
div.style.zIndex = window.getComputedStyle(div).zIndex - 1;
}
function handler(e) {
[].forEach.call(divs, setZIndex);
e.target.style.zIndex = 100;
}
[].forEach.call(divs, function(div) {
div.addEventListener('click', handler, false);
});
.classname {
width: 20px;
height: 20px;
border-style: solid;
border-width: 1px;
}
#one {
position: absolute;
top: 0px;
left: 0px;
background-color: red;
z-index: -1;
}
#two {
position: absolute;
top: 15px;
left: 10px;
background-color: blue;
z-index: -1;
}
#three {
position: absolute;
top: 30px;
left: 20px;
background-color: yellow;
z-index: -1;
}
#four {
position: absolute;
top: 45px;
left: 30px;
background-color: green;
z-index: -1;
}
#five {
position: absolute;
top: 60px;
left: 40px;
background-color: white;
z-index: -1;
}
<div id="one" class="classname"></div>
<div id="two" class="classname"></div>
<div id="three" class="classname"></div>
<div id="four" class="classname"></div>
<div id="five" class="classname"></div>

multiple pop up div's in the same page

In one of my projects, I have requirement of multiple pop up div's on the same page. That means when user clicks on a link, some content should open in a pop up. There will be many such links with their own pop ups. With little knowledge of javascript, I have tried to write a javascript for it but it works only for one pop up. When I click on second, third... links, only first pop up opens rather than opening second, third... pop ups. Here is my code. Please tell the modifications to it.
<!DOCTYPE html>
<html >
<head>
<script>
window.document.onkeydown = function (e)
{
if (!e)
{
e = event;
}
if (e.keyCode == 27)
{
lightbox_close();
}
}
function lightbox_open()
{
window.scrollTo(0,0);
document.getElementById('light').style.display='block';
document.getElementById('fade').style.display='block';
}
function lightbox_close()
{
document.getElementById('light').style.display='none';
document.getElementById('fade').style.display='none';
}
</script>
<style>
#fade
{
display: none;
position: fixed;
top: 0%;
left: 0%;
width: 100%;
height: 100%;
background-color: #000;
z-index:1001;
-moz-opacity: 0.7;
opacity:.70;
filter: alpha(opacity=70);
}
#light
{
display: none;
position: absolute;
top: 50%;
left: 50%;
width: 300px;
height: 200px;
margin-left: -150px;
margin-top: -100px;
padding: 10px;
border: 2px solid #FFF;
background: #CCC;
z-index:1002;
overflow:visible;
}
</style>
</head>
<body>
Open 1
<div id="light">div 1</div>
<div id="fade" onClick="lightbox_close();"></div>
Open 2
<div id="light">div 2</div>
<div id="fade" onClick="lightbox_close();"></div>
Open 3
<div id="light">div 3</div>
<div id="fade" onClick="lightbox_close();"></div>
</body>
</html>

Here's a way to achieve what you want. I'm sure it can be improved, but it's up to you then.
First, IDs should be unique across the page. If you want to group elements, give them a shared class instead.
With the changes, your HTML would look like this:
Open 1
<div class="light">div 1</div>
<div class="fade" onClick="lightbox_close()"></div>
Open 2
<div class="light">div 2</div>
<div class="fade" onClick="lightbox_close()"></div>
Open 3
<div class="light">div 3</div>
<div class="fade" onClick="lightbox_close()"></div>
Your CSS:
html, body {
width: 100%;
height: 100%;
}
.fade {
display: none;
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
background-color: #000;
z-index:1001;
-moz-opacity: 0.7;
opacity:.70;
filter: alpha(opacity=70);
}
.light {
display: none;
position: absolute;
top: 50%;
left: 50%;
width: 300px;
height: 200px;
margin-left: -150px;
margin-top: -100px;
padding: 10px;
border: 2px solid #FFF;
background: #CCC;
z-index:1002;
overflow:visible;
}
And your Javascript:
window.document.onkeydown = function (e) {
if (!e) {
e = event;
}
if (e.keyCode == 27) {
lightbox_close();
}
}
// Note that the function is receiving the clicked element reference.
function lightbox_open(el) {
window.scrollTo(0,0);
// All the anchors that have a class lightbox.
var anchors = document.querySelectorAll('a.lightbox');
// All the elements with class light.
var light = document.querySelectorAll('.light');
// All the elements with class fade.
var fade = document.querySelectorAll('.fade');
// Iterate over the anchors elements.
for (var i = 0; i < anchors.length; i++) {
// If the anchor matches the clicked one.
if (anchors[i] == el) {
// Look for the light and fade with the same index
// and display them.
light[i].style.display = 'block';
fade[i].style.display = 'block';
}
}
}
function lightbox_close() {
// All the elements with class light or fade.
var els = document.querySelectorAll('.light, .fade');
// Loop through the list.
for (var i = 0; i < els.length; i++) {
// Hide them.
els[i].style.display = 'none';
}
}
Demo