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HTML5 Drag&Drop: Implementing the whole sequence manually

I have a complex UI component with custom Drag & Drop behaviour and custom event code of several hundred lines, that basically works via regular MouseEvents.
I need to add additional functionality so that a user can Drag & Drop items from inside the component to outside the component. The "outside" works via regular HTML5 Drag & Drop events, meaning there is an element that already listens to the drop event, etc. Now, i just need to trigger the whole Drag & Drop chain, so that the drop zone element notices the event and during the drag operation i have the regular look drag image.
Simply setting my draggable item to draggable="true", does not work as the dragstart event is not thrown, it is prevented by our custom event code inside the component with event.preventDefault(). So my idea was to manually trigger dragstart events whenever the user wants to drag the item outside of the boundary of the component.
My current status is, that my onMoveStart() triggers the dragstart event, once it realizes the user wants to drag the item outside of the component. The onMove() triggers regular drag events while moving the mouse, the onMoveEnd() triggers the corresponding dragend / drop events. A somewhat similar and very boiled down version can be seen here: Fiddle.
Problem: A dragstart is properly triggered when dragging an item outside of the component, a drag event is properly triggered while regular mouse moving, the dragend event also works. The dropzone element however never receives the drop event. Additionally, there is no dragging image during the drag operation, which makes this look not working to the user. The datatransfer that i manually setup always has effectAllowed and dropOperation set to none even though i manually set them to proper values.
How can i get this working? How to implement the Drag & Drop behaviour manually? How to set the effectAllowed and dropEffect properties on DataTransfer manually so that i have a proper dragging effect without them being always overwritten to none?

Based on the code you provided, I produced something that built around it. From your post and my understanding, I believe there are 2 problems or maybe 3 that you are trying to solve
I start with dragging image. So I just clone the element that we want it to appear as dragging, set it with proper attributes
function draggingImage(el) {
let image = draggable.cloneNode(true);
image.setAttribute("id", "drag-image");
image.style.width = el.offsetWidth + 'px'
image.style.height = el.offsetHeight + 'px'
image.style.position = 'fixed'
image.style.left = el.getBoundingClientRect().left + 'px';
image.style.top = el.getBoundingClientRect().top + 'px';
image.style.background = getComputedStyle(draggable).backgroundColor
image.style.opacity = .5
let div = document.body.appendChild(image);
}
as for now, the dragging image is static, as expected, and we want it to move as our mouse move thus the dragging effect
let image = document.getElementById('drag-image')
image.style.left = image.getBoundingClientRect().left + e.movementX + 'px'
image.style.top = image.getBoundingClientRect().top + e.movementY + 'px'
while our mousemove with wasdragging = true, it's a good time to detect what's underneath our cursor, are we within the dropzone or outside
function getDropzone(e, left, top, id) {
// let rect1 = document.getElementById(id).getBoundingClientRect();
let rect1 = drop.getBoundingClientRect()
// to detect the overlap of mouse into the dropzone, as alternative of mouseover
var overlap = !(rect1.right < left ||
rect1.left > left ||
rect1.bottom < top ||
rect1.top > top)
return overlap
}
we can add the above function within onDrag function. The getDropzone function return true or false, and that's all needed. From there we can execute our logic or styling accordingly.
I also add global variable called as data, then a function that work almost similar to dataTransfer concept, naively.
let data = {}
function manualSetData(variable, v) {
data[variable] = v
}
with this, we can just get the data that we set with this function in ondragend
let getData = data[variable]
so this is DnD from scratch in a nutshell
const draggable = document.querySelector("#draggable");
const drop = document.querySelector("#drop");
const condition = true;
let wasDragging = false;
let allowedDrop = false
let data = {}
draggable.addEventListener("mousedown", onMoveStart)
document.addEventListener("mousemove", onMove)
document.addEventListener("mouseup", onMoveEnd)
function onMoveStart(e) {
event.preventDefault();
event.stopPropagation();
let el = e.target
// Other logic and custom event handling ...
if (condition) {
onDragStart(el);
}
}
function onMove(e) {
// Other logic ...
if (wasDragging) {
onDrag(e);
}
}
function onMoveEnd(event) {
// Other logic ...
if (wasDragging) {
onDragEnd(event.target);
}
}
function onDragStart(el) {
draggingImage(el)
wasDragging = true;
manualSetData('helloMom', el)
}
function draggingImage(el) {
let image = draggable.cloneNode(true);
image.setAttribute("id", "drag-image");
image.style.width = el.offsetWidth + 'px'
image.style.height = el.offsetHeight + 'px'
image.style.position = 'fixed'
image.style.left = el.getBoundingClientRect().left + 'px';
image.style.top = el.getBoundingClientRect().top + 'px';
image.style.background = getComputedStyle(draggable).backgroundColor
image.style.opacity = .5
let div = document.body.appendChild(image);
}
function manualSetData(variable, v) {
data[variable] = v
}
function onDrag(e) {
let image = document.getElementById('drag-image')
image.style.left = image.getBoundingClientRect().left + e.movementX + 'px'
image.style.top = image.getBoundingClientRect().top + e.movementY + 'px'
let left = e.pageX;
let top = e.pageY;
let overlap = getDropzone(e, left, top)
drop.style.backgroundColor = allowedDrop ? 'green' : 'blue'
if (overlap) {
document.body.style.cursor = 'copy';
allowedDrop = true
} else {
document.body.style.cursor = 'no-drop';
allowedDrop = false
}
}
function getDropzone(e, left, top, id) {
// let rect1 = document.getElementById(id).getBoundingClientRect();
let rect1 = drop.getBoundingClientRect()
// to detect the overlap of mouse into the dropzone, as alternative of mouseover
var overlap = !(rect1.right < left ||
rect1.left > left ||
rect1.bottom < top ||
rect1.top > top)
return overlap
}
function onDragEnd(element) {
let image = document.getElementById('drag-image')
image.remove()
document.body.style.cursor = 'default';
if (allowedDrop) {
let getData = data['helloMom']
drop.appendChild(getData)
}
wasDragging = false;
}
#draggable {
background-color: red;
}
#drop {
background-color: blue;
}
<div id="draggable" draggable="true">
fancy content
</div>
<div id="drop">
drop area
</div>

Prevent vertical scroll on swipe -vanilla JS

I wrote this code to add swipe function for an image slider. The slider is working correctly.
However when i perform a right or left swipe there is some vertical scrolling which is distracting and annoying.
I'm storing the reference to touchstart in the touch object.
And on touchend event, if vertical distance (lenY) is more than 50, i trigger preventDefault on the touchstart.
This isn't working.
Simplest option is to call preventDefault directly on touchStart. But the image slider occupies a large part of the mobile screen making scrolling down the page tricky.
I need to pass the lenY (vertical distance) to the touch start handler to prevent default action.
function triggerTouch() {
"use strict";
var tZone = document.getElementById('sl-m'),
touch = {},
startX = 0,
startY = 0,
endX = 0,
endY = 0;
if (tZone) {
tZone.addEventListener('touchstart', function (e) {
startX = e.changedTouches[0].screenX;
startY = e.changedTouches[0].screenY;
// store reference to touch event
touch.start = e;
}, false);
tZone.addEventListener('touchend', function (e) {
endX = e.changedTouches[0].screenX;
endY = e.changedTouches[0].screenY;
var lenX = Math.abs(endX - startX);
var lenY = Math.abs(endY - startY);
// check if user intended to scroll down
if (lenY < 50 && lenX > 50) {
touch.start.preventDefault();
e.preventDefault();
swipe(tZone, startX, endX);
}
}, false);
}
}

Since i haven't got an answer i am posting my own answer, hoping someone can provide the correct implementation.
I ended up using the css overflow property to temporarily disable vertical scroll.
This works perfectly though there is a small side effect. Once you swipe through the image slider, the scroll is disabled.
A swipe upwards is required to restore scroll to the page. Its not noticeable but i still want to figure the right way.
var touch = {};
window.onload = function () {
"use strict";
document.body.addEventListener("touchstart", touchHandler);
document.body.addEventListener("touchend", touchHandler);
};
function touchHandler(e) {
"use strict";
var el = e.target;
if (el.parentNode.id === "sl-m") {
if (e.type === "touchstart") {
touch.startX = e.changedTouches[0].screenX;
touch.startY = e.changedTouches[0].screenY;
} else {
touch.endX = e.changedTouches[0].screenX;
touch.endY = e.changedTouches[0].screenY;
touch.lenX = Math.abs(touch.endX - touch.startX);
touch.lenY = Math.abs(touch.endY - touch.startY);
if (touch.lenY < 20) {
// disable scroll
document.body.style.overflowY = "hidden";
// do swipe related stuff
swipe(el.parentNode);
} else {
// enable scroll if swipe was not intended
document.body.style.overflowY = "scroll";
}
}
} else {
// keep scroll enabled if touch is outside the image slider
document.body.style.overflowY = "scroll";
}
}

I want to share the solution that works for me. The above solution did not work on ios. I am sorry for my English. I do not know english.
function stop(e){
e=e || event;
e.preventDefault;
}
window.onscroll=stop(); //-->Yes, we will use it ..
For example, where you will use;
function move(event){
var finish=event.touches[0].clientX;
var verticalFinish=event.touches[0].clientY;
var diff=finish-strt;
var verticalDiff=verticalStrt-verticalFinish;
var f;
if(diff<0 && (Math.abs(diff)>Math.abs(verticalDiff)/3)){
f=verticalDiff+widthOffset;
slayt[x].style.left=diff+"px";
slayt[x].style.transition="none";
slayt[y].style.left=f+"px";
slayt[y].style.transition="none";
window.onscroll=stop(); //-->we used it here :)
}
else if(diff>0 && (Math.abs(diff)>Math.abs(verticalDiff)/3)){
f=diff-widthOffset;
slayt[x].style.left=diff+"px";
slayt[x].style.transition="none";
slayt[z].style.left=f+"px";
slayt[z].style.transition="none";
window.onscroll=stop();//-->we used it here :)
}
}
but there is a small problem. cancels if there is another function related to scrolling. return true; it does not work. I also write twice if I have a function related to the slider inside and outside the touchend.
function end(event){
//"touchend" related codes...
//bla bla
window.onscroll=function(){m=window.pageYOffset;console.log(m);if(m>=850)
{buton.style.display="block";}else{buton.style.display="none";}}
}
If it is useful, I will be happy...
Update :
I typed wrong. I want to fix. Actually, the scroll event cannot be canceled unfortunately. So the event we canceled above, scroll is not a vertical scroll event. All events.
window.onscroll=stop(); // ==>improper use
stop(); // ==> actually - Correct usage
It just needs to be written so stop().

html,
body {
overflow: hidden;
}
Did you try this?

How to change pop-up location of google charts tooltip

I currently have html enabled tooltips that also display "sub graphs". However, it would be nice if it was possible to have all tooltips pop up in a fixed location or have an offset that adjusted their relative poition.
This is an example of the kind of tooltip that I have (blank data). I'd like to move it to the right. Any suggestions would be appreciated, including any javascript trickery.

whilst the answer is very good it is a little outdated now. Google has implemented CSS control so there is greater flexibility without the need to hack the JavaScript.
.google-visualization-tooltip { position:relative !important; top:0 !important;right:0 !important; z-index:+1;}
will provide a tooltip fixed at the bottom of the chart, live example: http://www.taxformcalculator.com/federal-budget/130000.html
alternatively you could just tweak the left margin...
.google-visualization-tooltip { margin-left: 150px !important; z-index:+1;}
Note that pulling the container forward with z-index reduces (but does not stop entirely) visibility flicker as the mouse moves. The degree of flicker will vary on chart size, call etc. Personally, I prefer to fix the tool tip and make it part of the design as per the first example. Hope this helps those who are deterred by the JS hack (which is good but really no longer necessary).

The tooltip position is set inline, so you need to listen for DOM insertion of the tooltip and change the position manually. Mutation events are deprecated, so use a MutationObserver if it is available (Chrome, Firefox, IE11) and a DOMNodeInserted event handler if not (IE 9, 10). This will not work in IE8.
google.visualization.events.addOneTimeListener(myChart, 'ready', function () {
var container = document.querySelector('#myChartDiv > div:last-child');
function setPosition () {
var tooltip = container.querySelector('div.google-visualization-tooltip');
tooltip.style.top = 0;
tooltip.style.left = 0;
}
if (typeof MutationObserver === 'function') {
var observer = new MutationObserver(function (m) {
for (var i = 0; i < m.length; i++) {
if (m[i].addedNodes.length) {
setPosition();
break; // once we find the added node, we shouldn't need to look any further
}
}
});
observer.observe(container, {
childList: true
});
}
else if (document.addEventListener) {
container.addEventListener('DOMNodeInserted', setPosition);
}
else {
container.attachEvent('onDOMNodeInserted', setPosition);
}
});
The MutationObserver should be fine, but the events may need some work; I didn't test them.

I had more or less the same question as Redshift, having been trying to move the tooltip relative to the node being hovered over. Using asgallant's fantastic answer I've implemented his code as below.
I haven't been able to test whether this works with the MutationObserver because during my testing in Firefox, Chrome and IE11 it always fails that test and uses addEventListener. The docs suggest it should work though.
I had to introduce a timeout to actually manipulate the styles as otherwise the left and top position of the element was always reported as 0. My assumption is that the event fired upon addition of the node but the DOM wasn't quite ready. This is just a guess though and I'm not 100% happy with implementing it in this way.
var chart = new google.visualization.LineChart(document.getElementById('line_chart'));
google.visualization.events.addOneTimeListener(chart, 'ready', function () {
var container = document.querySelector('#line_chart > div:last-child');
function setPosition(e) {
if (e && e.target) {
var tooltip = $(e.target);
setTimeout(function () {
var left = parseFloat(tooltip.css('left')) - 49;
var top = parseFloat(tooltip.css('top')) - 40;
tooltip.css('left', left + 'px');
tooltip.css('top', top + 'px');
$(".google-visualization-tooltip").fadeIn(200);
}, 1);
}
else {
var tooltip = container.querySelector('.google-visualization-tooltip');
var left = parseFloat(tooltip.style.left) - 49;
var top = parseFloat(tooltip.style.top) - 40;
tooltip.style.left = left + 'px';
tooltip.style.top = top + 'px';
$(".google-visualization-tooltip").fadeIn(200);
}
}
if (typeof MutationObserver === 'function') {
var observer = new MutationObserver(function (m) {
if (m.length && m[0].addedNodes.length) {
setPosition(m);
}
});
observer.observe(container, {
childList: true
});
}
else if (document.addEventListener) {
container.addEventListener('DOMNodeInserted', setPosition);
}
else {
container.attachEvent('onDOMNodeInserted', setPosition);
}
});
chart.draw(data, options);
}
EDIT: Updated to get the MutationObserver working following asgallant's comment.

Handle feature on Drag and Drop, not working

I'm creating a Drag and Drop plugin.
Right now i'm trying to create as many as features as possible. My current goal is to get my handle feature to work.
This is the code i'm using:
$(o.handle).mousedown(function (event) {
down = true;
if (o.activeClass === true) {
$(oj).addClass(o.activeClass);
}
var dx = event.clientX - $(o.handle).position().left,
dy = event.clientY - $(o.handle).position().top;
$(o.handle).mousemove(function (event) {
if (down == true) {
if (o.dragClass === true) {
$(oj).addClass(o.dragClass);
}
$(oj).css({
cursor: 'move',
left: event.clientX - dx,
top: event.clientY - dy
});
}
});
});
About the Code. oj is referring to this. this is the element that is being dragged by the handle.
o.handle is referring to the element of the handle. The o in o.handle is referring to:
var o = $.extend(defaults, options);
Problem: The First time I try to click on o.handle. The element jumps to: top:0px;left:0px;. Then it drags fine. But then once I drop it and then pick up the element and try to drag it again. It jumps to: top:0px;left:1px;. This happens over and over, non stop. I don't why though. You can see the problem here. Do you see an error in my code?
Thanks for any help

Catch browser's "zoom" event in JavaScript

Is it possible to detect, using JavaScript, when the user changes the zoom in a page?
I simply want to catch a "zoom" event and respond to it (similar to window.onresize event).
Thanks.

There's no way to actively detect if there's a zoom. I found a good entry here on how you can attempt to implement it.
I’ve found two ways of detecting the
zoom level. One way to detect zoom
level changes relies on the fact that
percentage values are not zoomed. A
percentage value is relative to the
viewport width, and thus unaffected by
page zoom. If you insert two elements,
one with a position in percentages,
and one with the same position in
pixels, they’ll move apart when the
page is zoomed. Find the ratio between
the positions of both elements and
you’ve got the zoom level. See test
case.
http://web.archive.org/web/20080723161031/http://novemberborn.net/javascript/page-zoom-ff3
You could also do it using the tools of the above post. The problem is you're more or less making educated guesses on whether or not the page has zoomed. This will work better in some browsers than other.
There's no way to tell if the page is zoomed if they load your page while zoomed.

Lets define px_ratio as below:
px ratio = ratio of physical pixel to css px.
if any one zoom The Page, the viewport pxes (px is different from pixel ) reduces and should be fit to The screen so the ratio (physical pixel / CSS_px ) must get bigger.
but in window Resizing, screen size reduces as well as pxes. so the ratio will maintain.
zooming: trigger windows.resize event --> and change px_ratio
but
resizing: trigger windows.resize event --> doesn’t change px_ratio
//for zoom detection
px_ratio = window.devicePixelRatio || window.screen.availWidth / document.documentElement.clientWidth;
$(window).resize(function(){isZooming();});
function isZooming(){
var newPx_ratio = window.devicePixelRatio || window.screen.availWidth / document.documentElement.clientWidth;
if(newPx_ratio != px_ratio){
px_ratio = newPx_ratio;
console.log("zooming");
return true;
}else{
console.log("just resizing");
return false;
}
}
The key point is difference between CSS PX and Physical Pixel.
https://gist.github.com/abilogos/66aba96bb0fb27ab3ed4a13245817d1e

Good news everyone some people! Newer browsers will trigger a window resize event when the zoom is changed.

I'm using this piece of JavaScript to react to Zoom "events".
It polls the window width.
(As somewhat suggested on this page (which Ian Elliott linked to): http://novemberborn.net/javascript/page-zoom-ff3 [archive])
Tested with Chrome, Firefox 3.6 and Opera, not IE.
Regards, Magnus
var zoomListeners = [];
(function(){
// Poll the pixel width of the window; invoke zoom listeners
// if the width has been changed.
var lastWidth = 0;
function pollZoomFireEvent() {
var widthNow = jQuery(window).width();
if (lastWidth == widthNow) return;
lastWidth = widthNow;
// Length changed, user must have zoomed, invoke listeners.
for (i = zoomListeners.length - 1; i >= 0; --i) {
zoomListeners[i]();
}
}
setInterval(pollZoomFireEvent, 100);
})();

This works for me:
var deviceXDPI = screen.deviceXDPI;
setInterval(function(){
if(screen.deviceXDPI != deviceXDPI){
deviceXDPI = screen.deviceXDPI;
... there was a resize ...
}
}, 500);
It's only needed on IE8. All the other browsers naturally generate a resize event.

There is a nifty plugin built from yonran that can do the detection. Here is his previously answered question on StackOverflow. It works for most of the browsers. Application is as simple as this:
window.onresize = function onresize() {
var r = DetectZoom.ratios();
zoomLevel.innerHTML =
"Zoom level: " + r.zoom +
(r.zoom !== r.devicePxPerCssPx
? "; device to CSS pixel ratio: " + r.devicePxPerCssPx
: "");
}
Demo

Although this is a 9 yr old question, the problem persists!
I have been detecting resize while excluding zoom in a project, so I edited my code to make it work to detect both resize and zoom exclusive from one another. It works most of the time, so if most is good enough for your project, then this should be helpful! It detects zooming 100% of the time in what I've tested so far. The only issue is that if the user gets crazy (ie. spastically resizing the window) or the window lags it may fire as a zoom instead of a window resize.
It works by detecting a change in window.outerWidth or window.outerHeight as window resizing while detecting a change in window.innerWidth or window.innerHeight independent from window resizing as a zoom.
//init object to store window properties
var windowSize = {
w: window.outerWidth,
h: window.outerHeight,
iw: window.innerWidth,
ih: window.innerHeight
};
window.addEventListener("resize", function() {
//if window resizes
if (window.outerWidth !== windowSize.w || window.outerHeight !== windowSize.h) {
windowSize.w = window.outerWidth; // update object with current window properties
windowSize.h = window.outerHeight;
windowSize.iw = window.innerWidth;
windowSize.ih = window.innerHeight;
console.log("you're resizing"); //output
}
//if the window doesn't resize but the content inside does by + or - 5%
else if (window.innerWidth + window.innerWidth * .05 < windowSize.iw ||
window.innerWidth - window.innerWidth * .05 > windowSize.iw) {
console.log("you're zooming")
windowSize.iw = window.innerWidth;
}
}, false);
Note: My solution is like KajMagnus's, but this has worked better for me.

⬤ The resize event works on modern browsers by attaching the event on window, and then reading values of thebody, or other element with for example (.getBoundingClientRect()).
In some earlier browsers it was possible to register resize event
handlers on any HTML element. It is still possible to set onresize
attributes or use addEventListener() to set a handler on any element.
However, resize events are only fired on the window object (i.e.
returned by document.defaultView). Only handlers registered on the
window object will receive resize events.
⚠️ Do resize your tab, or zoom, to trigger this snippet:
window.addEventListener("resize", getSizes, false)
function getSizes(){
let body = document.body
body.width = window.innerWidth
body.height = window.innerHeight
console.log(body.width +"px x "+ body.height + "px")
}
getSizes()
⬤ An other modern alternative: the ResizeObserver API
Depending your layout, you can watch for resizing on a particular element.
This works well on «responsive» layouts, because the container box get resized when zooming.
function watchBoxchange(e){
info.textContent = e[0].contentBoxSize[0].inlineSize+" x "+e[0].contentBoxSize[0].blockSize + "px"
}
new ResizeObserver(watchBoxchange).observe(fluid)
#fluid {
width: 200px;
height:100px;
overflow: auto;
resize: both;
border: 3px black solid;
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
font-size: 8vh
}
<div id="fluid">
<info id="info"></info>
</div>
💡 Be careful to not overload javascript tasks from user gestures events. Use requestAnimationFrame whenever you needs redraws.

I'd like to suggest an improvement to previous solution with tracking changes to window width. Instead of keeping your own array of event listeners you can use existing javascript event system and trigger your own event upon width change, and bind event handlers to it.
$(window).bind('myZoomEvent', function() { ... });
function pollZoomFireEvent()
{
if ( ... width changed ... ) {
$(window).trigger('myZoomEvent');
}
}
Throttle/debounce can help with reducing the rate of calls of your handler.

According to MDN, "matchMedia" is the proper way to do this https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio#Monitoring_screen_resolution_or_zoom_level_changes
it's a bit finicky because each instance can only watch one MQ at a time, so if you're interested in any zoom level change you need to make a bunch of matchers.. but since the browser is in charge to emitting the events it's probably still more performant than polling, and you could throttle or debounce the callback or pin it to an animation frame or something - here's an implementation that seems pretty snappy, feel free to swap in _throttle or whatever if you're already depending on that.
Run the code snippet and zoom in and out in your browser, note the updated value in the markup - I only tested this in Firefox! lemme know if you see any issues.
const el = document.querySelector('#dppx')
if ('matchMedia' in window) {
function observeZoom(cb, opts) {
opts = {
// first pass for defaults - range and granularity to capture all the zoom levels in desktop firefox
ceiling: 3,
floor: 0.3,
granularity: 0.05,
...opts
}
const precision = `${opts.granularity}`.split('.')[1].length
let val = opts.floor
const vals = []
while (val <= opts.ceiling) {
vals.push(val)
val = parseFloat((val + opts.granularity).toFixed(precision))
}
// construct a number of mediamatchers and assign CB to all of them
const mqls = vals.map(v => matchMedia(`(min-resolution: ${v}dppx)`))
// poor person's throttle
const throttle = 3
let last = performance.now()
mqls.forEach(mql => mql.addListener(function() {
console.debug(this, arguments)
const now = performance.now()
if (now - last > throttle) {
cb()
last = now
}
}))
}
observeZoom(function() {
el.innerText = window.devicePixelRatio
})
} else {
el.innerText = 'unable to observe zoom level changes, matchMedia is not supported'
}
<div id='dppx'>--</div>

You can also get the text resize events, and the zoom factor by injecting a div containing at least a non-breakable space (possibly, hidden), and regularly checking its height. If the height changes, the text size has changed, (and you know how much - this also fires, incidentally, if the window gets zoomed in full-page mode, and you still will get the correct zoom factor, with the same height / height ratio).

<script>
var zoomv = function() {
if(topRightqs.style.width=='200px){
alert ("zoom");
}
};
zoomv();
</script>

On iOS 10 it is possible to add an event listener to the touchmove event and to detect, if the page is zoomed with the current event.
var prevZoomFactorX;
var prevZoomFactorY;
element.addEventListener("touchmove", (ev) => {
let zoomFactorX = document.documentElement.clientWidth / window.innerWidth;
let zoomFactorY = document.documentElement.clientHeight / window.innerHeight;
let pageHasZoom = !(zoomFactorX === 1 && zoomFactorY === 1);
if(pageHasZoom) {
// page is zoomed
if(zoomFactorX !== prevZoomFactorX || zoomFactorY !== prevZoomFactorY) {
// page is zoomed with this event
}
}
prevZoomFactorX = zoomFactorX;
prevZoomFactorY = zoomFactorY;
});

Here is a clean solution:
// polyfill window.devicePixelRatio for IE
if(!window.devicePixelRatio){
Object.defineProperty(window,'devicePixelRatio',{
enumerable: true,
configurable: true,
get:function(){
return screen.deviceXDPI/screen.logicalXDPI;
}
});
}
var oldValue=window.devicePixelRatio;
window.addEventListener('resize',function(e){
var newValue=window.devicePixelRatio;
if(newValue!==oldValue){
// TODO polyfill CustomEvent for IE
var event=new CustomEvent('devicepixelratiochange');
event.oldValue=oldValue;
event.newValue=newValue;
oldValue=newValue;
window.dispatchEvent(event);
}
});
window.addEventListener('devicepixelratiochange',function(e){
console.log('devicePixelRatio changed from '+e.oldValue+' to '+e.newValue);
});

Here is a native way (major frameworks cannot zoom in Chrome, because they dont supports passive event behaviour)
//For Google Chrome
document.addEventListener("mousewheel", event => {
console.log(`wheel`);
if(event.ctrlKey == true)
{
event.preventDefault();
if(event.deltaY > 0) {
console.log('Down');
}else {
console.log('Up');
}
}
}, { passive: false });
// For Mozilla Firefox
document.addEventListener("DOMMouseScroll", event => {
console.log(`wheel`);
if(event.ctrlKey == true)
{
event.preventDefault();
if(event.detail > 0) {
console.log('Down');
}else {
console.log('Up');
}
}
}, { passive: false });

I'am replying to a 3 year old link but I guess here's a more acceptable answer,
Create .css file as,
#media screen and (max-width: 1000px)
{
// things you want to trigger when the screen is zoomed
}
EG:-
#media screen and (max-width: 1000px)
{
.classname
{
font-size:10px;
}
}
The above code makes the size of the font '10px' when the screen is zoomed to approximately 125%. You can check for different zoom level by changing the value of '1000px'.