How to find the width of a div using vanilla JavaScript?

You can use clientWidth or offsetWidth Mozilla developer network reference

It would be like:

document.getElementById("yourDiv").clientWidth; // returns number, like 728

or with borders width :

document.getElementById("yourDiv").offsetWidth; // 728 + borders width

All Answers are right, but i still want to give some other alternatives that may work.

If you are looking for the assigned width (ignoring padding, margin and so on) you could use.

getComputedStyle(element).width; //returns value in px like "727.7px"

getComputedStyle allows you to access all styles of that elements. For example: padding, paddingLeft, margin, border-top-left-radius and so on.

Another option is to use the getBoundingClientRect function. Please note that getBoundingClientRect will return an empty rect if the element's display is 'none'.

var elem = document.getElementById("myDiv");
if(elem) {
   var rect = elem.getBoundingClientRect();
   console.log(rect.width);  
}

You can also search the DOM using ClassName. For example:

document.getElementsByClassName("myDiv")

This will return an array. If there is one particular property you are interested in. For example:

var divWidth = document.getElementsByClassName("myDiv")[0].clientWidth;

divWidth will now be equal to the the width of the first element in your div array.

Actually, you don't have to use document.getElementById("mydiv") .
You can simply use the id of the div, like:

var w = mydiv.clientWidth;
or
var w = mydiv.offsetWidth;
etc.

call below method on div or body tag onclick="show(event);" function show(event) {

        var x = event.clientX;
        var y = event.clientY;

        var ele = document.getElementById("tt");
        var width = ele.offsetWidth;
        var height = ele.offsetHeight;
        var half=(width/2);
       if(x>half)
        {
          //  alert('right click');
            gallery.next();
        }
        else
       {
           //   alert('left click');
            gallery.prev();
        }


    }

The correct way of getting computed style is waiting till page is rendered. It can be done in the following manner. Pay attention to timeout on getting auto values.

function getStyleInfo() {
    setTimeout(function() {
        const style = window.getComputedStyle(document.getElementById('__root__'));
        if (style.height == 'auto') {
            getStyleInfo();
        }
        // IF we got here we can do actual business logic staff
        console.log(style.height, style.width);
    }, 100);
};

window.onload=function() { getStyleInfo(); };

If you use just

window.onload=function() {
    var computedStyle = window.getComputedStyle(document.getElementById('__root__'));
}

you can get auto values for width and height because browsers does not render till full load is performed.