To set height equal to the dynamic width in JavaScript, you can try to run the following code −
Example
<!DOCTYPE html>
<html>
<head>
<style>
.wrap {
width:400px;
height:300px;
border:2px solid yellow;
}
.myChild{
width: 80%;
border:1px solid red;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
</script>
<script>
var res = $('.myChild').width();
$('.myChild').css({
'height': res + 'px'
});
</script>
</head>
<body>
<div class = "wrap">
<div class = "myChild"></div>
</div>
</body>
</html>