Use the flexBasis property in JavaScript to set the initial length of a flexible item. You can try to run the following code to set the initial length of a flexible item with JavaScript −
Example
<!DOCTYPE html>
<html>
<head>
<style>
#box {
border: 1px solid #000000;
width: 450px;
height: 150px;
display: flex;
}
#box div {
flex-grow: 0;
flex-shrink: 1;
flex-basis: 20px;
}
</style>
</head>
<body>
<div id = "box">
<div style = "background-color:orange;">DIV1</div>
<div style = "background-color:blue;">DIV2</div>
<div style = "background-color:yellow;" id="myID">DIV3</div>
</div>
<p>Using flexBasis property</p>
<button onclick = "display()">Set</button>
<script>
function display() {
document.getElementById("myID").style.flexBasis = "100px";
}
</script>
</body>
</html>