//include the cvss.js script 
<script src="http://cvssjs.github.io/cvssjs/cvss.js"></script>

craete an HTML element with an id for eg.,
<div id="cvssboard"/>

// create a new instance of CVSS calculator:
var c = new CVSS("cvssboard");

// create a new instance of CVSS calculator with some event handler callbacks
var c = new CVSS("cvssboard", {
            onchange: function() {....} //optional
            onsubmit: function() {....} //optional
            }
            
// set a vector
c.set('CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L');

// it is also backwards compatible with CVSS v2 vectors, 
// buts only sets the parameters that can be set without ambiguity.
c.set('AV:L/AC:L/Au:N/C:P/I:P/A:C');

//get the value
c.get() returns an object like:
  {
    score: 4.3,
    vector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L'
  }