JSFiddle - React, Tailwind, and code Playground

HTML

<div id="e1" class="someclass"></div>
<div id="e2" class="someclass"></div>
<div id="e3" class="someclass"></div>
<div id="e4" class="someclass"></div>

<div id="results"></div>

JavaScript

// Some preliminaries for formatting and output
r = document.getElementById("results");
function listIDs(a) {
    var b = [];
    for(var i=0;i<a.length;i+=1) { b[i]=a[i].id; }
    return b;
}

// 1, get all the elements of the class
A = document.getElementsByClassName("someclass");
r.innerHTML = "A = document.getElementsByClassName(\"someclass\");<br/><br/>";

// 2, print out these elements
r.innerHTML += "> A contains elements "+listIDs(A).join(', ')+ "<br/><br/>";

// 3, change the class of one element
document.getElementById("e2").className = "anotherclass";
r.innerHTML += "document.getElementById(\"e2\").className = \"anotherclass\";<br/><br/>";

// 4, print again the value of A
r.innerHTML += "> A contains elements "+listIDs(A).join(', ')+ "<br/><br/>";