JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id = "btn1" class = "btn-color" />
<input type="button" id = "btn2" class = "btn-color" />
<input type="button" id = "btn3" class = "btn-color" />
<input type="button" id = "btn4" class = "btn-color" />

JavaScript

var elements = document.querySelectorAll(".btn-color");
    for (var i = 0; i < elements.length; i++) {
        elements[i].style.backgroundColor = "blue";
    }