JSFiddle - React, Tailwind, and code Playground

HTML

<div id=a></div>

JavaScript

var isMouseDown = false;
document.onmousedown = function() { isMouseDown = true };
document.onmouseup   = function() { isMouseDown = false };
document.onmousemove = function() { if(isMouseDown) { 
    document.getElementById('a').innerHTML += 'a';
} };