JSFiddle - React, Tailwind, and code Playground

HTML

<input id="test">

JavaScript

var input = document.getElementById('test');
var m='';
input.onkeydown =  (function() {

    return function(e) {
    
    var mx = /^[A-Z]/.test(this.value.slice(-1));
    nt = e.which || e.keyCode;
    if(mx==true){
    m=this.value+this.value.slice(-1)
        return m;
    }else{
    m.slice(-1)
    }
    this.value=m
    
    }
})();