JSFiddle - React, Tailwind, and code Playground

JavaScript

var getAlpha = (function() {
    var alphas = [null, "a"],
        highest = [1];
    
    function toAlpha(val) {
    }
    
    return function(decNum) {
        if (alphas[decNum])
            return alphas[decNum];
        
        var d,
            next,
            carry,
            i = alphas.length;
        
        for(; i <= decNum; i++) {
            next = "";
            carry = true;
            for(d = 0; d < highest.length; d++){
                if (carry) {
                    if (highest[d] === 26) {
                        highest[d] = 1;
                    } else { 
                        highest[d]++;
                        carry = false;
                    }
                }
                next = String.fromCharCode(
                          highest[d] + 96)
                     + next;
            }
            if (carry) {
                highest.push(1);
                next = "a" + next;
            }
            alphas[i] = next;
        }
        
        return alphas[decNum];
    };
})();

alert(getAlpha(27));
alert(getAlpha(100000));
for (var i=1; i < 10000; i++)
   document.write(i + " " + getAlpha(i) + "<br>");