JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = "abc4sss5gggg10";

function searchIndex(index, strng) {
    var arr = strng.split("");
    for (var i = index; i < arr.length; i++) {
        if (!isNaN(arr[i])) {
            return i;
        }
    }
    return -1;
}
alert(searchIndex(4, str));