JSFiddle - React, Tailwind, and code Playground

by Roko

HTML

<input type="text" class="js_leading_zero" value="JS000000" maxlength=8>

JavaScript

const prefix = "JS";

const modifyVal = (ev) => {
	const EL = ev.currentTarget;
    let str = EL.value.replace(/\D/g, "");
    str +=  EL.dataset.key;
    const int = parseInt(str);
    str = String(int).padStart(6, "0");
    if (str.length === 6)
    EL.value = prefix + str;
};

const rememberKey = (ev) => {
	ev.preventDefault();
    const EL = ev.currentTarget;
    EL.dataset.key = ev.key;
};

const ELs_inp = document.querySelectorAll(".js_leading_zero");
ELs_inp.forEach(EL => EL.addEventListener("keydown", rememberKey));
ELs_inp.forEach(EL => EL.addEventListener("keyup", modifyVal));