En este script, intenta hacer que cuando elija la fecha y el formato dado, se muestre. Por supuesto, algo está mal con la validación. Si se selecciona la opción "Seleccionar formato de fecha", mostrará un error. Si elige los otros 2 formatos, normalmente debe hacer
var nazwamiesiecy = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]; var nazwadni = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]; let input = document.querySelector("input"); let guzik = document.querySelector("button"); guzik.addEventListener("click", () => { event.preventDefault(); let wybranaData = new Date(input.value); let format = document.querySelectorAll("select").value let opcja1 = document.querySelectorAll("option")[0].value let opcja2 = document.querySelectorAll("option")[1].value let opcja3 = document.querySelectorAll("option")[2].value if (!input.value) { alert("Enter the date"); return; } if (!opcja1) { alert("Enter the date format") return; } if (!opcja2) { document.getElementById("dataa").innerHTML = wybranaData.getDate() + " " + nazwamiesiecy[wybranaData.getMonth()] + " " + wybranaData.getFullYear().toString() + " roku"; return; } if (!opcja3) { document.getElementById("dataaa").innerHTML = wybranaData.getDate() + "." + wybranaData.getMonth() + "." + wybranaData.getFullYear().toString(); return; } }) <form name=""> <div> <label for="txtnum1">Date: </label> <input type="date" name="txtnum1" id="txtnum1"> </div> <label for="sztos">Date format: </label> <select name="sztos" id="sztos"> <option value="">Choose a date format</option> <option value="op">12/04/2000</option> <option value="xd">12 April 2000</option> </select> <button type="submit">Calculate</button> <div id="dataa"></div> <div id="dataaa"></div> </form> var nazwamiesiecy = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]; var nazwadni = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]; let input = document.querySelector("input"); let guzik = document.querySelector("button"); let dataa = document.getElementById("dataa") guzik.addEventListener("click", (event) => { event.preventDefault(); if (!input.value) { alert("Enter the date"); return; } let format = document.querySelector("select").value if (!format) { alert("Enter the date format") return; } let wybranaData = new Date(input.value); let dateString = ""; if (format === "xd") { dateString = wybranaData.getDate() + " " + nazwamiesiecy[wybranaData.getMonth()] + " " + wybranaData.getFullYear() + " roku"; } else if (format === "op") { dateString = wybranaData.getDate() + "/" + wybranaData.getMonth() + "/" + wybranaData.getFullYear(); } dataa.innerHTML = dateString; }) <form name=""> <div> <label for="txtnum1">Date: </label> <input type="date" name="txtnum1" id="txtnum1"> </div> <label for="sztos">Date format: </label> <select name="sztos" id="sztos"> <option value="">Choose a date format</option> <option value="op">dd/mm/yyyy</option> <option value="xd">dd MMMMMM yyyy</option> </select> <button type="submit">Calculate</button> <div id="dataa"></div> </form>