script.js

const textarea = document.querySelector("textarea"),
fileNameInput = document.querySelector(".file-name input"),
selectMenu = document.querySelector(".save-as select"),
saveBtn = document.querySelector(".save-btn");

selectMenu.addEventListener("change", () => {
     let selectedOption = selectMenu.option[selectMenu.selectedIndex].text;

});

saveBtn.addEventListener("click", () => {
    const blob = new Blob([textarea.value], {type: selectMenu.value});
    //
    const fileUrl = URL.createObjectURL(blob);
    const link = document.createElement("a");
    link.download = fileNameInput.value;
    link.href = fileUrl;
    link.click();
});