script.js

"use strict";

let inputText = document.getElementById('input-text');
let buttonUpperCase = document.getElementById('upper-case');
let buttonLowerCase = document.getElementById('lower-case');
let buttonProperCase = document.getElementById('proper-case');
let buttonSentenceCase = document.getElementById('sentence-case');
let buttonSaveTextFile = document.getElementById('save-text-file');

buttonUpperCase.addEventListener("click", () => {
    inputText.value = inputText.value.toUpperCase();
});

buttonLowerCase.addEventListener("click", () => {
    inputText.value = inputText.value.toLowerCase();
});

buttonProperCase.addEventListener("click", () => {
    let textArray = inputText.value.toLowerCase().split(' ');
    for (let i = 0; i < textArray.length; i++) {
        textArray[i] = textArray[i].charAt(0).toUpperCase() + textArray[i].slice(1);
    }
    inputText.value = textArray.join(' ');
});

buttonSentenceCase.addEventListener("click", () => {
    let textArray = inputText.value.toLowerCase().split('. ');
    for (let i = 0; i < textArray.length; i++) {
        textArray[i] = textArray[i].charAt(0).toUpperCase() + textArray[i].slice(1);
    }
    inputText.value = textArray.join('. ');
});

buttonSaveTextFile.addEventListener("click", () => {
    function download(filename, text) {
        let element = document.createElement('a');
        element.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(text));
        element.setAttribute('download', filename);

        element.style.display = 'none';
        document.body.appendChild(element);

        element.click();

        document.body.removeChild(element);
    }

// Start file download.
    download("text.txt", inputText.value);
});