todolist.js

const inputBox = document.getElementById("input-box");
const listContainer = document.getElementById("list-container");

function addTask() {
    if(inputBox.value === '') {
        alert("Write something!");
    }
    else {
        let li = document.createElement("li");
        li.innerHTML = inputBox.value;
        listContainer.appendChild(li);
        let span = document.createElement("span");
        span.innerHTML = "&#10006";
        li.appendChild(span);
    }
    inputBox.value = "";
    saveData();
}

listContainer.addEventListener("click",function(e) {
    if(e.target.tagName === "LI") {
        e.target.classList.toggle("Checked");
        saveData();
    }
    else if(e.target.tagName === "SPAN") {
        e.target.parentElement.remove();
        saveData();
    }
},false);

function saveData() {
    localStorage.setItem('data',listContainer.innerHTML);
}

function showTask() {
    listContainer.innerHTML = localStorage.getItem("data");
}
showTask();