app.js

const inputBox = document.getElementById("input-box");
const listContainer = document.getElementById("list-container");
function addTask(){
    if (inputBox.value === ''){
        alter("you must write something!");
    }
    else{
        let li = document.createElement("li");
        li.innerHTML = inputBox.value;
        listContainer.appendChild(li);
        let span = document.createElement("span");
        span.innerHTML ="\u00d7";
        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();