script.js

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

function addTask(){
   if(inputBox.value === '' ){
    alert("You must write a task!!!");
   }
   else{
    let li = document.createElement("li");
    li.innerHTML = inputBox.value;
    listBox.appendChild(li);
    let span = document.createElement("span");
    span.innerHTML = "\u00d7";
    li.appendChild(span);
   }

   inputBox.value = "";
   saveData()
}
listBox.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", listBox.innerHTML);
}
function showList(){
listBox.innerHTML = localStorage.getItem("data");
}
showList();