script.js

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

function addTask() {
  if (inputBox.value === "") {
    alert("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();