script.js

const inputBox = document.querySelector("#input-box");
const listContainer = document.querySelector("#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();