script.js

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

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();