localStorage.getItem(...).key is not a function

$(document).ready(function(){
    //var count=localStorage.getItem("todo").length;
    $(".submit").click(function(){
        //count++;

        localStorage.setItem("todo", JSON.stringify($('.data').val()));


        var i;
        while (localStorage.getItem("todo").key(i)) {
            var value = "<div class='todorow'><div class='todoitem'>"+JSON.parse(localStorage.getItem("todo").key(i))+"</div><div class='todoremove'>X</div></div>";
            $(".container").append(value);
            console.log(localStorage.getItem("todo").key(i));
            i++;
        }

    });
    $(".container").on("click", ".todoitem", function(){
        jQuery(this).addClass("strike");
    });
    $(".container").on("click", ".todoremove", function(){
        jQuery(this).closest('.todorow').addClass("hide");
    });

    var i;
    while (localStorage.getItem("todo").key(i)) {
            var value = "<div class='todorow'><div class='todoitem'>"+JSON.parse(localStorage.getItem("todo").key(i))+"</div><div class='todoremove'>X</div></div>";
            $(".container").append(value);
            i++;
    }

});