addItem() with localStorage

App.js

addItem() {
    // create a new item
    const newItem = {
      id: 1 + Math.random(),
      value: this.state.newItem.slice()
    };

    // copy current list of items
    const list = [...this.state.list];

    // add the new item to the list
    list.push(newItem);

    // update state with new list, reset the new item input
    this.setState({
      list,
      newItem: ""
    });

    // update localStorage
    localStorage.setItem("list", JSON.stringify(list));
    localStorage.setItem("newItem", "");
  }