Esta foi a forma como estruturei meu código para uma aplicação de manipulação do localStorage. Há algo desnecessário ou que poderia melhorar? Em alguns momentos fiquei perdido com tantas variáveis, mas acho que faz parte... Valeu :D

let currentState = []
const localStorageManager = new LocalStorage()
localStorageManager.pull()

function LocalStorage() {

  this.create = () =>{
    localStorage.setItem('tasks', JSON.stringify(currentState))
  }

  this.pull = () =>{
    const tasks = JSON.parse(localStorage.getItem('tasks'))

    !!tasks && 
      tasks.forEach((task)=>{
        new Task(task).create()
      })
  }

}

function Task(task) {
  this.create = () =>{
    const item = document.createElement('li')
    item.classList.add('item')

    const qtd = document.createElement('strong')
    qtd.id = task.id
    qtd.innerHTML = task.amount

    item.appendChild(qtd)
    item.insertAdjacentText('beforeend', task.name)

    item.appendChild(insertDelete(task.id))
    const list =  document.querySelector('.lista')
    list.appendChild(item)
    currentState.push(task)

    console.log(`Task ${task.id} criada`)
    return item
  }

  this.update = (index) =>{
    const amount = document.getElementById(task.id)
    amount.innerText = task.amount
    currentState[index] = task
  }

  this.removeTask = (element, index) =>{
    element.remove()
    currentState.splice(index,1)
    localStorageManager.create()
  }


}


function insertDelete(id) {
  const button = document.createElement('button');
  button.innerText = 'X'

  button.onclick = (e) =>{
    const element = e.target.parentNode
    const index = currentState.findIndex(item => item.id === id)
    new Task().removeTask(element, index)
  }
  return button
}

function checkingExistence(name) {
  const oldTask =  {}

  const item = currentState.find((task) => task.name === name)
  oldTask.item = item

  if(item) {
    const index =  currentState.findIndex(element => element.id === item.id)
    oldTask.index =  index
  }

  return oldTask
}

function handleSubmit(e) {
  e.preventDefault();
  const inputName = e.target.elements['name']
  const inputAmount = e.target.elements['amount']

  const task = {
    name: inputName.value,
    amount: inputAmount.value
  }

  const oldTask = checkingExistence(inputName.value)

  if(!oldTask.item && inputName.value && inputAmount.value){
    task.id = currentState[currentState.length-1] ? currentState[currentState.length-1].id + 1 : 0
    new Task(task).create()
    localStorageManager.create() 
  }

  else if (!!oldTask.item && inputName.value && inputAmount.value){
    task.id = oldTask.item.id 
    new Task(task).update(oldTask.index)
    localStorageManager.create() 
  }

  else{
    console.log('Valor inválido')
  }
}

const form = document.querySelector('.adicionar');
form.onsubmit = handleSubmit