ES6

let list = [2,3,1];

const addCounter = (list)=>{
  return[...list, 0];
}

const removeCounter = (list, index) => {
  return [
    ...list.slice(0,index),
    ...list.slice(index+1)
  ];
}

const incrementCounter = (list, index) => {
  return [
    ...list.slice(0,index),
    list[index] + 1,
    ...list.slice(index + 1)
  ];
}

//console.log(incrementCounter(list,1))

let todo = {completed:true}

const toggleTodo = (todo) => {
  //todo.completed = !todo.completed;
  //let newTodo = {completed: !todo.completed}
//   let newTodo = Object.assign({}, todo, {
//     completed: !todo.completed
//   })
  let newTodo = {...todo, completed: !todo.completed}
  return newTodo;
};

console.log(toggleTodo(todo),todo)