const orders = [
  {
    name: "Janni",
    id: 123456789,
  },
  {
    name: "Unayo",
    id: 987654321,
  }
];

// 大括號沒有拿掉要加上 return 
let index = orders.findIndex((i)=>{
  return i.id === 123456789 
});
console.log(index);
// 0

let index1 = orders.findIndex((item) => item.id === 123456789 );
console.log(index1);
// 0

// 刪除選取到該 id 的物件
orders.splice(index1, 1);
console.log(orders)