reduce

arr.reduce((參數一,參數二) ⇒ { }, 參數三)

• 參數一,為累加的暫存
• 參數二,為每個元素
• 參數三為初始值

const arr = [1,2,3,4,5,6,7,8]
arr.reduce((final, item) =>{
    return final + item 
},0)
// 36

將以下陣列取出重複值並相加

const data = [
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 7, size: "草莓", price: "3000"},
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 1, size: "草莓", price: "3000"},
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 1, size: "巧克力", price: "3000"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "L", price: "1800"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "M", price: "1800"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "L", price: "1800"}
]

1. 用reduce做數值的加總
2. 用some判斷資料是否重複,若有重複數量加總,若無重複則新增到新陣列中

const data = [
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 7, size: "草莓", price: "3000"},
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 1, size: "草莓", price: "3000"},
  {product_id: "-Lr8YllJkIewLhA7DCNU", title: "碗豆乳清蛋白粉", qty: 1, size: "巧克力", price: "3000"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "L", price: "1800"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "M", price: "1800"},
  {product_id: "-LyWmWDrUg0qWmvJmHmJ", title: "皮革腰帶", qty: 1, size: "L", price: "1800"}
]

const filteredArr = data.reduce((final, current) => {
  // 判斷數值(id, size)是否重複
  const repeatItem = final.some(item => item.product_id === current.product_id && item.size === current.size)
  console.log('final' ,final)
  if(repeatItem){
    // 重複時總數(qty)相加
    // current為重複的值,forEach的item為不重複的值
    // 再次判斷有重複的值才相加
    final.forEach(item => { if(item.product_id === current.product_id && item.size === current.size) console.log('current',current),console.log('item',item)})
  } else {
    // 不重複時代表累加陣列(final)無該筆資料,做push
    final.push(current)
  }
  return final
}, [])