map():處理陣列

map()

透過函式產生一個新陣列

不會改變原陣列

回傳等於原陣列

如不回傳是undefined

語法

const newArr = arr.map(function (value, index, array){
  return  ...
});

回調函式（callback）與三個參數:value,index,arr

const shoppingCart = [
  {itemName: "Book",price : 220 },
  {itemName: "Bag",price : 350 }
];
// 使用箭頭函示的縮寫
const itemNames = shoppingCart.map(x => 
  x.itemName
); 
console.log(itemNames )
// 只把 itemNames 提取出來
itemNames; // ["Book", "Bag"]

// 把 price 打七九折後提取出來
const discounts = shoppingCart.map(x => x.price * 0.79); 
discounts; 
console.log('折扣:discounts',discounts)
// [173.8, 276.5]



//小於 50標示無折扣
const prices = [42, 57, 89, 23, 78, 12]
const newPrices = prices.map( function(elem) {
  if(elem < 50) return "無折扣";
return elem
});

newPrices; 
console.log(newPrices)
//["無折扣",57,89,"無折扣",78,"無折扣"]

價格大於999元，均8折

map() & Math.max 取出最大值