reduce使用技巧

const numbers = [10, 20, 30, 40];
const doubledOver50 = numbers.reduce((finalList, num) => {
  
  num = num * 2; //(map)
  
  //filter number > 50
  if (num > 50) {
    finalList.push(num);
  }
  return finalList;
}, []);
doubledOver50; // [60, 80]

var cars = [‘‘宝马‘，‘奔驰‘，‘奔驰‘，‘特斯拉‘，‘宝马‘，‘丰田‘];
var carsObj = cars.reduce（function（obj，name）{ 
   obj [name] = obj [name]？++ obj [name]：1; 
  return obj; 
}，{}）;
carsObj; // => {宝马：2，奔驰：2，特斯拉：1，丰田：1}