js reduce用法

let books = [
  0,
  {bookName:"python",price:10,count:1},
  {bookName:"Ruby",count:2,price:20},
  {bookName:"Java",count:4,price:30},
  {bookName:"php",count:2,price:40},
  {bookName:"Go",count:1,price:50},
];
// pre 代表第一项， next代码第二项，
// 第二次循环，pre为undefined next为第三项
// return 结果是下一次next的值
let sum = books.reduce(function (prev,next,index,item) {
  return prev+(next.price* next.count);
});

console.log(sum);