如何访问 .reduce() 传递的数组

如何解决如何访问 .reduce() 传递的数组

我有一个 JavaScript 对象：

const movies = [
  {'title' : 'Inception','rating' : 8.8},{'title' : 'Interstellar','rating' : 8.6},{'title' : 'The Dark Knight','rating' : 9.0},{'title' : 'Batman Begins','rating' : 8.3}
]

例如，我想做的是使用 .reduce() 计算平均评分，所以我被困在这里：

const averagerating = movies.filter(movie => movie['rating'] > 8.5).reduce((acc,movie) => (acc+movie['rating'])/2,/* The rating of the first value of this array*/)

如何访问由 reduce() 处理的数组以获取第一部电影的评分（使用它的第一个 ELEMENT.rating 作为初始值）？我知道默认分配给 acc 的第一个值是第一个值，但在这种情况下它是一个对象。

我知道有更好的方法来解决这种特殊情况，我实际上已经解决了，但我不知道这样是否可以达到相同的结果？

const movies = [{
    'title': 'Inception','rating': 8.8
  },{
    'title': 'Interstellar','rating': 8.6
  },{
    'title': 'The Dark Knight','rating': 9.0
  },{
    'title': 'Batman Begins','rating': 8.3
  }
]
const averagerating = movies.filter(movie => movie['rating'] > 8.5).reduce((acc,movie) => (acc + movie['rating']) / 2,)

console.log(averagerating)

解决方法

如果您注意到 reduce 函数的 documentation，它接受四个参数。

累加器
当前值
当前索引
数组

因此，在您的情况下，您可以执行以下操作：

const averageRating = movies
                       .filter(movie => movie['rating'] > 8.5)
                       .reduce((acc,movie,idx,arr) => (acc+movie['rating'])/2)

Array.reduce() 的第四个参数是传递给它的数组。

将累加器的初始值设置为 0 - 这将防止当数组为空时 reduce 抛出错误。将每个数字除以过滤后的数组的长度，并将结果添加到累加器中以获得平均值。

const movies = [{"title":"Inception","imdbRating":8.8},{"title":"Interstellar","imdbRating":8.6},{"title":"The Dark Knight","imdbRating":9},{"title":"Batman Begins","imdbRating":8.3}]

const averageRating = movies
  .filter(movie => movie.imdbRating > 8.5)
  .reduce((acc,i,arr) => acc + movie.imdbRating / arr.length,0)
  
console.log(averageRating)

完整的 .reduce 签名是：

arr.reduce(callback( accumulator,currentValue,[,index[,array]] )[,initialValue])

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

基于此，您可以编写：

const averageRating = movies
  .filter((movie) => movie["rating"] > 8.5)
  .reduce((acc,index,array) => (acc + movie["rating"]) / array.length,0);

并且可以访问reduce函数内的array

查询docs可以看到reduce的回调有如下签名

function(accumulator,currentIndex,array)

因此

array
  .filter(x => ...)
  .reduce((ac,cv,ar) => {
    //here ar is the array you are calling reduce,ie the array resulting from filter
  })

好的，试试这个

const movies = [
  {'title' : 'Inception','rating' : 8.8},{'title' : 'Interstellar','rating' : 8.6},{'title' : 'The Dark Knight','rating' : 9.0},{'title' : 'Batman Begins','rating' : 8.3}
]

// On each reduce iteration you modifying object property,// not array value. To calculate average,you adding acc.rating
// to current movie.rating and when current iteration equals
// array length (last iteration) you divide accumulated value
// by array length (number of entries that was accumulated).
const averageRating = movies.filter(movie => movie['rating'] > 8.5)
.reduce((acc,a) => ({
  'rating': (acc.rating + movie.rating)/(i+1==a.length?a.length:1)
}));

console.log(averageRating);