数组常用方法记录

reduce()函数

reduce() 方法为数组的每个值（从左到右）执行提供的函数，将数组缩减为单个值。函数的返回值存储在累加器中（结果/总计）。

注释：对没有值的数组元素，不执行 reduce() 方法。reduce() 方法不会改变原始数组。

实例1：返回一个包含用户名称做为属性，其年龄做为值的对象。

const users = [
  { name: 'John', age: 34 },
  { name: 'Amy', age: 20 },
  { name: 'camperCat', age: 10 }
];

const usersObj = users.reduce((obj, user) => {
  obj[user.name] = user.age;
  return obj;
}, {});
console.log(usersObj);

实例2：求由 Christopher Nolan 导演的电影的 IMDB 评级平均值。

// 全局变量
const watchList = [
  {
    "Title": "Inception",
    "Year": "2010",
    "Rated": "PG-13",
    "Released": "16 Jul 2010",
    "Runtime": "148 min",
    "Genre": "Action, Adventure, Crime",
    "Director": "Christopher Nolan",
    "Writer": "Christopher Nolan",
    "Actors": "Leonardo DiCaprio, Joseph Gordon-Levitt, Elliot Page, Tom Hardy",
    "Plot": "A thief, who steals corporate secrets through use of dream-sharing technology, is given the inverse task of planting an idea into the mind of a CEO.",
    "Language": "English, Japanese, French",
    "Country": "USA, UK",
    "Awards": "Won 4 Oscars. Another 143 wins & 198 nominations.",
    "Poster": "http://ia.media-imdb.com/images/M/MV5BMjAxMzY3NjcxNF5BMl5BanBnXkFtZTcwNTI5OTM0Mw@@._V1_SX300.jpg",
    "Metascore": "74",
    "imdbRating": "8.8",
    "imdbVotes": "1,446,708",
    "imdbID": "tt1375666",
    "Type": "movie",
    "Response": "True"
  },
  {
    "Title": "Interstellar",
    "Year": "2014",
    "Rated": "PG-13",
    "Released": "07 Nov 2014",
    "Runtime": "169 min",
    "Genre": "Adventure, Drama, Sci-Fi",
    "Director": "Christopher Nolan",
    "Writer": "Jonathan Nolan, Christopher Nolan",
    "Actors": "Ellen Burstyn, Matthew McConaughey, Mackenzie Foy, John Lithgow",
    "Plot": "A team of explorers travel through a wormhole in space in an attempt to ensure humanity's survival.",
    "Language": "English",
    "Country": "USA, UK",
    "Awards": "Won 1 Oscar. Another 39 wins & 132 nominations.",
    "Poster": "http://ia.media-imdb.com/images/M/MV5BMjIxNTU4MzY4MF5BMl5BanBnXkFtZTgwMzM4ODI3MjE@._V1_SX300.jpg",
    "Metascore": "74",
    "imdbRating": "8.6",
    "imdbVotes": "910,366",
    "imdbID": "tt0816692",
    "Type": "movie",
    "Response": "True"
  },
  {
    "Title": "The Dark Knight",
    "Year": "2008",
    "Rated": "PG-13",
    "Released": "18 Jul 2008",
    "Runtime": "152 min",
    "Genre": "Action, Adventure, Crime",
    "Director": "Christopher Nolan",
    "Writer": "Jonathan Nolan (screenplay), Christopher Nolan (screenplay), Christopher Nolan (story), David S. Goyer (story), Bob Kane (characters)",
    "Actors": "Christian Bale, Heath Ledger, Aaron Eckhart, Michael Caine",
    "Plot": "When the menace known as the Joker wreaks havoc and chaos on the people of Gotham, the caped crusader must come to terms with one of the greatest psychological tests of his ability to fight injustice.",
    "Language": "English, Mandarin",
    "Country": "USA, UK",
    "Awards": "Won 2 Oscars. Another 146 wins & 142 nominations.",
    "Poster": "http://ia.media-imdb.com/images/M/MV5BMTMxNTMwODM0NF5BMl5BanBnXkFtZTcwODAyMTk2Mw@@._V1_SX300.jpg",
    "Metascore": "82",
    "imdbRating": "9.0",
    "imdbVotes": "1,652,832",
    "imdbID": "tt0468569",
    "Type": "movie",
    "Response": "True"
  },
  {
    "Title": "Batman Begins",
    "Year": "2005",
    "Rated": "PG-13",
    "Released": "15 Jun 2005",
    "Runtime": "140 min",
    "Genre": "Action, Adventure",
    "Director": "Christopher Nolan",
    "Writer": "Bob Kane (characters), David S. Goyer (story), Christopher Nolan (screenplay), David S. Goyer (screenplay)",
    "Actors": "Christian Bale, Michael Caine, Liam Neeson, Katie Holmes",
    "Plot": "After training with his mentor, Batman begins his fight to free crime-ridden Gotham City from the corruption that Scarecrow and the League of Shadows have cast upon it.",
    "Language": "English, Urdu, Mandarin",
    "Country": "USA, UK",
    "Awards": "Nominated for 1 Oscar. Another 15 wins & 66 nominations.",
    "Poster": "http://ia.media-imdb.com/images/M/MV5BNTM3OTc0MzM2OV5BMl5BanBnXkFtZTYwNzUwMTI3._V1_SX300.jpg",
    "Metascore": "70",
    "imdbRating": "8.3",
    "imdbVotes": "972,584",
    "imdbID": "tt0372784",
    "Type": "movie",
    "Response": "True"
  },
  {
    "Title": "Avatar",
    "Year": "2009",
    "Rated": "PG-13",
    "Released": "18 Dec 2009",
    "Runtime": "162 min",
    "Genre": "Action, Adventure, Fantasy",
    "Director": "James Cameron",
    "Writer": "James Cameron",
    "Actors": "Sam Worthington, Zoe Saldana, Sigourney Weaver, Stephen Lang",
    "Plot": "A paraplegic marine dispatched to the moon Pandora on a unique mission becomes torn between following his orders and protecting the world he feels is his home.",
    "Language": "English, Spanish",
    "Country": "USA, UK",
    "Awards": "Won 3 Oscars. Another 80 wins & 121 nominations.",
    "Poster": "http://ia.media-imdb.com/images/M/MV5BMTYwOTEwNjAzMl5BMl5BanBnXkFtZTcwODc5MTUwMw@@._V1_SX300.jpg",
    "Metascore": "83",
    "imdbRating": "7.9",
    "imdbVotes": "876,575",
    "imdbID": "tt0499549",
    "Type": "movie",
    "Response": "True"
  }
];

function getRating(watchList) {
  // 只修改这一行下面的代码
  let averageRating = watchList
  .filter (v=> v.Director == 'Christopher Nolan')
  .map( v=> Number(v.imdbRating))
  .reduce((sumOfRatings, rating) => sumOfRatings + rating) / watchList.filter(film => film.Director === "Christopher Nolan").length;
  // 只修改这一行上面的代码
  return averageRating;
}

console.log(getRating(watchList));

sort排序

sort() 方法默认按字母和升序将值作为字符串进行排序。
注意：这适用于字符串（“Apple” 出现在 “Banana” 之前）。但是，如果数字按字符串排序，则 “25” 大于 “100” ，因为 “2” 大于 “1”。
因此，建议提供一个回调函数来指定如何对数组项目排序 array.sort(compareFunction)。 这个回调函数通常叫做 compareFunction，它根据 compareFunction 的返回值决定数组元素的排序方式： 如果两个元素 a 和 b，compareFunction(a,b) 返回一个比 0 小的值，那么 a 会在 b 的前面。 如果返回一个比 0 大的值，那么 b 会在 a 的前面。 如果返回等于 0的值，那么 a 和 b 的位置保持不变。

function alphabeticalOrder(arr) {
  return arr.sort(function(a,b){
     return a < b ? -1 : a > b ? 1 : 0;
  })
}
alphabeticalOrder(["a", "d", "c", "a", "z", "g"]);

ps：补充一个替换字符串的方法

replace替换字符串

替换字符串中的特殊字符（并非全部，若有需要可再添加）

str.replace(/\\|\/|\,|\-|\*|\.|\"|\“|\”|\'|\‘|\’|\<|\>|\{|\}|\[|\]|\【|\】|\：|\:|\、|\^|\$|\!|\~|\`|\|/g,' ')

join 方法将数组组合成字符串

const arr = ["Hello", "World"];
const str = arr.join(" "); // 打印出来是 Hello World

every 方法检查数组

every()方法用于检测数组中所有元素是否都符合指定条件。 如果所有元素满足条件，返回布尔值 true，反之返回 false。

const numbers = [1, 5, 8, 0, 10, 11];

numbers.every(function(currentValue) { // 返回 false
  return currentValue < 10;
});

some 方法检查数组

some()方法用于检测数组中任何元素是否满足指定条件。 如果有一个元素满足条件，返回布尔值 true，反之返回 false。

const numbers = [10, 50, 8, 220, 110, 11];

numbers.some(function(currentValue) { // 返回true
  return currentValue < 10;
});

from()

from() 方法从具有 length 属性或可迭代对象的任何对象返回 Array 对象。

function destroyer(arr) {
    const valsToRemove = Array.from(arguments)
    console.log(valsToRemove);
}

destroyer([1, 2, 3, 1, 2, 3], 4, 3);

查找两个数组的差值

concat合并两个数组，includes查找数组中是否有某个元素，如果有返回true，没有返回false

function diffArray(arr1, arr2) {
    return arr1
    .concat(arr2)
    .filter(item => !arr1.includes(item) || !arr2.includes(item));
}

diffArray([1, 2, 3, 7, 5], [1, 2, 3, 4, 5, 6]);