let donuts = ["glazed", "chocolate frosted", "Boston creme", "glazed cruller", "cinnamon sugar", "sprinkled"];
donuts.push("powdered");
donuts.length   // 7

let fruits = ["Orange", "Pear"];
fruits.unshift('Apple');
alert( fruits ); // Apple, Orange, Pear

let fruits = ["Apple"];
fruits.push("Orange", "Peach");
fruits.unshift("Pineapple", "Lemon");
alert( fruits );   // ["Pineapple", "Lemon", "Apple", "Orange", "Peach"]

let donuts = ["glazed", "strawberry frosted", "powdered", "Boston creme"];
donuts.pop();   // "Boston creme"

// 删除第二个元素
let arr = ["I", "study", "JavaScript"];
arr.splice(1, 1); // 从索引 1 开始删除 1 个元素
alert( arr ); // ["I", "JavaScript"]
// 删除部分元素并插入新的元素
let donuts = ["cookies", "cinnamon sugar", "creme de leche"];
donuts.splice(-2, 0, "chocolate frosted", "glazed");   // ["cookies", "chocolate frosted", "glazed", "cinnamon sugar", "creme de leche"]
// 向数组的末尾处添加元素
let rainbow = ['Red', 'Orange', 'Yellow', 'Blue'];
rainbow.splice(-1, 1, rainbow[rainbow.length-1], "Purple");   // ['Red', 'Orange', 'Yellow', 'Blue', 'Purple']，相当于方法 arr.push()

let arr = [1, 2];
// create an array from: arr and [3,4], then add values 5 and 6
alert( arr.concat([3, 4], 5, 6) ); // 1,2,3,4,5,6

let arr = [1, 2];
let arrayLike1 = {
  0: "something",
  length: 1
};
alert( arr.concat(arrayLike1) ); // 1,2,[object Object]
let arrayLike2 = {
  0: "something",
  1: "else",
  [Symbol.isConcatSpreadable]: true,
  length: 2
};
alert( arr.concat(arrayLike2) ); // 1,2,something,else

let users = [
  {id: 1, name: "John"},
  {id: 2, name: "Pete"},
  {id: 3, name: "Mary"}
];
let user = users.find(item => item.id == 1);
alert(user.name); // John

let users = [
  {id: 1, name: "John"},
  {id: 2, name: "Pete"},
  {id: 3, name: "Mary"}
];
// 返回前两个用户的数组
let someUsers = users.filter(item => item.id < 3);
alert(someUsers.length); // 2

let donuts = ["jelly donut", "chocolate donut", "glazed donut"];
// 返回的新数组赋予给变量 improvedDonuts
let improvedDonuts = donuts.map(function(donut) {
  donut += " hole";
  donut = donut.toUpperCase();
  return donut;
});   // ["JELLY DONUT HOLE", "CHOCOLATE DONUT HOLE", "GLAZED DONUT HOLE"]

const people = [
  {
    name: '小明',
    money: 500
  },
  {
    name: '漂亮阿姨',
    money: 3000
  },
  {
    name: '杰倫',
    money: 60000
  },
  {
    name: '老媽',
    money: Infinity
  }
];
// every（所有元素处理结果都为 true，最终返回才是 true）
let ans = people.every(function(item, index) {
  return item.money > 1000;
});
console.log(ans);   // false，因为 小明 只有 500
// some（部分元素的处理结果为 true，最终返回就是 true）
let ans = people.some(function (item, index) {
  return item.money > 10000;
});
console.log(ans);   // true

let arr = [ 1, 2, 15 ];
// 该方法重新排列 arr 的内容
arr.sort();
// 默认将元素转换为字符串再进行排序
alert( arr );  // 1, 15, 2

alert( countries.sort( (a, b) => a.localeCompare(b) ) ); // Andorra,Österreich,Vietnam（对的！）

let arr = [ 1, 2, 15 ];
// 显式规定返回值
function compareNumeric(a, b) {
  if (a > b) return 1;
  if (a == b) return 0;
  if (a < b) return -1;
}
arr.sort(compareNumeric);   // [1, 2, 15]
// 优化版
arr.sort(function(a, b) { return a - b; });   // 1, 2, 15
// 箭头函数优化版
arr.sort( (a, b) => a - b );   // 1, 2, 15

let arr = [1, 2, 3, 4, 5];
arr.reverse();
alert( arr ); // 5,4,3,2,1

// 数组元素累加求和
let arr = [1, 2, 3, 4, 5];
let result = arr.reduce((sum, current) => sum + current, 0);
alert(result); // 15
// 按属性将对象分类
let users = [
  {id: 'john', name: "John Smith", age: 20},
  {id: 'ann', name: "Ann Smith", age: 24},
  {id: 'pete', name: "Pete Peterson", age: 31},
];
let usersById = groupById(users);
function groupById(array) {
  return array.reduce((obj, value) => {
    obj[value.id] = value;
    return obj;
  }, {})
}
/*
调用函数后，我们应该得到：
usersById = {
  john: {id: 'john', name: "John Smith", age: 20}
  ann: {id: 'ann', name: "Ann Smith", age: 24},
  pete: {id: 'pete', name: "Pete Peterson", age: 31},
}
*/

const people = [
  {
    name: '小明',
    money: 500
  },
  {
    name: '杰倫',
    money: 60000
  },
  {
    name: '漂亮阿姨',
    money: 3000
  },
];
let newNumber = people.reduce(function(pre, item, index) {
  return Math.max(pre, item.money);
}, 0)
console.log(newNumber);   // 60000

const arr1 = [0, 1, 2, [3, 4]];
arr1.flat(); // [0, 1, 2, 3, 4]
arr1.flat(1); // 与前面的作用一样
const arr2 = [0, 1, 2, [[[3, 4]]]];
arr2.flat(Infinity); // [0, 1, 2, [3, 4]]
const arr3 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

const arr1 = [1, 2, [3], [4, 5], 6, []];
const flattened = arr1.flatMap(num => {
  if(Array.isArray(num)) {
    return num.map(item=> item*2)
  } else {
    return num*2
  }
});
console.log(flattened); // [2, 4, 6, 8, 10, 12]