前端JS数组进阶教程

时间:2022-09-28 云牧人气:0

1.批量制造数据

一、创建新数组使用 for 循环批量 push 数据

function createData() {
  const data = [];
  for (let i = 0; i < 1000; i++) {
    data.push({
      name: `name${i + 1}`,
    });
  }
  return data;
}
const data = createData();
console.log(data);

二、创建空数组，填充full，然后map

function createData() {
  // 如果不 fill 循环默认会跳过空值
  return new Array(1000).fill(null).map((v, i) => ({ name: `name${i + 1}` }));
}
const data = createData();
console.log(data);

三、Array.from 第二个初始化函数返回数据

function createData() {
  return Array.from({ length: 1000 }, (v, i) => ({ name: `name${i + 1}` }));
}
const data = createData();
console.log(data);

2.数组合并去重

一、Set去重

const arr1 = [1, 2, 3];
const arr2 = [3, 4, 5];
console.log(new Set([...arr1, ...arr2]));

二、for循环，indexOf判断是否存在

const arr1 = [1, 2, 3];
const arr2 = [3, 4, 5];
function mergeArray(arr1, arr2) {
  // 克隆
  const cloneArr1 = arr1.slice(0);
  let v;
  for (let i = 0; i < arr2.length; i++) {
    v = arr2[i];
    // 按位非，反转操作数的位，表象是对后面数字取负减一
    // 当数组中不存在此项 indexOf 返回 -1 按位非得 0 不走 if 逻辑
    // 如果两个数组都包含NaN，想要去重可使用includes
    if (~cloneArr1.indexOf(v)) {
      continue;
    }
    cloneArr1.push(v);
  }
  return cloneArr1;
}
console.log(mergeArray(arr1, arr2));

去重对象？

const arr1 = [{ id: 1 }, { id: 2 }, { id: 3 }];
const arr2 = [{ id: 3 }, { id: 4 }, { id: 5 }];
console.log(Array.from(new Set([...arr1, ...arr2]))); 
// [ { id: 1 }, { id: 2 }, { id: 3 }, { id: 3 }, { id: 4 }, { id: 5 } ] 
// 这样对象都是独立的引用，肯定无法去除属性相同的数据啦

如果是相同引用呢？

const obj3 = { id: 3 };
const arr1 = [{ id: 1 }, { id: 2 }, obj3];
const arr2 = [obj3, { id: 4 }, { id: 5 }];
console.log(Array.from(new Set([...arr1, ...arr2]))); // 确实可以，但是你开发这样做？

我们可以这样做

const arr1 = [{ id: 1 }, { id: 2 }, { id: 3 }];
const arr2 = [{ id: 3 }, { id: 4 }, { id: 5 }];
function mergeArray(arr1, arr2) {
  // 克隆
  const cloneArr1 = arr1.slice(0);
  let v;
  for (let i = 0; i < arr2.length; i++) {
    v = arr2[i];
    // 能找到相同 id 属性值的数据则进入判断
    if (~cloneArr1.findIndex((el) => el.id === v.id)) {
      continue;
    }
    cloneArr1.push(v);
  }
  return cloneArr1;
}
console.log(mergeArray(arr1, arr2)); // [ { id: 1 }, { id: 2 }, { id: 3 }, { id: 4 }, { id: 5 } ]

3.创建数组的几种方式

字面量

// 字面量
const arr1 = [1, 2, 3, ...[4, 5, 6]]; // 1,2,3,4,5,6
const arr2 = [, , , , ,]; // [empty × 5]

new Array（当参数只有一个且是数字时，new Array()表示数组的长度，其余参数则是数组的内容）

const arr3 = new Array(5); // [empty × 5]
const arr4 = new Array(1, 2, 3); // 1,2,3
const arr5 = new Array("a"); // ["a"]

Array.of（参数只用来作为数组中的内容）

const arr6 = Array.of(5); // [5]
const arr7 = Array.of(1, 'abc', true); // [1, "abc", true]

Array.from 可传入类数组和可遍历对象转换为真数组
（第一个参数传入对应类数组和可遍历对象，第二个函数参数则相当于对生成的数组做一次map）
可遍历和类数组 ==> 数组、字符串、Set、Map、NodeList、HTMLCollection、arguments以及拥有 length 属性的任意对象

const arr8 = Array.from([1, 2, 3]); // [1,2,3]
const arr9 = Array.from({ length: 3 }, (value, index) => {
  return index + 1;    
}); // [1,2,3]
const arr10 = Array.from({ 0: "a", 1: "b", 2: "c", length: 3 }); // ["a", "b", "c"]

其他的很多可以返回数组的方法都算

// Array.prototype.slice
const arr11 = Array.prototype.slice.call(document.querySelectorAll("div")); // [div, div, div....]
// Array.prototype.concat
const arr12 = Array.prototype.concat.call([], [1, 2, 3]); // [1, 2, 3]

4.类数组

是一个普通对象，不具备数组自带丰富的内建方法
key是以数字或者字符串数字组成
必须有length属性

const arrayLike = {
  0: "a",
  1: "b",
  2: "c",
  name: "test",
  length: 3,
  push: Array.prototype.push, //自己实现
  splice: Array.prototype.splice,
};
//由于类数组对象length属性声明了对象有多少个属性,所以可以使用for遍历对象属性:
for (let i = 0; i < arrayLike.length; i++) {
  console.log(i + ":" + arrayLike[i]);
}

常见的类数组

arguments

function person(name, age, sex) {
  console.log("person arguments:", arguments);
  console.log("person type:", Object.prototype.toString.call(arguments));
}
person("name", "age", "sex");

打印结果如下：

NodeList、HTMLCollection、DOMTokenList等

const nodeList = document.querySelectorAll("box");
console.log("querySelectorAll type:", Object.prototype.toString.call(nodeList));
const htmlCollection = document.getElementsByTagName("div");
console.log("getElementsByTagName type:", Object.prototype.toString.call(htmlCollection)); 
const DOMTokenList = document.querySelector("div").classList;
console.log("classList:", DOMTokenList);

奇特：字符串（具备类数组的特性，但一般类数组指对象）

const str = "abc";
console.log(Object.keys(str)); // ['0', '1', '2']
console.log(Array.from(str)); // ['a', 'b', 'c']

判断是否是类数组

function isArrayLikeObject(arr) {
  // 不是对象直接返回
  if (arr == null || typeof arr !== "object") return false;
  const lengthMaxValue = Math.pow(2, 53) - 1;
  //  是否有 length 属性
  if (!Object.prototype.hasOwnProperty.call(arr, "length")) return false;
  //  length 属性是否是number类型
  if (typeof arr.length != "number") return false;
  //使用 isFinite() 判断是否在正常数字范围
  if (!isFinite(arr.length)) return false;
  // 构造函数等于Array
  if (Array === arr.constructor) return false;
  // 长度有效值
  if (arr.length >= 0 && arr.length < lengthMaxValue) {
    return true;
  } else {
    return false;
  }
}
console.log(isArrayLikeObject(null)); // false
console.log(isArrayLikeObject({ 0: "a", 1: "b", length: 2 })); // true
console.log(isArrayLikeObject({ 0: 1, 2: 3, length: "" })); // false
console.log(isArrayLikeObject({ 0: 1, 2: 3 })); // false
console.log(isArrayLikeObject([1, 2])); // false

类数组如何转换为数组

复制遍历

const arr = [];
const arrayLike = {
  0: 1,
  1: 2,
  length: 2,
};
for (let i = 0; i < arrayLike.length; i++) {
  arr[i] = arrayLike[i];
}
console.log(arr); //  [1, 2]

slice, concat等

const arrayLike = {
  0: 1,
  1: 2,
  length: 2,
};
const array1 = Array.prototype.slice.call(arrayLike);
console.log(array1); // [ 1, 2 ]
const array2 = Array.prototype.concat.apply([], arrayLike);
console.log(array2); // [ 1, 2 ]

Array.from

const arrayLike = {
  0: 1,
  1: 2,
  length: 2,
};
console.log(Array.from(arrayLike)); // [ 1, 2 ]

Array.apply

const arrayLike = {
  0: 1,
  1: 2,
  length: 2,
};
console.log(Array.apply(null, arrayLike)); // [ 1, 2 ]

扩展运算符

console.log([...document.body.childNodes]); // [div, script, script...]
// arguments
function argumentsTest() {
  console.log([...arguments]); // [ 1, 2, 3 ]
}
argumentsTest(1, 2, 3);

如何让类数组使用上数组丰富的内建方法

在类数组对象上直接定义数组原型的方法
运用call或者apply显示绑定this的指向

例如我想通过 filter 方法过滤出类数组中元素包含 "i" 这个字符的所有元素。

const arrayLike = {
  0: "i love",
  1: "you",
  length: 1,
};
console.log([].filter.call(arrayLike, (item) => item.includes("i"))); // [ 'i love' ]

为什么会这样？其实可以想想 filter 是如何实现的。

[].__proto__.myfilter = function (callback) {
  let newArr = [];
  for (let i = 0; i < this.length; i++) {
    if (callback(this[i])) {
      newArr.push(this[i]);
    }
  }
  return newArr;
};

可以看出因为 filter 实现是通过 this 进行绑定的，哪个数组调用了这个filter，filter中的 this 就指向哪个数组

类数组和数组的区别

方法/特征	数组	类数组
自带方法	多个方法	无
length属性	有	有
toString返回	[object Array]	[object Object]
instanceof	Array	Object
constructor	[Function: Array]	[Function: Object]
Array.isArray	true	false

5.数组方法的使用注意事项

数组的长度

const arr1 = [1];
const arr2 = [1, ,];
const arr3 = new Array("10");
const arr4 = new Array(10);
console.log("arr1 length: " + arr1.length); // arr1 length: 1
console.log("arr2 length: " + arr2.length); // arr2 length: 2
console.log("arr3 length: " + arr3.length); // arr3 length: 1
console.log("arr4 length: " + arr4.length); // arr4 length: 10

数组的空元素 empty

empty:数组的空位，指数组的某一位置没有任何值，有空位的数组也叫稀疏数组

稀疏数组性能会较差，可以避免创建

Array.apply(null,Array(3))

[...new Array(3)]

Array.from(Array(3))

一般遍历如forEach、map、reduce 会自动跳过空位

const arr = [1, ,];
arr.forEach((item) => console.log(item)); // 1
console.log("arr", arr);// arr [ 1, <1 empty item> ]

基于值进行运算，空位的值作为undefined

find,findIndex,includes等, indexOf除外
当被作为迭代的时候，参与Object.entries、扩展运算符、for of 等

join和toString，空位怎么处理

视为空字符串
toString 内部其实会调用 join 方法

数组不会自动添加分号

(，[， + , -,/，其作为一行代码的开头，很可能产生意外的情况，所以，没事代码最后写个分号，保准没错

const objA = { a: 1 }
["a"];
console.log(objA); // 1
const objB = ["a"]
["a"];
console.log(objB);  // undefined
const a = [[1, 2], 2, 3];
console.log(a) 
[0, 2, 3].map((v) => console.log(v * v)); // 报错
console.log(a);

indexOf与includes

方法	返回值	是否能查找NaN	[, ,]空位	undefined
indexOf	number	×	×	√
includes	boolean	√	√	√

const array1 = [NaN];
console.log("array.includes NaN:", array1.includes(NaN)); //  true
console.log("array.indexOf NaN:", array1.indexOf(NaN) > -1); // false
const array2 = [1, ,];
console.log("array.includes ,,:", array2.includes(undefined)); // true
console.log("array.indexOf ,,:", array2.indexOf(undefined) > -1); // false
const array3 = [undefined];
console.log("array.includes undefined:", array3.includes(undefined)); // true
console.log("array.indexOf undefined:", array3.indexOf(undefined) > -1);  // true
console.log(Object.prototype.hasOwnProperty.call(array2, 1)); // 区分空位和undefined，判断此位上是否有值

数组可变长度问题

length 代表数组中元素个数，数组额外附加属性不计算在内
length 可写，可以通过修改length改变数组的长度
数组操作不存在越界，找不到下标，返回undefined

const array = [1, 2, 3, 4, 5, 6];
array[10] = 10;  // 尽量不要这样破坏数组默认线性存储的结构
console.log("array.length:", array.length); // 11
array["test"] = "test";
console.log("array.length:", array.length); // 11
array.length = 3;
console.log("array.length:", array.length); // 3
console.log("array value:", array[Number.MAX_VALUE + 1000]); // undefined

数组查找和过滤

方法	返回结果类型	是否能短路操作	是否需要全部满足条件	遍历空元素
some	boolean	√	×	×
find	undefined \| object	√	×	√
findelndex	number	√	×	√
every	boolean	√	√	×
filter	array	×	×	×

改变自身的方法

push、pop、unshift、shift

sort、splice、reverse

ES6: copyWithin、fill

let array = [1, 2, 3, 4, 5, 6, 7];
array.push("push");
console.log("array push:", array);
array.pop();
console.log("array pop:", array);
array.unshift("unshift");
console.log("array unshift:", array);
array.shift();
console.log("array shift:", array);
array.reverse();
console.log("array reverse:", array);
array.sort();
console.log("array sort:", array);
array.splice(2, 1);
console.log("array splice:", array);
array.copyWithin(2, 0);
console.log("array copyWithin:", array);
array.fill("fill", 3);
console.log("array fill:", array);

delete误区

delete删除数组元素，后面元素不会补齐，delete删除引用

const array = [1, 2, 3, 4, 5];
delete array[2];
console.log("delete array:", array); // delete array: [ 1, 2, <1 empty item>, 4, 5 ]

push vs concat

大量数据操作的时候 push 性能会比 concat 性能高很多

const count = 10000;
const array1 = [1, 2, 4, 5, 6];
let newArray = [];
console.time("push");
for (let i = 0; i < count; i++) {
  newArray.push(array1[0], array1[1], array1[2], array1[3], array1[4]);
}
console.timeEnd("push");
console.time("concat");
for (let i = 0; i < count; i++) {
  newArray = newArray.concat(array1[0], array1[1], array1[2], array1[3], array1[4]);
}
console.timeEnd("concat");

6.数组的高级用法

1.万能数据生成器

const createValues = (creator, length = 10) => Array.from({ length }, creator);
// 第一个参数控制随机数生成，第二个控制其数组长度
const createRandomValues = (len) => createValues(Math.random, len);
const values = createRandomValues();
console.log("values:", values.length, values);

2.序列生成器

const createValues = (creator, length = 10) => Array.from({ length }, creator);
const createRange = (start, stop, step) =>
  createValues((_, i) => start + i * step, (stop - start) / step + 1);
// 生成数组，里面元素是 1 ~ 100 以内每次从 1 开始每次递增 3 的数字
const values = createRange(1, 100, 3);
console.log(values);

3.数据生成器

const createValues = (creator, length = 10) => Array.from({ length }, creator);
function createUser(v, index) {
  return {
    name: `user-${index}`,
    age: (Math.random() * 100) >> 0, // 取整
  };
}
const users = createValues(createUser, 100);
console.log("users:", users);

4.清空数组

const arr = [1, 2, 3];
arr.splice(0);
console.log("splice:", arr); // []
const arr1 = [1, 2, 3];
arr1.length = 0;
console.log("length:", arr1); // []

5.数组去重

const arr = [
  "apple",
  "banana",
  1,
  1,
  3,
  3,
  undefined,
  undefined,
  ,
  ,
  NaN,
  NaN,
  null,
  null,
  "true",
  true,
  { a: 1 },
];
const arr1 = Array.from(new Set(arr));  // 正常去重
console.log("set:", arr1);

对于数组里面对象去重

function uniqueArray(arr) {
  return Array.from(new Set(arr));
}
const arr = [{ a: 1 }, { a: 1 }];
console.log("set 不同引用：", uniqueArray(arr));
const obj1 = { a: 1 };
const arr2 = [obj1, obj1];
console.log("set 同一引用：", uniqueArray(arr2));

如果我们想认为两个对象里面的 a 属性的值相同就认为是同一数组的话，可以使用 filter

function uniqueArray(arr = [], key) {
  const keyValues = new Set();
  let val;
  return arr.filter((obj) => {
    val = obj[key];
    if (keyValues.has(val)) {
      return false;
    }
    keyValues.add(val);
    return true;
  });
}
const arr = [{ a: 1 }, { a: 1 }, { a: 2 }];
console.log("filter 去重：", uniqueArray(arr, "a")); // filter 去重： [ { a: 1 }, { a: 2 } ]

6.数组交集

Array.prototype.filter + includes判断
但是会存在性能和引用类型相同的判断的问题

const arr1 = [0, 1, 2];
const arr2 = [3, 2, 0];
function intersectSet(arr1, arr2) {
  return [...new Set(arr1)].filter((item) => arr2.includes(item));
}
const values = intersectSet(arr1, arr2);
console.log(values); // [ 0, 2 ]

我们可以这样做：

// 引用类型
function intersect(arr1, arr2, key) {
  const map = new Map();
  arr1.forEach((val) => map.set(val[key]));
  return arr2.filter((val) => map.has(val[key]));
}
// 原始数据类型
function intersectBase(arr1, arr2) {
  const map = new Map();
  arr1.forEach((val) => map.set(val));
  return arr2.filter((val) => map.has(val));
}
const arr1 = [{ p: 0 }, { p: 1 }, { p: 2 }];
const arr2 = [{ p: 3 }, { p: 2 }, { p: 1 }];
const result = intersect(arr1, arr2, "p");
console.log("result:", result); // result: [ { p: 2 }, { p: 1 } ]
const arr3 = [0, 1, 2];
const arr4 = [3, 2, 0];
const result1 = intersectBase(arr3, arr4);
console.log("result1:", result1); // result1: [ 2, 0 ]

性能比对：

function createData(length) {
  return Array.from({ length }, (val, i) => {
    return ~~(Math.random() * length);
  });
}
function intersectSet(arr1, arr2) {
  return [...new Set(arr1)].filter((item) => arr2.includes(item));
}
// 原始数据类型
function intersectMap(arr1, arr2) {
  const map = new Map();
  arr1.forEach((val) => map.set(val));
  return arr2.filter((val) => {
    return map.has(val);
  });
}
console.time("createData");
const data1 = createData(100000);
const data2 = createData(100000);
console.timeEnd("createData");
console.time("intersectMap");
intersectMap(data1, data2);
console.timeEnd("intersectMap");
console.time("intersectSet");
intersectSet(data1, data2);
console.timeEnd("intersectSet");

7.数组差集

// 引用类型
function difference(arr1, arr2, key) {
  const map = new Map();
  arr1.forEach((val) => map.set(val[key]));
  return arr2.filter((val) => !map.has(val[key]));
}
// 原始数据类型
function differenceBase(arr1, arr2) {
  const map = new Map();
  arr1.forEach((val) => map.set(val));
  return arr2.filter((val) => !map.has(val));
}
const arr1 = [{ p: 0 }, { p: 1 }, { p: 2 }];
const arr2 = [{ p: 3 }, { p: 2 }, { p: 1 }];
const result = difference(arr1, arr2, "p");
console.log("result:", result); // result: [ { p: 3 } ]
const arr3 = [0, 1, 2];
const arr4 = [3, 2, 0];
const result1 = differenceBase(arr3, arr4);
console.log("result1:", result1); // result1: [ 3 ]

8.数组删除虚（假）值

const array = [false, 0, undefined, , "", NaN, 9, true, undefined, null, "test"];
const newArray = array.filter(Boolean);
console.log(newArray); // [ 9, true, 'test' ]

9.获取数组中最大值和最小值

const numArray = [1, 3, 8, 666, 22, 9982, 11, 0];
const max = Math.max.apply(Math, numArray);
const min = Math.min.apply(Math, numArray);
console.log("max:", max + ",min:" + min); // max: 9982,min:0
console.log(Math.max(...numArray)); // 9982
console.log(Math.min(...numArray)); // 0

来看一个实际的例子，我们去获取用户对象中最大和最小的年龄：

const createValues = (creator, length = 10) => Array.from({ length }, creator);
function createUser(v, index) {
  return {
    name: `user-${index}`,
    age: (Math.random() * 100) >> 0,
  };
}
const users = createValues(createUser, 10);
const ages = users.map((u) => u.age);
const max = Math.max.apply(Math, ages);
const min = Math.min.apply(Math, ages);
console.log(ages);
console.log("max:", max + ",min:" + min);

10.reduce高级用法

querystring

作用∶页面传递参数
规律∶地址url问号(?)拼接的键值对

URLSearchParams：

const urlSP = new URLSearchParams(location.search);
function getQueryString(key) {
    return urlSP.get(key);
}
// 获取页面上查询参数 words 和 wordss 的值
console.log("words:", getQueryString("words"));
console.log("wordss:", getQueryString("wordss"));

URL：

const urlObj = new URL(location.href);
function getQueryString(key) {
    return urlObj.searchParams.get(key);
}
// urlObj.searchParams instanceof URLSearchParams 为 true，证明是其实例
console.log("words:", getQueryString("words")); 
console.log("wordss:", getQueryString("wordss"));

使用 reduce 手写查询：

const urlObj = location.search
.slice(1)
.split("&")
.filter(Boolean)
.reduce((obj, cur) => {
    const arr = cur.split("=");
    if (arr.length != 2) {
        return obj;
    }
    obj[decodeURIComponent(arr[0])] = decodeURIComponent(arr[1]);
    return obj;
}, {});
function getQueryString(key) {
    return urlObj[key];
}
console.log("words:", getQueryString("words")); 
console.log("wordss:", getQueryString("wordss"));

折上折

优惠1：9折
优惠2：200减50

草民版：

function discount(x) {
  return x * 0.9;
}
function reduce(x) {
  return x > 200 ? x - 50 : x;
}
const print = console.log;
// 享受九折
print(reduce(discount(100))); // 90
// 享受九折 + 满减
print(reduce(discount(250))); // 175

黄金版：

function discount(x) {
  return x * 0.9;
}
function reduce(x) {
  return x > 200 ? x - 50 : x;
}
function getPriceMethod(discount, reduce) {
  return function _getPrice(x) {
    return reduce(discount(x));
  };
}
const method = getPriceMethod(discount, reduce);
const print = console.log;
print(method(100));
print(method(250));

王者版：

function compose(...funcs) {
  if (funcs.length === 0) {
    return (arg) => arg;
  }
  return funcs.reduce(
    (a, b) =>
      (...args) =>
        a(b(...args))
  );
}
function discount(x) {
  console.log("discount");
  return x * 0.9;
}
function reduce(x) {
  console.log("reduce");
  return x > 200 ? x - 50 : x;
}
function discountPlus(x) {
  console.log("discountPlus");
  return x * 0.95;
}
// 从后往前执行传入的函数
const getPrice = compose(discountPlus, reduce, discount);
const print = console.log;
print(getPrice(200));
print(getPrice(250));

打印结果如下图：

Promise顺序执行

function runPromises(promiseCreators, initData) {
  return promiseCreators.reduce(function (promise, next) {
    return promise.then((data) => next(data));
  }, Promise.resolve(initData));
}
function login(data) {
  console.log("login: data", data);
  return new Promise((resolve) => {
    setTimeout(() => {
      return resolve({
        token: "token",
      });
    }, 500);
  });
}
function getUserInfo(data) {
  console.log("getUserInfo: data", data);
  return new Promise((resolve) => {
    setTimeout(() => {
      return resolve({
        name: "user-1",
        id: 988,
      });
    }, 300);
  });
}
function getOrders(data) {
  console.log("getOrders: data", data);
  return new Promise((resolve) => {
    setTimeout(() => {
      return resolve([
        {
          orderId: 1,
          productId: 100,
          price: 100,
        },
      ]);
    }, 100);
  });
}
const initData = { name: "name", pwd: "pwd" };
Promise.resolve(initData)
  .then((data) => login(data))
  .then((data) => getUserInfo(data))
  .then((data) => getOrders(data))
  .then((data) => console.log("orders", data));
// 使用 reduce 封装的 runPromises 方法，确保返回 Promise 且执行结果是下一个函数的入参
runPromises([login, getUserInfo, getOrders], initData).then((res) => {
  console.log("res", res);
});

数组分组

const hasOwn = Object.prototype.hasOwnProperty;
function group(arr, fn) {
  // 不是数组
  if (!Array.isArray(arr)) {
    return arr;
  }
  // 不是函数
  if (typeof fn !== "function") {
    throw new TypeError("fn必须是一个函数");
  }
  let v;
  return arr.reduce((obj, cur, index) => {
    v = fn(cur, index);
    if (!hasOwn.call(obj, v)) {
      obj[v] = [];
    }
    obj[v].push(cur);
    return obj;
  }, {});
}
// 按照长度分组
let result = group(["apple", "pear", "orange", "peach"], (v) => v.length);
console.log(result);
// 按照份数分组
result = group(
  [
    {
      name: "tom",
      score: 60,
    },
    {
      name: "Jim",
      score: 40,
    },
    {
      name: "Nick",
      score: 88,
    },
  ],
  (v) => v.score >= 60
);
console.log(result);

打印结果如下：

7.手写数组方法

Array.isArray

判断是否是数组

const arr = ["1"];
console.log("isArray:", Array.isArray(arr));

非基本使用：

const arr = ["1"];
const proxy = new Proxy(arr, {});
console.log("isArray:", Array.isArray(proxy)); // true

为什么上面 Array.isArray 判断代理对象是否数组返回 true 呢？

const arr = ["1"];
const proxy = new Proxy(arr, {});
const log = console.log;
log("__proto__:", proxy.__proto__ === Array.prototype); // __proto__: true
log("instanceof:", proxy instanceof Array); // instanceof: true
log("toString", Object.prototype.toString.call(Proxy)); // toString [object Function]
log("Proxy.prototype:", Proxy.prototype); // Proxy.prototype: undefined
log("proxy instanceof Proxy:", proxy instanceof Proxy); // 报错

实际 Array.isArray 判断的是 Proxy里面的 target 属性

接下来我们真正手写下 Array.isArray 的方法

Object.prototype.toString

Array.isArray = function (obj) {
  return Object.prototype.toString.call(obj) === "[object Array]";
};
const arr = ["1"];
const proxy = new Proxy(arr, {});
console.log(Array.isArray(arr));
console.log(Array.isArray(proxy));

instanceof

Array.isArray = function (obj) {
  if (typeof obj !== "object" || obj === null) {
    return false;
  }
  return obj instanceof Array;
};
const arr = ["1"];
const proxy = new Proxy(arr, {});
console.log(Array.isArray(arr));
console.log(Array.isArray(proxy));

其实还有很多方法可以判断其数据类型，比如 constructor、isPrototypeOf等，不过我还是更推荐上面两种

Array.prototype.entries

作用：返回一个新的 Array Iterator 对象，该对象包含数组中每个索引的键/值对

const arr = ["a", "b", "c"];
const iter = arr.entries();
console.log("iter:", iter);
// next函数访问
console.log("iter.next():", iter.next());
console.log("iter.next():", iter.next());
console.log("iter.next():", iter.next());
console.log("iter.next():", iter.next());
// for of迭代
for (let [k, v] of arr.entries()) {
  console.log(k, v);
}

打印结果如下：

done 表示遍历是否结束，value 返回当前遍历的值

自己来实现下这个方法：

Array.prototype.entries = function () {
  // 转换对象(引用数据类型返回自身)
  const O = Object(this);
  let index = 0;
  const length = O.length;
  return {
    next() {
      if (index < length) {
        return { value: [index, O[index++]], done: false };
      }
      return { value: undefined, done: true };
    },
  };
};
const arr = ["a", "b", "c"];
const iter = arr.entries();
console.log("iter.next():", iter.next());
console.log("iter.next():", iter.next());
console.log("iter.next():", iter.next());
// 不能正常执行，因为如果要能 for...of 遍历需要去实现 Symbol.iterator
for (let [k, v] of arr.entries()) {
  console.log(`k:${k}`, `v:${v}`);
}

下面添加 Symbol.iterator 方法返回 next 即可for...of

Array.prototype.entries = function () {
  const O = Object(this);
  let index = 0;
  const length = O.length;
  function next() {
    if (index < length) {
      return { value: [index, O[index++]], done: false };
    }
    return { value: undefined, done: true };
  }
  return {
    next,
    [Symbol.iterator]() {
      return {
        next,
      };
    },
  };
};

数组还有 Array.prototype.keys，Array.prototype.keys，如果我们像上面这样写等于每个方法里面都要实现[Symbol.iterator]，我们可以抽离其逻辑，代码如下：

Array.prototype[Symbol.iterator] = function () {
  const O = Object(this);
  let index = 0;
  const length = O.length;
  function next() {
    if (index < length) {
      return { value: O[index++], done: false };
    }
    return { value: undefined, done: true };
  }
  return {
    next,
  };
};
Array.prototype.entries = function () {
  const O = Object(this);
  const length = O.length;
  let entries = [];
  for (let i = 0; i < length; i++) {
    entries.push([i, O[i]]);
  }
  const itr = this[Symbol.iterator].bind(entries)();
  return {
    next: itr.next,
    [Symbol.iterator]() {
      return itr;
    },
  };
};
Array.prototype.keys = function () {
  const O = Object(this);
  const length = O.length;
  let keys = [];
  for (let i = 0; i < length; i++) {
    keys.push([i]);
  }
  const itr = this[Symbol.iterator].bind(keys)();
  return {
    next: itr.next,
    [Symbol.iterator]() {
      return itr;
    },
  };
};
Array.prototype.values = function () {
  const O = Object(this);
  const length = O.length;
  let keys = [];
  for (let i = 0; i < length; i++) {
    keys.push([O[i]]);
  }
  const itr = this[Symbol.iterator].bind(keys)();
  return {
    next: itr.next,
    [Symbol.iterator]() {
      return itr;
    },
  };
};
const arr = ["a", "b", "c"];
var iter = arr.entries();
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
for (let [k, v] of arr.entries()) {
  console.log(`k:${k}`, `v:${v}`);
}
var iter = arr.keys();
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
for (let k of arr.keys()) {
  console.log(`k:${k}`);
}
var iter = arr.values();
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
console.log("iter.next().value:", iter.next().value);
for (let k of arr.values()) {
  console.log(`k:${k}`);
}

Array.prototype.includes

判断数组是否含有某值，可判断NaN

const arr = [1, 2, 3, { a: 1 }, null, undefined, NaN, ""];
console.log("includes null:", arr.includes(null)); // includes null: true
console.log("indexOf null:", arr.indexOf(null)); // indexOf null: 4
console.log("includes NaN:", arr.includes(NaN)); // includes NaN: true
console.log("indexOf NaN:", arr.indexOf(NaN)); // indexOf NaN: -1

手写该方法

Number.isNaN = function (param) {
  if (typeof param === "number") {
    return isNaN(param);
  }
  return false;
};
Array.prototype.includes = function (item, fromIndex) {
  // call, apply调用，严格模式
  if (this == null) {
    throw new TypeError("无效的this");
  }
  let O = Object(this);
  let len = O.length >> 0;
  if (len <= 0) {
    return false;
  }
  const isNAN = Number.isNaN(item);
  for (let i = 0; i < len; i++) {
    if (O[i] === item) {
      return true;
    } else if (isNAN && Number.isNaN(O[i])) {
      return true;
    }
  }
  return false;
};
const obj = { a: 3 };
const arr = [1, 2, 3, { a: 1 }, null, undefined, NaN, "", 0, obj, obj];
console.log("includes null:", arr.includes(null));
console.log("includes NaN:", arr.includes(NaN));

其实 includes 还有第二个参数，表示从哪个下标开始检查，我们也来写写该方法

注意参数的情况

转为整数:TolntegerOrlnfinity
+lnfinity , -Infinity
可能为负数

Number.isNaN = function (params) {
  if (typeof params === "number") {
    return isNaN(params);
  }
  return false;
};
// 转换整数
function ToIntegerOrInfinity(argument) {
  const num = Number(argument);
  // NaN 和 +0、-0 
  if (Number.isNaN(num) || num == 0) {
    return 0;
  }
  if (num === Infinity || num == -Infinity) {
    return num;
  }
  let inter = Math.floor(Math.abs(num));
  if (num < 0) {
    inter = -inter;
  }
  return inter;
}
Array.prototype.includes = function (item, fromIndex) {
  // 严格模式
  if (this == null) {
    throw new TypeError("无效的this");
  }
  const O = Object(this);
  const len = O.length >> 0;
  if (len <= 0) {
    return false;
  }
  let n = ToIntegerOrInfinity(fromIndex);
  if (fromIndex === undefined) {
    n = 0;
  }
  if (n === +Infinity) {
    return false;
  }
  // 负无穷转换为0
  if (n === -Infinity) {
    n = 0;
  }
  let k = n >= 0 ? n : len + n;
  if (k < 0) {
    k = 0;
  }
  const isNAN = Number.isNaN(item);
  for (let i = k; i < len; i++) {
    if (O[i] === item) {
      return true;
    } else if (isNAN && Number.isNaN(O[i])) {
      return true;
    }
  }
  return false;
};
const arr = ["a", "b", "c"];
console.log("arr include -100->0:", arr.includes("c", -100)); // true
console.log("arr include -100->0:", arr.includes("a", -1)); // false
console.log("arr include 1:", arr.includes("a", -Infinity)); // true
console.log("arr include 1:", arr.includes("a", Infinity)); // false

Array.from

有三个参数

arrayLike：类数组对象或者可遍历对象（Map、Set）等
mapFn：可选参数，在最后生成数组后执行一次map方法后返回
thisArg：可选参数，实际是Array.from(obj).map(mapFn, thisArg)

特殊值处理

console.log("Array.from1:", Array.from({})); 
console.log("Array.from2:", Array.from("")); 
console.log("Array.from3:", Array.from({ a: 1, length: "10" }));
console.log("Array.from4:", Array.from({ a: 1, length: "ss" }));
console.log("Array.from5:", Array.from([NaN, null, undefined, 0]));
// 长度极限问题
// const max = Math.pow(2, 32);
// console.log("Array.from:", Array.from({ 0: 1, 1: 2, length: max - 1 })); // 极限
// console.log("Array.from:", Array.from({ 0: 1, 1: 2, length: max })); // 失败

执行结果如下：

自己实现一个：

//类数组的特征
let maxSafeInteger = Math.pow(2, 32) - 1;
let ToIntegerOrInfinity = function (value) {
  let number = Number(value);
  if (isNaN(number)) {
    return 0;
  }
  if (number === 0 || !isFinite(number)) {
    return number;
  }
  return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
};
let ToLength = function (value) {
  let len = ToIntegerOrInfinity(value);
  return Math.min(Math.max(len, 0), maxSafeInteger);
};
let isCallable = function (fn) {
  return typeof fn === "function" || toStr.call(fn) === "[object Function]";
};
Array.from = function (arrayLike, mapFn, thisArg) {
  let C = this;
  //判断对象是否为空
  if (arrayLike == null) {
    throw new TypeError("Array.from requires an array-like object - not null or undefined");
  }
  //检查mapFn是否是方法
  if (typeof mapFn !== "function" && typeof mapFn !== "undefined") {
    throw new TypeError(mapFn + "is not a function");
  }
  let items = Object(arrayLike);
  //判断 length 为数字，并且在有效范围内。
  let len = ToLength(items.length);
  if (len <= 0) return [];
  let A = isCallable(C) ? Object(new C(len)) : new Array(len);
  for (let i = 0; i < len; i++) {
    let value = items[i];
    if (mapFn) {
      A[i] = typeof thisArg === "undefined" ? mapFn(value, i) : mapFn.call(thisArg, value, i);
    } else {
      A[i] = value;
    }
  }
  return A;
};
console.log("Array.from1:", Array.from({ a: 1, length: "10" }));
console.log("Array.from2:", Array.from({ a: 1, length: "ss" }));
console.log(
  "Array.from3:",
  Array.from({ 0: 1, 1: 2, 4: 5, length: 4 }, (x) => x + x)
);
function MyArray(length) {
  const len = length * 2;
  return new Array(len);
}
function MyObject(length) {
  return {
    length,
  };
}
console.log("Array.from:MyArray", Array.from.call(MyArray, { length: 5 }));
console.log("Array.from:MyObject", Array.from.call(MyObject, { length: 5 }));

打印结果如下：

Array.prototype.flat

指定的深度递归遍历数组，并将所有元素与遍历到的子数组中的元素合并为一个新数组返回

const array = [1, 3, 4, [4, 5], [6, [7, 8]], [, ,], [undefined, null, NaN]];
console.log("flat 1:", array.flat(1));
console.log("flat 2:", array.flat(2));

执行结果如下：

reduce + 递归

const array = [1, [1, , ,]];
const flat = (arr) =&gt; {
  return arr.reduce((pre, cur) =&gt; {
    return pre.concat(Array.isArray(cur) ? flat(cur) : cur);
  }, []);
};
console.log(flat(array)); // [ 1, 1 ]

上面的实现存在几个弊端：

无法指定躺平深度
性能差的一批（递归 + concat）
丢数据（空值reduce无法遍历）

正规军入场：

let has = Object.prototype.hasOwnProperty;
let maxSafeInteger = Math.pow(2, 32) - 1;
let toInteger = function (value) {
  const number = Number(value);
  if (isNaN(number)) {
    return 0;
  }
  if (number === 0 || !isFinite(number)) {
    return number;
  }
  return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
};
let toLength = function (value) {
  let len = toInteger(value);
  return Math.min(Math.max(len, 0), maxSafeInteger);
};
let push = Array.prototype.push;
Array.prototype.flat = function (deep) {
  let O = Object(this);
  let sourceLen = toLength(O.length);
  let depthNum = 1;
  if (deep !== undefined) {
    depthNum = toLength(deep);
  }
  if (depthNum <= 0) {
    return O;
  }
  let arr = [];
  let val;
  for (let i = 0; i < sourceLen; i++) {
    if (has.call(O, i)) {
      val = O[i];
      if (Array.isArray(val)) {
        push.apply(arr, val.flat(depthNum - 1));
      } else {
        arr.push(val);
      }
    } else {
      arr.push(undefined);
    }
  }
  return arr;
};
let array = [1, 3, [4, 5], [6, [7, 8, [9, , 10]]], [, ,], [undefined, null, NaN]];
console.log(array.flat(2));

打印结果如下：

8.实战：数组合并

准备好两条数据，对 uid 相同的数据进行合并

export const usersInfo = Array.from({ length: 200 }, (val, index) =&gt; {
  return {
    uid: `${index + 1}`,
    name: `user-name-${index}`,
    age: index + 10,
    avatar: `http://www.my-avatar.com/${index + 1}`,
  };
});
export const scoresInfo = Array.from({ length: 10 }, (val, index) =&gt; {
  return {
    uid: `${index + 1}`,
    score: ~~(Math.random() * 10000),
    comments: ~~(Math.random() * 10000),
    stars: ~~(Math.random() * 1000),
  };
});

基础版本：

两层for循环，通过key关联

import * as data from "./data.js";
const { usersInfo, scoresInfo } = data;
console.time("merge data");
for (let i = 0; i < usersInfo.length; i++) {
  let user: any = usersInfo[i];
  for (let j = 0; j < scoresInfo.length; j++) {
    let score = scoresInfo[j];
    if (user.uid == score.uid) {
      user.score = score.score;
      user.comments = score.comments;
      user.stars = score.stars;
    }
  }
}
console.timeEnd("merge data");
console.log(usersInfo);

hash基础版：

数组转换为map对象。数组查找变为属性查找

import * as data from "./data.js";
const { usersInfo, scoresInfo } = data;
console.time("merge data");
const scoreMap = scoresInfo.reduce((obj, cur) => {
  obj[cur.uid] = cur;
  return obj;
}, Object.create(null));
for (let i = 0; i < usersInfo.length; i++) {
  const user: any = usersInfo[i];
  const score = scoreMap[user.uid];
  if (score != null) {
    user.score = score.score;
    user.comments = score.comments;
    user.stars = score.stars;
  }
}
console.timeEnd("merge data");
console.log(usersInfo);

hash跳出版：

import * as data from "./data.js";
const { usersInfo, scoresInfo } = data;
console.time("merge data");
const scoreMap = scoresInfo.reduce((obj, cur) => {
  obj[cur.uid] = cur;
  return obj;
}, Object.create(null));
// 被合并数据的条数
const len = scoresInfo.length;
// 已合并的条数
let count = 0;
// 已遍历的次数
let walkCount = 0;
for (let i = 0; i < usersInfo.length; i++) {
  const user: any = usersInfo[i];
  const score = scoreMap[user.uid];
  walkCount++;
  if (score != null) {
    count++;
    user.score = score.score;
    user.comments = score.comments;
    user.stars = score.stars;
    if (count >= len) {
      break;
    }
  }
}
console.timeEnd("merge data");
console.log(`合并完毕:遍历次数${walkCount}, 实际命中次数${count}, 预期命中次数${len}`);
console.log(usersInfo);

数据合并-基础 hash 跳出-倒叙版

在跳出版的基础上，一个是从前向后，一个是从后往前
适应场景∶分页拉取数据，新数组添加在最后，倒叙更快

import * as data from "./data.js";
const { usersInfo, scoresInfo } = data;
console.time("merge data");
const scoreMap = scoresInfo.reduce((obj, cur) => {
  obj[cur.uid] = cur;
  return obj;
}, Object.create(null));
const len = scoresInfo.length;
let count = 0;
let walkCount = 0;
for (let i = usersInfo.length - 1; i >= 0; i--) {
  const user: any = usersInfo[i];
  const score = scoreMap[user.uid];
  walkCount++;
  if (score != null) {
    count++;
    user.score = score.score;
    user.comments = score.comments;
    user.stars = score.stars;
    if (count >= len) {
      break;
    }
  }
}
console.timeEnd("merge data");
console.log(`合并完毕：遍历次数${walkCount}, 实际命中次数${count}, 预期命中次数${len}`);
console.log(usersInfo);

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