玩转数组高级技法,成为JS高手

发布于:2023-01-20 ⋅ 阅读:(16) ⋅ 点赞:(0) ⋅ 评论:(0)

在这里插入图片描述

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]);
}

image

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:

image

  • 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);

image

  • 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
length属性
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-<span class="hljs-subst">${index}</span>,
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”);

image

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-<span class="hljs-subst">${index}</span>,
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:

image

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:

image

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

image

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

image

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:

image

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:<span class="hljs-subst">${k}</span>, v:<span class="hljs-subst">${v}</span>);
}

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:<span class="hljs-subst">${k}</span>, v:<span class="hljs-subst">${v}</span>);
}

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:<span class="hljs-subst">${k}</span>);
}

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:<span class="hljs-subst">${k}</span>);
}

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:

image

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© ? 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:

image

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:

image

reduce + 递归

const array = [1, [1, , ,]];

const flat = (arr) => {
return arr.reduce((pre, cur) => {
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:

image

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) => {
  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) => {
return {
uid: <span class="hljs-subst">${index + <span class="hljs-number">1</span>}</span>,
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;

<span class="hljs-keyword">if</span> (count &gt;= len) {
  <span class="hljs-keyword">break</span>;
}

}
}

console.timeEnd(“merge data”);

console.log(合并完毕:遍历次数<span class="hljs-subst">${walkCount}</span>, 实际命中次数<span class="hljs-subst">${count}</span>, 预期命中次数<span class="hljs-subst">${len}</span>);
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;

<span class="hljs-keyword">if</span> (count &gt;= len) {
  <span class="hljs-keyword">break</span>;
}

}
}
console.timeEnd(“merge data”);

console.log(合并完毕:遍历次数<span class="hljs-subst">${walkCount}</span>, 实际命中次数<span class="hljs-subst">${count}</span>, 预期命中次数<span class="hljs-subst">${len}</span>);

console.log(usersInfo);