前端進階JS數組高級用法大全教程示例

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1.批量制造數據

一、創建新數組使用 for 循環批量 push 數據

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、創建空數組，填充full，然後map

function createData() {
  // 如果不 fill 循環默認會跳過空值
  return new Array(1000).fill(null).map((v, i) => ({ name: `name${i + 1}` }));
}
const data = createData();
console.log(data);

三、Array.from 第二個初始化函數返回數據

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

2.數組合並去重

一、Set去重

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

二、for循環，indexOf判斷是否存在

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，反轉操作數的位，表象是對後面數字取負減一
    // 當數組中不存在此項 indexOf 返回 -1 按位非得 0 不走 if 邏輯
    // 如果兩個數組都包含NaN，想要去重可使用includes
    if (~cloneArr1.indexOf(v)) {
      continue;
    }
    cloneArr1.push(v);
  }
  return cloneArr1;
}
console.log(mergeArray(arr1, arr2));

去重對象？

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 } ] 
// 這樣對象都是獨立的引用，肯定無法去除屬性相同的數據啦

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]))); // 確實可以，但是你開發這樣做？

我們可以這樣做

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 屬性值的數據則進入判斷
    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.創建數組的幾種方式

  • 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（當參數隻有一個且是數字時，new Array()表示數組的長度，其餘參數則是數組的內容）
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（參數隻用來作為數組中的內容）
const arr6 = Array.of(5); // [5]
const arr7 = Array.of(1, 'abc', true); // [1, "abc", true]
  • Array.from 可傳入類數組和可遍歷對象轉換為真數組
  • （第一個參數傳入對應類數組和可遍歷對象，第二個函數參數則相當於對生成的數組做一次map）
  • 可遍歷和類數組 ==> 數組、字符串、Set、Map、NodeList、HTMLCollection、arguments以及擁有 length 屬性的任意對象
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數組的方法都算
// 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.類數組

  • 是一個普通對象，不具備數組自帶豐富的內建方法
  • key是以數字或者字符串數字組成
  • 必須有length屬性
const arrayLike = {
  0: "a",
  1: "b",
  2: "c",
  name: "test",
  length: 3,
  push: Array.prototype.push, //自己實現
  splice: Array.prototype.splice,
};
//由於類數組對象length屬性聲明瞭對象有多少個屬性,所以可以使用for遍歷對象屬性:
for (let i = 0; i < arrayLike.length; i++) {
  console.log(i + ":" + arrayLike[i]);
}

常見的類數組

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

打印結果如下：

  • 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（具備類數組的特性，但一般類數組指對象）
const str = "abc";
console.log(Object.keys(str)); // ['0', '1', '2']
console.log(Array.from(str)); // ['a', 'b', 'c']

判斷是否是類數組

function isArrayLikeObject(arr) {
  // 不是對象直接返回
  if (arr == null || typeof arr !== "object") return false;
  const lengthMaxValue = Math.pow(2, 53) - 1;
  //  是否有 length 屬性
  if (!Object.prototype.hasOwnProperty.call(arr, "length")) return false;
  //  length 屬性是否是number類型
  if (typeof arr.length != "number") return false;
  //使用 isFinite() 判斷是否在正常數字范圍
  if (!isFinite(arr.length)) return false;
  // 構造函數等於Array
  if (Array === arr.constructor) return false;
  // 長度有效值
  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

類數組如何轉換為數組

  • 復制遍歷
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 ]
  • 擴展運算符
console.log([...document.body.childNodes]); // [div, script, script...]
// arguments
function argumentsTest() {
  console.log([...arguments]); // [ 1, 2, 3 ]
}
argumentsTest(1, 2, 3);

如何讓類數組使用上數組豐富的內建方法

  • 在類數組對象上直接定義數組原型的方法
  • 運用call或者apply顯示綁定this的指向

例如我想通過 filter 方法過濾出類數組中元素包含 "i" 這個字符的所有元素。

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

為什麼會這樣？其實可以想想 filter 是如何實現的。

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

可以看出因為 filter 實現是通過 this 進行綁定的，哪個數組調用瞭這個filter，filter中的 this 就指向哪個數組

類數組和數組的區別

方法/特征 數組 類數組
自帶方法 多個方法 無
length屬性 有 有
toString返回 [object Array] [object Object]
instanceof Array Object
constructor [Function: Array] [Function: Object]
Array.isArray true false

5.數組方法的使用註意事項

數組的長度

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

數組的空元素 empty

empty:數組的空位，指數組的某一位置沒有任何值，有空位的數組也叫稀疏數組

稀疏數組性能會較差，可以避免創建

Array.apply(null,Array(3))

[…new Array(3)]

Array.from(Array(3))

一般遍歷如forEach、map、reduce 會自動跳過空位

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

基於值進行運算，空位的值作為undefined

  • find,findIndex,includes等, indexOf除外
  • 當被作為迭代的時候，參與Object.entries、擴展運算符、for of 等

join和toString，空位怎麼處理

  • 視為空字符串
  • toString 內部其實會調用 join 方法

數組不會自動添加分號

  • (，[， + , -,/，其作為一行代碼的開頭，很可能產生意外的情況，所以，沒事代碼最後寫個分號，保準沒錯
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)); // 報錯
console.log(a);

indexOf與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)); // 區分空位和undefined，判斷此位上是否有值

數組可變長度問題

  • length 代表數組中元素個數，數組額外附加屬性不計算在內
  • length 可寫，可以通過修改length改變數組的長度
  • 數組操作不存在越界，找不到下標，返回undefined
const array = [1, 2, 3, 4, 5, 6];
array[10] = 10;  // 盡量不要這樣破壞數組默認線性存儲的結構
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

數組查找和過濾

方法 返回結果類型 是否能短路操作 是否需要全部滿足條件 遍歷空元素
some boolean √ × ×
find undefined | object √ × √
findelndex number √ × √
every boolean √ √ ×
filter array × × ×

改變自身的方法

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誤區

  • delete刪除數組元素，後面元素不會補齊，delete刪除引用
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

  • 大量數據操作的時候 push 性能會比 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.數組的高級用法

1.萬能數據生成器

const createValues = (creator, length = 10) => Array.from({ length }, creator);
// 第一個參數控制隨機數生成，第二個控制其數組長度
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);
// 生成數組，裡面元素是 1 ~ 100 以內每次從 1 開始每次遞增 3 的數字
const values = createRange(1, 100, 3);
console.log(values);

3.數據生成器

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.清空數組

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.數組去重

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);

對於數組裡面對象去重

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));

如果我們想認為兩個對象裡面的 a 屬性的值相同就認為是同一數組的話，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.數組交集

  • Array.prototype.filter + includes判斷
  • 但是會存在性能和引用類型相同的判斷的問題
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 ]

我們可以這樣做：

// 引用類型
function intersect(arr1, arr2, key) {
  const map = new Map();
  arr1.forEach((val) => map.set(val[key]));
  return arr2.filter((val) => map.has(val[key]));
}
// 原始數據類型
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 ]

性能比對：

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));
}
// 原始數據類型
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.數組差集

// 引用類型
function difference(arr1, arr2, key) {
  const map = new Map();
  arr1.forEach((val) => map.set(val[key]));
  return arr2.filter((val) => !map.has(val[key]));
}
// 原始數據類型
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.數組刪除虛（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.獲取數組中最大值和最小值

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

來看一個實際的例子，我們去獲取用戶對象中最大和最小的年齡：

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高級用法

querystring

  • 作用∶頁面傳遞參數
  • 規律∶地址url問號(?)拼接的鍵值對

URLSearchParams：

const urlSP = new URLSearchParams(location.search);
function getQueryString(key) {
    return urlSP.get(key);
}
// 獲取頁面上查詢參數 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 為 true，證明是其實例
console.log("words:", getQueryString("words")); 
console.log("wordss:", getQueryString("wordss"));

使用 reduce 手寫查詢：

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

  • 優惠1：9折
  • 優惠2：200減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 + 滿減
print(reduce(discount(250))); // 175

黃金版：

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;
}
// 從後往前執行傳入的函數
const getPrice = compose(discountPlus, reduce, discount);
const print = console.log;
print(getPrice(200));
print(getPrice(250));

打印結果如下圖：

Promise順序執行

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 封裝的 runPromises 方法，確保返回 Promise 且執行結果是下一個函數的入參
runPromises([login, getUserInfo, getOrders], initData).then((res) => {
  console.log("res", res);
});

數組分組

const hasOwn = Object.prototype.hasOwnProperty;
function group(arr, fn) {
  // 不是數組
  if (!Array.isArray(arr)) {
    return arr;
  }
  // 不是函數
  if (typeof fn !== "function") {
    throw new TypeError("fn必須是一個函數");
  }
  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;
  }, {});
}
// 按照長度分組
let result = group(["apple", "pear", "orange", "peach"], (v) => v.length);
console.log(result);
// 按照份數分組
result = group(
  [
    {
      name: "tom",
      score: 60,
    },
    {
      name: "Jim",
      score: 40,
    },
    {
      name: "Nick",
      score: 88,
    },
  ],
  (v) => v.score >= 60
);
console.log(result);

打印結果如下：

7.手寫數組方法

Array.isArray

  • 判斷是否是數組
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

為什麼上面 Array.isArray 判斷代理對象是否數組返回 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); // 報錯

實際 Array.isArray 判斷的是 Proxy裡面的 target 屬性

接下來我們真正手寫下 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));

其實還有很多方法可以判斷其數據類型，比如 constructor、isPrototypeOf等，不過我還是更推薦上面兩種

Array.prototype.entries

  • 作用：返回一個新的 Array Iterator 對象，該對象包含數組中每個索引的鍵/值對
const arr = ["a", "b", "c"];
const iter = arr.entries();
console.log("iter:", iter);
// next函數訪問
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);
}

打印結果如下：

done 表示遍歷是否結束，value 返回當前遍歷的值

自己來實現下這個方法：

Array.prototype.entries = function () {
  // 轉換對象(引用數據類型返回自身)
  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());
// 不能正常執行，因為如果要能 for...of 遍歷需要去實現 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,
      };
    },
  };
};

數組還有 Array.prototype.keys，Array.prototype.keys，如果我們像上面這樣寫等於每個方法裡面都要實現[Symbol.iterator]，我們可以抽離其邏輯，代碼如下：

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

  • 判斷數組是否含有某值，可判斷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

手寫該方法

Number.isNaN = function (param) {
  if (typeof param === "number") {
    return isNaN(param);
  }
  return false;
};
Array.prototype.includes = function (item, fromIndex) {
  // call, apply調用，嚴格模式
  if (this == null) {
    throw new TypeError("無效的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));

其實 includes 還有第二個參數，表示從哪個下標開始檢查，我們也來寫寫該方法

註意參數的情況

  • 轉為整數:TolntegerOrlnfinity
  • +lnfinity , -Infinity
  • 可能為負數
Number.isNaN = function (params) {
  if (typeof params === "number") {
    return isNaN(params);
  }
  return false;
};
// 轉換整數
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) {
  // 嚴格模式
  if (this == null) {
    throw new TypeError("無效的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;
  }
  // 負無窮轉換為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

有三個參數

  • arrayLike：類數組對象或者可遍歷對象（Map、Set）等
  • mapFn：可選參數，在最後生成數組後執行一次map方法後返回
  • thisArg：可選參數，實際是Array.from(obj).map(mapFn, thisArg)

特殊值處理

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]));
// 長度極限問題
// const max = Math.pow(2, 32);
// console.log("Array.from:", Array.from({ 0: 1, 1: 2, length: max - 1 })); // 極限
// console.log("Array.from:", Array.from({ 0: 1, 1: 2, length: max })); // 失敗

執行結果如下：

自己實現一個：

//類數組的特征
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;
  //判斷對象是否為空
  if (arrayLike == null) {
    throw new TypeError("Array.from requires an array-like object - not null or undefined");
  }
  //檢查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);
  //判斷 length 為數字，並且在有效范圍內。
  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 }));

打印結果如下：

Array.prototype.flat

  • 指定的深度遞歸遍歷數組，並將所有元素與遍歷到的子數組中的元素合並為一個新數組返回
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));

執行結果如下：

reduce + 遞歸

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 ]

上面的實現存在幾個弊端：

  • 無法指定躺平深度
  • 性能差的一批（遞歸 + concat）
  • 丟數據（空值reduce無法遍歷）

正規軍入場：

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));

打印結果如下：

8.實戰：數組合並

準備好兩條數據，對 uid 相同的數據進行合並

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),
  };
});

基礎版本：

  • 兩層for循環，通過key關聯
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基礎版：

  • 數組轉換為map對象。數組查找變為屬性查找
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));
// 被合並數據的條數
const len = scoresInfo.length;
// 已合並的條數
let count = 0;
// 已遍歷的次數
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(`合並完畢:遍歷次數${walkCount}, 實際命中次數${count}, 預期命中次數${len}`);
console.log(usersInfo);

數據合並-基礎 hash 跳出-倒敘版

  • 在跳出版的基礎上，一個是從前向後，一個是從後往前
  • 適應場景∶分頁拉取數據，新數組添加在最後，倒敘更快
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(`合並完畢：遍歷次數${walkCount}, 實際命中次數${count}, 預期命中次數${len}`);
console.log(usersInfo);

以上就是前端進階JS數組高級用法教程示例的詳細內容，更多關於前端JS數組進階教程的資料請關註WalkonNet其它相關文章！

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