32个手撕JS，彻底摆脱初级前端（面试高频）

const arr = [1, [2, [3, [4, 5]]], 6];// => [1, 2, 3, 4, 5, 6]

const res1 = arr.flat(Infinity);

const res2 = JSON.stringify(arr).replace(/\[|\]/g, '').split(',');

const res3 = JSON.parse('[' + JSON.stringify(arr).replace(/\[|\]/g, '') + ']');

const flatten = arr => {  return arr.reduce((pre, cur) => {    return pre.concat(Array.isArray(cur) ? flatten(cur) : cur);  }, [])}const res4 = flatten(arr);

const res5 = [];const fn = arr => {  for (let i = 0; i < arr.length; i++) {    if (Array.isArray(arr[i])) {      fn(arr[i]);    } else {      res5.push(arr[i]);    }  }}fn(arr);

const arr = [1, 1, '1', 17, true, true, false, false, 'true', 'a', {}, {}];// => [1, '1', 17, true, false, 'true', 'a', {}, {}]

const res1 = Array.from(new Set(arr));

const unique1 = arr => {  let len = arr.length;  for (let i = 0; i < len; i++) {    for (let j = i + 1; j < len; j++) {      if (arr[i] === arr[j]) {        arr.splice(j, 1);        // 每删除一个树，j--保证j的值经过自加后不变。同时，len--，减少循环次数提升性能        len--;        j--;      }    }  }  return arr;}

const unique2 = arr => {  const res = [];  for (let i = 0; i < arr.length; i++) {    if (res.indexOf(arr[i]) === -1) res.push(arr[i]);  }  return res;}

const unique3 = arr => {  const res = [];  for (let i = 0; i < arr.length; i++) {    if (!res.includes(arr[i])) res.push(arr[i]);  }  return res;}

const unique4 = arr => {  return arr.filter((item, index) => {    return arr.indexOf(item) === index;  });}

const unique5 = arr => {  const map = new Map();  const res = [];  for (let i = 0; i < arr.length; i++) {    if (!map.has(arr[i])) {      map.set(arr[i], true)      res.push(arr[i]);    }  }  return res;}

Array.from(document.querySelectorAll('div'))

Array.prototype.slice.call(document.querySelectorAll('div'))

[...document.querySelectorAll('div')]

Array.prototype.concat.apply([], document.querySelectorAll('div'));

Array.prototype.filter = function(callback, thisArg) {  if (this == undefined) {    throw new TypeError('this is null or not undefined');  }  if (typeof callback !== 'function') {    throw new TypeError(callback + 'is not a function');  }  const res = [];  // 让O成为回调函数的对象传递（强制转换对象）  const O = Object(this);  // >>>0 保证len为number，且为正整数  const len = O.length >>> 0;  for (let i = 0; i < len; i++) {    // 检查i是否在O的属性（会检查原型链）    if (i in O) {      // 回调函数调用传参      if (callback.call(thisArg, O[i], i, O)) {        res.push(O[i]);      }    }  }  return res;}

Array.prototype.map = function(callback, thisArg) {  if (this == undefined) {    throw new TypeError('this is null or not defined');  }  if (typeof callback !== 'function') {    throw new TypeError(callback + ' is not a function');  }  const res = [];  // 同理  const O = Object(this);  const len = O.length >>> 0;  for (let i = 0; i < len; i++) {    if (i in O) {      // 调用回调函数并传入新数组      res[i] = callback.call(thisArg, O[i], i, this);    }  }  return res;}

Array.prototype.forEach = function(callback, thisArg) {  if (this == null) {    throw new TypeError('this is null or not defined');  }  if (typeof callback !== "function") {    throw new TypeError(callback + ' is not a function');  }  const O = Object(this);  const len = O.length >>> 0;  let k = 0;  while (k < len) {    if (k in O) {      callback.call(thisArg, O[k], k, O);    }    k++;  }}

Array.prototype.reduce = function(callback, initialValue) {  if (this == undefined) {    throw new TypeError('this is null or not defined');  }  if (typeof callback !== 'function') {    throw new TypeError(callbackfn + ' is not a function');  }  const O = Object(this);  const len = this.length >>> 0;  let accumulator = initialValue;  let k = 0;  // 如果第二个参数为undefined的情况下  // 则数组的第一个有效值作为累加器的初始值  if (accumulator === undefined) {    while (k < len && !(k in O)) {      k++;    }    // 如果超出数组界限还没有找到累加器的初始值，则TypeError    if (k >= len) {      throw new TypeError('Reduce of empty array with no initial value');    }    accumulator = O[k++];  }  while (k < len) {    if (k in O) {      accumulator = callback.call(undefined, accumulator, O[k], k, O);    }    k++;  }  return accumulator;}

Function.prototype.apply = function(context = window, args) {  if (typeof this !== 'function') {    throw new TypeError('Type Error');  }  const fn = Symbol('fn');  context[fn] = this;
  const res = context[fn](...args);  delete context[fn];  return res;}

Function.prototype.call = function(context = window, ...args) {  if (typeof this !== 'function') {    throw new TypeError('Type Error');  }  const fn = Symbol('fn');  context[fn] = this;
  const res = this[fn](...args);  delete this.fn;  return res;}

Function.prototype.bind = function(context, ...args) {  if (typeof this !== 'function') {    throw new Error("Type Error");  }  // 保存this的值  var self = this;
  return function F() {    // 考虑new的情况    if(this instanceof F) {      return new self(...args, ...arguments)    }    return self.apply(context, [...args, ...arguments])  }}

const debounce = (fn, time) => {  let timeout = null;  return function() {    clearTimeout(timeout)    timeout = setTimeout(() => {      fn.apply(this, arguments);    }, time);  }};

const throttle = (fn, time) => {  let flag = true;  return function() {    if (!flag) return;    flag = false;    setTimeout(() => {      fn.apply(this, arguments);      flag = true;    }, time);  }}

function add() {  const _args = [...arguments];  function fn() {    _args.push(...arguments);    return fn;  }  fn.toString = function() {    return _args.reduce((sum, cur) => sum + cur);  }  return fn;}

function newOperator(ctor, ...args) {  if (typeof ctor !== 'function') {    throw new TypeError('Type Error');  }  const obj = Object.create(ctor.prototype);  const res = ctor.apply(obj, args);
  const isObject = typeof res === 'object' && res !== null;  const isFunction = typeof res === 'function';  return isObject || isFunction ? res : obj;}

const myInstanceof = (left, right) => {  // 基本数据类型都返回false  if (typeof left !== 'object' || left === null) return false;  let proto = Object.getPrototypeOf(left);  while (true) {    if (proto === null) return false;    if (proto === right.prototype) return true;    proto = Object.getPrototypeOf(proto);  }}

function Parent5) {  this.name = 'parent';}function Child() {  Parent.call(this);  this.type = 'children';}Child.prototype = Object.create(Parent.prototype);Child.prototype.constructor = Child;

+0 === -0  // trueNaN === NaN // false

const is= (x, y) => {  if (x === y) {    // +0和-0应该不相等    return x !== 0 || y !== 0 || 1/x === 1/y;  } else {    return x !== x && y !== y;  }}

Object.defineProperty(Object, 'assign', {  value: function(target, ...args) {    if (target == null) {      return new TypeError('Cannot convert undefined or null to object');    }
    // 目标对象需要统一是引用数据类型，若不是会自动转换    const to = Object(target);
    for (let i = 0; i < args.length; i++) {      // 每一个源对象      const nextSource = args[i];      if (nextSource !== null) {        // 使用for...in和hasOwnProperty双重判断，确保只拿到本身的属性、方法（不包含继承的）        for (const nextKey in nextSource) {          if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {            to[nextKey] = nextSource[nextKey];          }        }      }    }    return to;  },  // 不可枚举  enumerable: false,  writable: true,  configurable: true,})

const cloneDeep1 = (target, hash = new WeakMap()) => {  // 对于传入参数处理  if (typeof target !== 'object' || target === null) {    return target;  }  // 哈希表中存在直接返回  if (hash.has(target)) return hash.get(target);
  const cloneTarget = Array.isArray(target) ? [] : {};  hash.set(target, cloneTarget);
  // 针对Symbol属性  const symKeys = Object.getOwnPropertySymbols(target);  if (symKeys.length) {    symKeys.forEach(symKey => {      if (typeof target[symKey] === 'object' && target[symKey] !== null) {        cloneTarget[symKey] = cloneDeep1(target[symKey]);      } else {        cloneTarget[symKey] = target[symKey];      }    })  }
  for (const i in target) {    if (Object.prototype.hasOwnProperty.call(target, i)) {      cloneTarget[i] =        typeof target[i] === 'object' && target[i] !== null        ? cloneDeep1(target[i], hash)        : target[i];    }  }  return cloneTarget;}

const PENDING = 'PENDING';      // 进行中const FULFILLED = 'FULFILLED';  // 已成功const REJECTED = 'REJECTED';    // 已失败
class Promise {  constructor(exector) {    // 初始化状态    this.status = PENDING;    // 将成功、失败结果放在this上，便于then、catch访问    this.value = undefined;    this.reason = undefined;    // 成功态回调函数队列    this.onFulfilledCallbacks = [];    // 失败态回调函数队列    this.onRejectedCallbacks = [];
    const resolve = value => {      // 只有进行中状态才能更改状态      if (this.status === PENDING) {        this.status = FULFILLED;        this.value = value;        // 成功态函数依次执行        this.onFulfilledCallbacks.forEach(fn => fn(this.value));      }    }    const reject = reason => {      // 只有进行中状态才能更改状态      if (this.status === PENDING) {        this.status = REJECTED;        this.reason = reason;        // 失败态函数依次执行        this.onRejectedCallbacks.forEach(fn => fn(this.reason))      }    }    try {      // 立即执行executor      // 把内部的resolve和reject传入executor，用户可调用resolve和reject      exector(resolve, reject);    } catch(e) {      // executor执行出错，将错误内容reject抛出去      reject(e);    }  }  then(onFulfilled, onRejected) {    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;    onRejected = typeof onRejected === 'function'? onRejected:      reason => { throw new Error(reason instanceof Error ? reason.message:reason) }    // 保存this    const self = this;    return new Promise((resolve, reject) => {      if (self.status === PENDING) {        self.onFulfilledCallbacks.push(() => {          // try捕获错误          try {            // 模拟微任务            setTimeout(() => {              const result = onFulfilled(self.value);              // 分两种情况：              // 1. 回调函数返回值是Promise，执行then操作              // 2. 如果不是Promise，调用新Promise的resolve函数              result instanceof Promise ? result.then(resolve, reject) : resolve(result);            })          } catch(e) {            reject(e);          }        });        self.onRejectedCallbacks.push(() => {          // 以下同理          try {            setTimeout(() => {              const result = onRejected(self.reason);              // 不同点：此时是reject              result instanceof Promise ? result.then(resolve, reject) : reject(result);            })          } catch(e) {            reject(e);          }        })      } else if (self.status === FULFILLED) {        try {          setTimeout(() => {            const result = onFulfilled(self.value);            result instanceof Promise ? result.then(resolve, reject) : resolve(result);          });        } catch(e) {          reject(e);        }      } else if (self.status === REJECTED){        try {          setTimeout(() => {            const result = onRejected(self.reason);            result instanceof Promise ? result.then(resolve, reject) : reject(result);          })        } catch(e) {          reject(e);        }      }    });  }  catch(onRejected) {    return this.then(null, onRejected);  }  static resolve(value) {    if (value instanceof Promise) {      // 如果是Promise实例，直接返回      return value;    } else {      // 如果不是Promise实例，返回一个新的Promise对象，状态为FULFILLED      return new Promise((resolve, reject) => resolve(value));    }  }  static reject(reason) {    return new Promise((resolve, reject) => {      reject(reason);    })  }}

Promise.myAll = function(promiseArr) {  return new Promise((resolve, reject) => {    const ans = [];    let index = 0;    for (let i = 0; i < promiseArr.length; i++) {      promiseArr[i]      .then(res => {        ans[i] = res;        index++;        if (index === promiseArr.length) {          resolve(ans);        }      })      .catch(err => reject(err));    }  })}

Promise.race = function(promiseArr) {  return new Promise((resolve, reject) => {    promiseArr.forEach(p => {      // 如果不是Promise实例需要转化为Promise实例      Promise.resolve(p).then(        val => resolve(val),        err => reject(err),      )    })  })}

class Scheduler {  constructor() {    this.queue = [];    this.maxCount = 2;    this.runCounts = 0;  }  add(promiseCreator) {    this.queue.push(promiseCreator);  }  taskStart() {    for (let i = 0; i < this.maxCount; i++) {      this.request();    }  }  request() {    if (!this.queue || !this.queue.length || this.runCounts >= this.maxCount) {      return;    }    this.runCounts++;
    this.queue.shift()().then(() => {      this.runCounts--;      this.request();    });  }}
const timeout = time => new Promise(resolve => {  setTimeout(resolve, time);})
const scheduler = new Scheduler();
const addTask = (time,order) => {  scheduler.add(() => timeout(time).then(()=>console.log(order)))}

addTask(1000, '1');addTask(500, '2');addTask(300, '3');addTask(400, '4');scheduler.taskStart()// 2// 3// 1// 4

const jsonp = ({ url, params, callbackName }) => {  const generateUrl = () => {    let dataSrc = '';    for (let key in params) {      if (Object.prototype.hasOwnProperty.call(params, key)) {        dataSrc += `${key}=${params[key]}&`;      }    }    dataSrc += `callback=${callbackName}`;    return `${url}?${dataSrc}`;  }  return new Promise((resolve, reject) => {    const scriptEle = document.createElement('script');    scriptEle.src = generateUrl();    document.body.appendChild(scriptEle);    window[callbackName] = data => {      resolve(data);      document.removeChild(scriptEle);    }  })}

const getJSON = function(url) {  return new Promise((resolve, reject) => {    const xhr = XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Mscrosoft.XMLHttp');    xhr.open('GET', url, false);    xhr.setRequestHeader('Accept', 'application/json');    xhr.onreadystatechange = function() {      if (xhr.readyState !== 4) return;      if (xhr.status === 200 || xhr.status === 304) {        resolve(xhr.responseText);      } else {        reject(new Error(xhr.responseText));      }    }    xhr.send();  })}

function EventEmitter() {  this.events = new Map();}
// 需要实现的一些方法：// addListener、removeListener、once、removeAllListeners、emit
// 模拟实现addlistener方法const wrapCallback = (fn, once = false) => ({ callback: fn, once });EventEmitter.prototype.addListener = function(type, fn, once = false) {  const hanlder = this.events.get(type);  if (!hanlder) {    // 没有type绑定事件    this.events.set(type, wrapCallback(fn, once));  } else if (hanlder && typeof hanlder.callback === 'function') {    // 目前type事件只有一个回调    this.events.set(type, [hanlder, wrapCallback(fn, once)]);  } else {    // 目前type事件数>=2    hanlder.push(wrapCallback(fn, once));  }}// 模拟实现removeListenerEventEmitter.prototype.removeListener = function(type, listener) {  const hanlder = this.events.get(type);  if (!hanlder) return;  if (!Array.isArray(this.events)) {    if (hanlder.callback === listener.callback) this.events.delete(type);    else return;  }  for (let i = 0; i < hanlder.length; i++) {    const item = hanlder[i];    if (item.callback === listener.callback) {      hanlder.splice(i, 1);      i--;      if (hanlder.length === 1) {        this.events.set(type, hanlder[0]);      }    }  }}// 模拟实现once方法EventEmitter.prototype.once = function(type, listener) {  this.addListener(type, listener, true);}// 模拟实现emit方法EventEmitter.prototype.emit = function(type, ...args) {  const hanlder = this.events.get(type);  if (!hanlder) return;  if (Array.isArray(hanlder)) {    hanlder.forEach(item => {      item.callback.apply(this, args);      if (item.once) {        this.removeListener(type, item);      }    })  } else {    hanlder.callback.apply(this, args);    if (hanlder.once) {      this.events.delete(type);    }  }  return true;}EventEmitter.prototype.removeAllListeners = function(type) {  const hanlder = this.events.get(type);  if (!hanlder) return;  this.events.delete(type);}

function lazyload() {  const imgs = document.getElementsByTagName('img');  const len = imgs.length;  // 视口的高度  const viewHeight = document.documentElement.clientHeight;  // 滚动条高度  const scrollHeight = document.documentElement.scrollTop || document.body.scrollTop;  for (let i = 0; i < len; i++) {    const offsetHeight = imgs[i].offsetTop;    if (offsetHeight < viewHeight + scrollHeight) {      const src = imgs[i].dataset.src;      imgs[i].src = src;    }  }}
// 可以使用节流优化一下window.addEventListener('scroll', lazyload);

window.addEventListener('scroll', function() {  const clientHeight = document.documentElement.clientHeight;  const scrollTop = document.documentElement.scrollTop;  const scrollHeight = document.documentElement.scrollHeight;  if (clientHeight + scrollTop >= scrollHeight) {    // 检测到滚动至页面底部，进行后续操作    // ...  }}, false);

setTimeout(() => {  // 插入十万条数据  const total = 100000;  // 一次插入的数据  const once = 20;  // 插入数据需要的次数  const loopCount = Math.ceil(total / once);  let countOfRender = 0;  const ul = document.querySelector('ul');  // 添加数据的方法  function add() {    const fragment = document.createDocumentFragment();    for(let i = 0; i < once; i++) {      const li = document.createElement('li');      li.innerText = Math.floor(Math.random() * total);      fragment.appendChild(li);    }    ul.appendChild(fragment);    countOfRender += 1;    loop();  }  function loop() {    if(countOfRender < loopCount) {      window.requestAnimationFrame(add);    }  }  loop();}, 0)

const fn = () => {  return [...new Set([...document.querySelectorAll('*')].map(el => el.tagName))].length;}

// vnode结构：// {//   tag,//   attrs,//   children,// }
//Virtual DOM => DOMfunction render(vnode, container) {  container.appendChild(_render(vnode));}function _render(vnode) {  // 如果是数字类型转化为字符串  if (typeof vnode === 'number') {    vnode = String(vnode);  }  // 字符串类型直接就是文本节点  if (typeof vnode === 'string') {    return document.createTextNode(vnode);  }  // 普通DOM  const dom = document.createElement(vnode.tag);  if (vnode.attrs) {    // 遍历属性    Object.keys(vnode.attrs).forEach(key => {      const value = vnode.attrs[key];      dom.setAttribute(key, value);    })  }  // 子数组进行递归操作  vnode.children.forEach(child => render(child, dom));  return dom;}

var a = {    b: 123,    c: '456',    e: '789',}var str=`a{a.b}aa{a.c}aa {a.d}aaaa`;// => 'a123aa456aa {a.d}aaaa'

const fn1 = (str, obj) => {    let res = '';    // 标志位，标志前面是否有{    let flag = false;    let start;    for (let i = 0; i < str.length; i++) {        if (str[i] === '{') {            flag = true;            start = i + 1;            continue;        }        if (!flag) res += str[i];        else {            if (str[i] === '}') {                flag = false;                res += match(str.slice(start, i), obj);            }        }    }    return res;}// 对象匹配操作const match = (str, obj) => {    const keys = str.split('.').slice(1);    let index = 0;    let o = obj;    while (index < keys.length) {        const key = keys[index];        if (!o[key]) {            return `{${str}}`;        } else {            o = o[key];        }        index++;    }    return o;}