「面试题」100+道JS代码实现题

前言

前几个星期在找工作，刷了100多道题目，在这里总结一下

1. 实现curry

const join = (a, b, c) => {
   return `${a}_${b}_${c}`
}

const curriedJoin = curry(join)

curriedJoin(1, 2, 3) // '1_2_3'

curriedJoin(1)(2, 3) // '1_2_3'

curriedJoin(1, 2)(3) // '1_2_3'

// 实现curry
const curry = (fn, ...initParams) => {
  return (...args) => {
    return ((params) => {
      return params.length >= fn.length ? fn(...params) : curry(fn, ...params) 
    })([ ...initParams, ...args])
  }
}

2. 实现支持placeholder的curry()

/**
 * 合并参数（替换占位符）
 */
function merge(params1, params2) {
  for (let i = 0; i < params1.length; i += 1) {
    if (params2.length) {
      if (typeof params1[i] === 'symbol') {
        const first = params2.shift()
        params1[i] = first
      }
    } else {
        break
    }
  }
  return [...params1, ...params2]
}

/**
 * @param { (...args: any[]) => any } fn
 * @returns { (...args: any[]) => any }
 */
function curry(fn, ...initParams) {
  return (...args) => {
    const params = merge([...initParams], [...args])
    return ((params) => {
      params = params.slice(0, fn.length)
      // 判断是否可以执行
      const isCall = params.filter((item) => typeof item !== 'symbol').length >= fn.length
      return isCall ? fn(...params) : curry(fn, ...params) 
    })(params)
  }
}

curry.placeholder = Symbol()

3. 实现Array.prototype.flat()

/**
 * @param { Array } arr
 * @param { number } depth
 * @returns { Array }
 */
function flat(arr, depth = 1) {
  const result = []
  if (depth) {
    let isExistArray = false;
    for (let i = 0; i < arr.length; i += 1) {
      const item = arr[i]
      if (Array.isArray(item)) {
        isExistArray = true
        result.push(...item)
      } else {
        result.push(item)
      }
    }
    if (isExistArray) {
      return flat(result, depth - 1)
    } else {
      return result
    }
  } else {
    return arr
  }
}

4. 手写throttle()

/**
 * @param {Function} func
 * @param {number} wait
 */
function throttle(func, wait) {
  let timer
  let lastArgs
  return function (...param) {
    if (!timer) {
      func.call(this, ...param)
      timer = window.setTimeout(() => {
        lastArgs && func.call(this, ...lastArgs)
        lastArgs = null
        timer = null
      }, wait)
    } else {
      lastArgs = [...param]
    }
  }
}

6. 手写debounce()

/**
 * @param {Function} func
 * @param {number} wait
 */
function debounce(func, wait) {
  let timer
  return function (...param) {
    function clear() {
      window.clearTimeout(timer)
      timer = null
    }
    function run() {
      timer = window.setTimeout(() => {
        func.call(this, ...param)
        clear()
      }, wait)
    }
    if (!timer) {
      run()
    } else {
      clear()
      run()
    }
  }
}

8. 手写shuffle()随机打乱一个数组（实现一个洗牌算法）

/**
  * @param {any[]} arr
  */
function shuffle(arr) {
  for (let i = 0; i < arr.length; i += 1) {
    const j = i + Math.floor(Math.random() * (arr.length - 1));
    [arr[i], arr[j]] = [arr[j], arr[i]]
  }
  return arr
}

9. 解密消息

/**
 * @param {string[][]} message
 * @return {string}
 */
function decode(message) {
  // 为空的判断
  if (!message.length || !message[0].length) {
    return ''
  }

  let power = true
  let str = ''
  let i = 0
  let j = 0
  let direction = 'LowerRight' // LowerRight | UpperRight
  let w = message[0].length
  let h = message.length

  function lowerRight() {
    if (i + 1 < h && j + 1 < w) {
      i = i + 1
      j = j + 1
    } else {
      if (i - 1 > 0 && j + 1 < w) {
        direction = 'UpperRight'
        i = i - 1
        j = j + 1
      } else {
        power = false
      }
    }
  }

  function upperRight() {
    if (i - 1 > 0 && j + 1 < w) {
      i = i - 1
      j = j + 1
    } else {
      if (i + 1 < h && j + 1 < w) {
        direction = 'LowerRight'
        i = i + 1
        j = j + 1
      } else {
        power = false
      }
    }
  }

  while (power) {
    str += message[i][j]
    if (direction === 'LowerRight') {
      lowerRight()
    } else if (direction === 'UpperRight') {
      upperRight()
    }
  }
  return str
}

10. 找出第一个不良版本

/*
 type TypIsBad = (version: number) => boolean
 */

/**
 * @param {TypIsBad} isBad 
 */
function firstBadVersion(isBad) {
  return (version) => {
    let start = 0
    let end = version
    let result = isBad(version) ? version : -1

    while (end >= start) {
      let midd = Math.floor((start + end) / 2)
      if (isBad(midd)) {
        // 如果是坏的
        end = midd - 1
        result = midd
      } else {
        // 如果是好的
        start = midd + 1
      }
    }
    return result
  }
}

11. 什么是Composition?实现pipe()

/**
 * @param {Array<(arg: any) => any>} funcs 
 * @return {(arg: any) => any}
 */
function pipe(funcs) {
    return function (arg) {
        let param = arg
        for (let i = 0; i < funcs.length; i += 1) {
            const func = funcs[i]
            param = func(param)
        }
        return param
    }
}

13. 利用栈(Stack)创建队列(Queue)


class Stack {
  constructor () {
    this.stack = []
  }
  push(element) {
    this.stack.push(element)
  }
  peek() {
    return this.stack[this.stack.length - 1]
  }
  pop() {
    return this.stack.pop()
  }
  size() {
    return this.stack.length
  }
}

// 使用Stack实现Queue
class Queue {
  constructor () {
    this.enqueueStack = new Stack()
    this.dequeueStack = new Stack()
  }

  _enqueueSyncDequeue () {
    const dequeueTemp = new Stack()
    const enqueueTemp = new Stack()
    while (this.enqueueStack.size()) {
      const p = this.enqueueStack.pop()
      dequeueTemp.push(p)
      enqueueTemp.push(p)
    }
    while (enqueueTemp.size()) {
      this.enqueueStack.push(enqueueTemp.pop())
    }
    this.dequeueStack = dequeueTemp
  }

  _dequeueSyncEnqueue () {
    const dequeueTemp = new Stack()
    const enqueueTemp = new Stack()
    while (this.dequeueStack.size()) {
      const p = this.dequeueStack.pop()
      dequeueTemp.push(p)
      enqueueTemp.push(p)
    }
    while (dequeueTemp.size()) {
      this.dequeueStack.push(dequeueTemp.pop())
    }
    this.enqueueStack = enqueueTemp
  }

  enqueue(element) { 
    this.enqueueStack.push(element)
    this._enqueueSyncDequeue()
  }

  peek() { 
    return this.dequeueStack.peek()
  }
  
  dequeue() {
    const p = this.dequeueStack.pop()
    this._dequeueSyncEnqueue()
    return p
  }

  size() { 
    return this.enqueueStack.size()
  }
}

// 改进版
class Queue {
  constructor () {
    this.stack = new Stack()
    this.queue = new Stack()
  }

  enqueue(element) { 
    while (this.queue.size()) {
      this.stack.push(this.queue.pop())
    }
    this.queue.push(element)
    while (this.stack.size()) {
      this.queue.push(this.stack.pop())
    }
  }

  peek() { 
    return this.queue.peek()
  }
  
  dequeue() {
    return this.queue.pop()
  }

  size() { 
    return this.queue.size()
  }
}

14. 实现`memo()`

/**
 * @param {Function} func
 * @param {(args:[]) => string }  [resolver] - cache key generator
 */
function memo(func, resolver) {
  const map = new Map();
  return function (...params) {
    let key
    if (typeof resolver === 'function') {
      key = resolver(...params)
    } else {
      key = [...params].join('-')
    }
    if (map.has(key)) {
      return map.get(key)
    } else {
      const val = func.apply(this, [...params])
      map.set(key, val)
      return val
    }
  }
}

15. 实现类似jQuery的DOM wrapper


/**
 * @param {HTMLElement} el - element to be wrapped
 */
function $(el) {
  el.css = function (key, value) {
    el.style[key] = value
    return el
  }

  return el
}

16. 实现一个Event Emitter

// please complete the implementation
class EventEmitter {
  constructor () {
    this.map = {}
  }

  subscribe(eventName, callback) {
    const event = {eventName, callback}
    const that = this
    if (this.map[eventName]) {
      this.map[eventName].push(event)
    } else {
      this.map[[eventName]] = [event]
    }

    return {
      release () {
        that.map = {
          ...that.map,
          [eventName]: that.map[eventName].filter((e) => e !== event)
        }
      }
    }
  }
  
  emit(eventName, ...args) {
      if (this.map[eventName]) {
      this.map[eventName].forEach((event) => {
        const { callback } = event
        callback(...args)
      })
    }
  }
}

17. 实现一个DOM element store

class NodeStore {
  constructor () {
    this.map = {}
  }

  /**
  * @param {Node} node
  * @param {any} value
  */
  set(node, value) {
    if (!node.__mapKey__) {
      node.__mapKey__ = Symbol()
      this.map[node.__mapKey__] = value
    } else {
      this.map[node.__mapKey__] = value
    }
  }

 /**
  * @param {Node} node
  * @return {any}
  */
  get(node) {
    return this.map[node.__mapKey__]
  }
 
 /**
  * @param {Node} node
  * @return {Boolean}
  */
 has(node) {
   return !!node.__mapKey__ && node.__mapKey__ in this.map
 }
}

18. 优化一个function

优化前的时间复杂度O(m * n)

// items是一个array
// 包含的元素有 >=3 个属性

let items = [
  {color: 'red', type: 'tv', age: 18}, 
  {color: 'silver', type: 'phone', age: 20},
  {color: 'blue', type: 'book', age: 17}
] 

// 一个由key和value组成的array
const excludes = [ 
  {k: 'color', v: 'silver'}, 
  {k: 'type', v: 'tv'}, 
  ...
] 

function excludeItems(items, excludes) { 
  excludes.forEach( pair => { 
    items = items.filter(item => item[pair.k] === item[pair.v])
  })
 
  return items
}

优化后：

function excludeItems(items, excludes) { 
  const excludesMap = {}
  excludes.forEach(({k, v}) => {
    if (excludesMap[k]) {
      excludesMap[k].add(v)
    } else {
      excludesMap[k] = new Set()
      excludesMap[k].add(v)
    }
  })

  return items.filter((item) => {
    return Object.keys(item).every((key) => {
      if (excludesMap[key]) {
        return !excludesMap[key].has(item[key])
      }
      return true
    })
  })
}

19. 相同结构的DOM tree上面寻找对应的节点

/**
 * @param {HTMLElement} rootA
 * @param {HTMLElement} rootB - rootA and rootB are clone of each other
 * @param {HTMLElement} nodeA
 */
const findCorrespondingNode = (rootA, rootB, target) => {
  const paths = []
  let isSearchEnd = false
  let targetB = rootB

  const find = (node, index) => {
    if (index !== undefined && !isSearchEnd) {
      paths.push(index)
    }
    if (node !== target) {
      const children = [...node.children]
      children.forEach((item, index) => {
        find(item, index)
      })
      if (!isSearchEnd) {
        paths.pop()
      }
    } else {
      isSearchEnd = true
    }
  }

  find(rootA)

  if (paths.length !== 0) {
    while (paths.length) {
      const index = paths.shift()
      targetB = [...targetB.children][index]
    }
  }

  return targetB
}

20. 检测 data type

/**
 * @param {any} data
 * @return {string}
 */
function detectType(data) {
  if(data instanceof FileReader) {
    return 'object'
  }
  const type = Object.prototype.toString.call(data);
  return /^\[object\s+([A-Za-z]+)\]$/.exec(type)[1].toLocaleLowerCase()
}

21. 手写JSON.stringify()

/**
 * @param {any} data
 * @return {string}
 */
function stringify(data) {
  if(typeof data === 'bigint') {
    throw new Error()
  } 
  if(typeof data === 'string') {
    return `"${data}"`
  } 
  if(typeof data === 'function') {
    return undefined;
  }
  if(data !== data) {
    return 'null'
  }
  if(data === Infinity) {
    return 'null'
  }
  if(data === -Infinity) {
    return 'null'
  }
  if(typeof data === 'number') {
   return `${data}`
  }
  if(typeof data === 'boolean') {
    return `${data}`
  }
  if(data === null) {
    return 'null'
  }
  if(data === undefined) {
    return 'null'
  }
  if(typeof data === 'symbol') {
    return 'null'
  }
  if(data instanceof Date) {
    return `"${data.toISOString()}"`
  }
  if (Array.isArray(data)) {
    const arr = data.map((item) => stringify(item))
    return `[${arr.join(',')}]`
  }
  if (typeof data === 'object') {
    const arr = []
    Object.entries(data).forEach(([key, value]) => {
      if (value !== undefined) {
        arr.push(`"${key}":${stringify(value)}`)
      }
    })
    return `{${arr.join(',')}}`
  }
}

22. 手写JSON.parse()

/**
 * @param {any} data
 * @return {string}
 */
function parse(data) {

  function equal(text) {
    if (text && text !== current) {
      throw new Error()
    }
    return true
  }

  function next() {
    current = text.charAt(index)
    index += 1
    return current
  }

  function white() {
    while (current && current == ' ') {
      next()
    }
  }

  function object() {
    let key
    let obj = Object.create(null)

    if (current === '{') {
      next()
      white()
      if (current === '}') {
        next()
        return obj
      }

      while (current) {
        key = string()
        // 读取下一个非空的字符
        white()
        // 下一个非空字符应该是":", 否则就抛出错误
        equal(':')
        next()
        obj[key] = value()
        // 读取下一个非空的字符
        white()
        if (current === '}') {
          next()
          return obj
        }
        // 如果不是'}', 下一个字符应该是',', 否则就抛出错误
        equal(',')
        // 读取下一个非空的字符
        white()
      }
    }

    throw new Error()
  }

  function string() {
    let str = ''
    if (current === '"') {
      let startIndex = index
      while (next()) {
        if (current === '"') {
          if (index - 1 > startIndex) {
            str += text.substring(startIndex, index - 1)
          }
          next()
          return str
        }
      }
    } else {
      throw new Error()
    }
  }

  function array() {
    let arr = []
    if (current === '[') {
      next()
      // 读取下一个非空字符串
      white()

      if (current === ']') {
        next()
        return arr
      }

      while (current) {
        // 读取数组的内容
        arr.push(value())
        // 读取下一个字符
        white()
        if (current === ']') {
          next()
          return arr
        }
        equal(',')
        next()
        white()
      }
    }
    throw new Error()
  }

  function number() {
    
    let number = ''
    let string = ''
    if (current === '-') {
      string = ''
      next()
    }

    let temp = Number(current)

    while (temp >= 0 && temp <= 9 && current) {
      string += current
      next()
      temp = Number(current)
    }

    if (current === '.') {
      string += '.'
      next()
      temp = Number(current)
      while (temp >= 0 && temp <= 9 && current) {
        string += current
        next()
        temp = Number(current)
      }
    }

    number = Number(string)
    return number
  }

  function word() {
    switch (current) {
      case 't':
        next()
        equal('r')
        next()
        equal('u')
        next()
        equal('e')
        next()
        return true
      case 'f':
        next()
        equal('a')
        next()
        equal('l')
        next()
        equal('s')
        next()
        equal('e')
        next()
        return false
      case 'n':
        next()
        equal('u')
        next()
        equal('l')
        next()
        equal('l')
        next()
        return null
    }
    throw new Error()
  }

  function value() {
    white()
    switch (current) {
      // 如果是对象
      case '{':
        return object()
      // 如果是数组
      case '[':
        return array()
      // 如果是字符串
      case '"':
        return string()
      // 如果包含负号，说明是数字
      case "-":
        return number()
      default:
        let temp = Number(current)
        if (temp >= 0 && temp <= 9) {
          return number()
        } else {
          return word()
        }
    }
  }

  let text = data + ''
  let index = 0
  let current = ' '
  let result = value()

  white()

  if (current) {
    throw new Error()
  }

  return result
}

23. 实现一个sum()方法

/**
 * @param {number} num
 */
function sum(num) {
  const fn = function (arg) {
    return sum(num + arg)
  }
  fn.toString = function () {
    return num
  }
  return fn
}

24. 用JavaScript手写一个Priority Queue


// complete the implementation
class PriorityQueue {
  /**
   * @param {(a: any, b: any) => -1 | 0 | 1} compare - 
   * compare function, similar to parameter of Array.prototype.sort
   */
  constructor(compare) {
    this.compare = compare;
  }

  /**
   * return {number} amount of items
   */
  size() {

  }

  /**
   * returns the head element
   */
  peek() {

  }

  /**
   * @param {any} element - new element to add
   */
  add(element) {
   
  }

  /**
   * remove the head element
   * @return {any} the head element
   */
  poll() {
    
  }
}

25. 更新数组的顺序

/**
 * @param {any[]} items
 * @param {number[]} newOrder
 * @return {void}
 */
// 不实用额外空间
function sort(items, newOrder) {
  // 使用简单的冒泡排序
  for (let i = 0; i < items.length; i += 1) {
    for (let j = i + 1; j < items.length; j += 1) {
      if (newOrder[i] > newOrder[j]) {
        // 更新items的顺序以及newOrder的顺序
        let otemp = newOrder[j]
        let itemp = items[j]
        newOrder[j] = newOrder[i]
        newOrder[i] = otemp
        items[j] = items[i]
        items[i] = itemp
      }
    }
  }
  return items
}

26. 实现Object.assign()

/**
 * @param {any} target
 * @param {any[]} sources
 * @return {object}
 */
 function objectAssign(target, ...sources) {
  if (target === null || target === undefined) {
    throw new Error()
  } 
  if (typeof target === 'number') {
    target = new Number(target)
  }
  if (typeof target === 'boolean') {
    target = new Boolean(target)
  }
  if (typeof target === 'string') {
    target = new String(target)
  }
  sources.forEach(source => {
    if (source === undefined || source === null) {
      return
    }
    Object.defineProperties(
      target,
      Object.getOwnPropertyDescriptors(source)
    )
  })
  return target
}

27. 实现completeAssign()

function completeAssign(target, ...sources) {
  if (target === null || target === undefined) {
    throw new Error()
  } 
  if (typeof target === 'number') {
    target = new Number(target)
  }
  if (typeof target === 'boolean') {
    target = new Boolean(target)
  }
  if (typeof target === 'string') {
    target = new String(target)
  }
  sources.forEach(source => {
    if (source === undefined || source === null) {
      return
    }
    Object.defineProperties(
      target,
      Object.getOwnPropertyDescriptors(source)
    )
  })
  return target
}

28. 实现clearAllTimeout()

const beforeSetTimeout = window.setTimeout

window.timers = new Set()

// 重写setTimeout
window.setTimeout = function (handler, timeout, ...arg) {
  const that = this
  const timer = beforeSetTimeout(
    function (...arg) {
      handler.apply(that, [...arg])
    },
    timeout,
    ...arg
  )
  window.timers.add(timer)
  return timer
}


/**
 * cancel all timer from window.setTimeout
 */
function clearAllTimeout() {
  window.timers.forEach((timer) => {
    window.clearTimeout(timer)
  })
}

29. 实现async helper - `sequence()`

/*
type Callback = (error: Error, data: any) => void

type AsyncFunc = (
   callback: Callback,
   data: any
) => void
*/

function promisify(func) {
  return function (num) {
    return new Promise(function(resolve, reject) {
      func(function (err, data) {
        if (err) {
          reject(err)
        } else {
          resolve(data)
        }
      }, num)
    })
  }
}

/**
 * @param {AsyncFunc[]} funcs
 * @return {(callback: Callback) => void}
 */
function sequence(funcs) {
  funcs = funcs.map(func => promisify(func))
  return async function (callback, data) {
    try {
      for (let i = 0; i < funcs.length; i += 1) {
        data = await funcs[i](data)
      }
      callback(undefined, data)
    } catch (error) {
      callback(error, undefined)
    }
  }
}

30. 实现async helper - `parallel()`

/*
type Callback = (error: Error, data: any) => void

type AsyncFunc = (
   callback: Callback,
   data: any
) => void

*/

function promisify(func) {
  return function (...args) {
    return new Promise(function(resolve, reject) {
      func(function (err, data) {
        if (err) {
          reject(err)
        } else {
          resolve(data)
        }
      }, ...args)
    })
  }
}

/**
 * @param {AsyncFunc[]} funcs
 * @return {(callback: Callback) => void}
 */
function parallel(funcs){
  funcs = funcs.map(func => promisify(func))
  return function (callback, data) {
    let count = 0
    let handleError = false
    const result = []
    for (let i = 0; i < funcs.length; i += 1) {
      funcs[i](data).then(function (res) {
        result[i] = res
        count += 1
        if (count === funcs.length) {
          callback(undefined, result)
        }
      }).catch(function (err) {
        if (!handleError) {
          callback(err, undefined)
          handleError = true
        }
      })
    }
  }
}

31. 实现async helper - `race()`

/*
type Callback = (error: Error, data: any) => void

type AsyncFunc = (
   callback: Callback,
   data: any
) => void

*/
function promisify(func) {
  return function (...args) {
    return new Promise(function(resolve, reject) {
      func(function (err, data) {
        if (err) {
          reject(err)
        } else {
          resolve(data)
        }
      }, ...args)
    })
  }
}

/**
 * @param {AsyncFunc[]} funcs
 * @return {(callback: Callback) => void}
 */
function race(funcs){
  funcs = funcs.map(func => promisify(func))
  return function (callback, data) {
    let handleEnd = false
    for (let i = 0; i < funcs.length; i += 1) {
      funcs[i](data).then(function (res) {
        if (!handleEnd) {
          callback(undefined, res)
        }
        handleEnd = true
      }).catch(function (err) {
        if (!handleEnd) {
          callback(err, undefined)
        }
        handleEnd = true
      })
    }
  }
}

32. 实现`Promise.all()`

/**
 * @param {Array<any>} promises - notice input might have non-Promises
 * @return {Promise<any[]>}
 */
function all(promises) {
  return new Promise((resolve, reject) => {
    if (!promises.length) {
      return resolve([])
    }
    const result = [];
    let count = 0;
    for (let i = 0; i < promises.length; i++) {
      const promise = promises[i] instanceof Promise ? promises[i] : Promise.resolve(promises[i])
      promise.then((res) => {
        result[i] = res
        count += 1
        if (count === promises.length) {
          resolve(result)
        }
      }).catch((err) => {
        reject(err)
      })
    }
  })
}

33. 实现`Promise.allSettled()`

/**
 * @param {Array<any>} promises - notice that input might contains non-promises
 * @return {Promise<Array<{status: 'fulfilled', value: any} | {status: 'rejected', reason: any}>>}
 */
function allSettled(promises) {
  return new Promise((resolve, reject) => {
    if (!promises.length) {
      return resolve([])
    }
    const result = [];
    let count = 0;
    for (let i = 0; i < promises.length; i++) {
      const promise = promises[i] instanceof Promise ?
        promises[i] :
        Promise.resolve(promises[i])
      promise.then((res) => {
        result[i] = {status:"fulfilled",value: res}
        count += 1
        if (count === promises.length) {
          resolve(result)
        }
      }).catch((err) => {
        result[i] = {status:"rejected",reason: err}
        count += 1
        if (count === promises.length) {
          resolve(result)
        }
      })
    }
  })
}

34. 实现`Promise.any()`

/**
 * @param {Array<Promise>} promises
 * @return {Promise}
 */
function any(promises) {
  return new Promise((resolve, reject) => {
    if (!promises.length) {
      return reject(
        new AggregateError(
          'No Promise in Promise.any was resolved', 
          []
        )
      )
    }
    const errors = [];
    let count = 0;
    for (let i = 0; i < promises.length; i += 1) {
      const promise = promises[i] instanceof Promise ? promises[i] : Promise.resolve(promises[i])
      promise.then((res) => {
        resolve(res)
      }).catch((err) => {
        errors[i] = err
        count += 1
        if (count === promises.length) {
          reject(
            new AggregateError(
              'No Promise in Promise.any was resolved', 
              errors
            )
          )
        }
      })
    }
  })
}

35. 实现`Promise.race()`

/**
 * @param {Array<Promise>} promises
 * @return {Promise}
 */
function race(promises) {
  return new Promise((resolve, reject) => {
    if (promises.length) {
      for (let i = 0; i < promises.length; i += 1) {
        const promise = promises[i] instanceof Promise ?
          promises[i]
          :
          Promise.resolve(promises[i])
        promise.then((res) => {
          resolve(res)
        }).catch((err) => {
          reject(err)
        })
      }
    }
  })
}

37. 手写Binary Search (unique)

/**
 * @param {number[]} arr - ascending unique array
 * @param {number} target
 * @return {number}
 */
function binarySearch(arr, target){
  if (arr.length === 0) return -1
  
  const _binarySearch = (arr, base) => {
    if (arr.length) {
      const midd = Math.floor(arr.length / 2)
      if (arr[midd] === target) {
        return midd + base
      } else if (arr[midd] > target) {
        return _binarySearch(arr.splice(midd + 1), midd)
      } else {
        return _binarySearch(arr.splice(0, midd), base)
      }
    } else {
      return -1
    }
  }

  return _binarySearch([...arr], 0)
}

38. 实现`jest.spyOn()`

/**
 * @param {object} obj
 * @param {string} methodName
 */
function spyOn(obj, methodName) {
  if (!(obj[methodName] instanceof Function)) {
    throw new Error()
  }

  const before = obj[methodName]
  const calls = []

  obj[methodName] = function (...args) {
    calls.push([...args])
    before.call(this, ...args)
  }

  return {
    calls
  }
}

39. 手写range()

/**
 * @param {integer} from
 * @param {integer} to
 */
function range(from, to) {
  const result = []
  while (from <= to) {
    result.push(from)
    from += 1
  }
  return result
}

40. 手写冒泡排序

/**
 * @param {number[]} arr
 */
 function bubbleSort(arr) {
  for (let i = 0; i < arr.length; i += 1) {
    for (let j = i + 1; j < arr.length; j += 1) {
      if (arr[i] > arr[j]) {
        [arr[i], arr[j]] = [arr[j], arr[i]]
      }
    }
  }
  return arr
}

41. 手写归并排序

// Note: 需要直接在原数组上修改
function merge (left, right) {
  const arr = []
  while (left.length && right.length) {
    if (left[0] > right[0]) {
      arr.push(right.shift())
    } else {
      arr.push(left.shift())
    }
  }
  return [...arr, ...left, ...right]
}

/**
 * @param {number[]} arr
 */
function mergeSort(arr) {
  if (arr.length <= 1) {
    return arr
  }

  const midd = Math.floor(arr.length / 2)
  const left = arr.splice(0, midd)

  return merge(mergeSort(left), mergeSort(arr))
}

// 原地修改
// 递归最外层的right就是原数组
function merge (left, right) {
  const arr = []
  while (left.length && right.length) {
    if (left[0] > right[0]) {
      arr.push(right.shift())
    } else {
      arr.push(left.shift())
    }
  }

  while (arr.length) {
    right.unshift(arr.pop())
  }

  if (left.length) {
    right.push(left.pop())
  }

  return right
}

/**
 * @param {number[]} arr
 */
function mergeSort(arr) {
  if (arr.length <= 1) {
    return arr
  }

  const midd = Math.floor(arr.length / 2)
  const left = arr.splice(0, midd)

  return merge(mergeSort(left), mergeSort(arr))
}

42. 手写插入排序

/**
 * @param {number[]} arr
 */
function insertionSort(arr) {
  for (let i = 0; i < arr.length; i += 1) {
    for (let j = i + 1; j < arr.length; j += 1) {
      if (arr[i] > arr[j]) {
        const item = arr.splice(j,1)[0]
        arr.splice(i,0,item)
      }
    }
  }

  return arr
}

43. 手写快速排序

// Note: 需要直接在原数组上修改
/**
 * @param {number[]} arr
 */
function quickSort(arr) {
  if (arr.length <= 1) {
    return arr
  }

  const referenceValue = arr[0]
  const max = []
  const min = []

  for (let i = 1; i < arr.length; i += 1) {
    if (arr[i] >= referenceValue) {
      max.push(arr[i])
    } else {
      min.push(arr[i])
    }
  }

  return quickSort(min).concat(referenceValue, quickSort(max))
}

// 原地修改
/**
 * @param {number[]} arr
 */
function quickSort(arr) {
  if (arr.length <= 1) {
    return arr
  }

  const referenceValue = arr[0]
  let max = []
  let min = []

  for (let i = 1; i < arr.length; i += 1) {
    if (arr[i] >= referenceValue) {
      max.push(...arr.splice(i,1))
    } else {
      min.push(...arr.splice(i,1))
    }
    i -= 1
  }

  min = quickSort(min)
  max = quickSort(max)

  while (max.length) {
    arr.push(max.shift())
  }

  while (min.length) {
    arr.unshift(min.pop())
  }

  return arr
}

44. 手写选择排序

/**
 * @param {number[]} arr
 */
function selectionSort(arr) {
  for (let i = 0; i < arr.length; i += 1) {
    let minIndex = i
    for (let j = i + 1; j < arr.length; j += 1) {
      if (arr[j] < arr[minIndex]) {
        minIndex = j
      }
    }
    const min = arr.splice(minIndex, 1)[0]
    arr.splice(i, 0, min)
  }
  return arr
}

45. 在未排序的数组中找到第K大的元素

// 分治

/**
 * @param {number[]} arr
 * @param {number} k
 */
function findKThLargest(arr, k) {
  let result

  const divideAndConquer = (arr, base) => {
    if (arr.length <= 1) {
      result = arr[0]
    }
    const min = []
    const max = []
    let maxLen
    const referenceValue = arr[0]
    for (let i = 1; i < arr.length; i += 1) {
      if (arr[i] > referenceValue) {
        max.push(arr[i])
      } else {
        min.push(arr[i])
      }
    }
    max.push(arr[0])
    maxLen = max.length + base
    if (maxLen >= k && max.length) {
      divideAndConquer(max, base)
    } else if (maxLen < k && min.length) {
      divideAndConquer(min, maxLen)
    }
  }

  divideAndConquer(arr, 0)

  return result
}

46. 实现`_.once()`

/**
 * @param {Function} func
 * @return {Function}
 */
function once(func) {
  let result
  let once = true
  return function (...args) {
    if (once) {
      result = func.call(this, ...args)
      once = false
    }
    return result
  }
}

47. 反转链表

/** 
 * class Node {
 *  new(val: number, next: Node);
 *    val: number
 *    next: Node
 * }
 */

/**
 * @param {Node} list
 * @return {Node} 
 */
const reverseLinkedList = (list) => {
    if (!list) return
    const initNode = list
    let newHead = initNode
    let newHeadNextNode = initNode

    while (initNode.next) {
        newHead = initNode.next
        if (newHead.next) {
            initNode.next = newHead.next
        } else {
            initNode.next = null
        }
        newHead.next = newHeadNextNode
        newHeadNextNode = newHead
    }

    return newHead
}

48. 含有重复元素的数组中返回特定元素的首次出现的位置

/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function firstIndex(arr, target){
  if (arr.length === 0) return -1
  let index = -1

  const _firstIndex = (start, end) => {
    if (start <= end) {
      const midd = Math.floor((start + end) / 2)
      if (arr[midd] === target) {
        if (index === -1) {
          index = midd
        } else {
          index = Math.min(index, midd)
        }
        _firstIndex(start, midd - 1)
      } else if (arr[midd] > target) {
        _firstIndex(start, midd - 1)
      } else {
        _firstIndex(midd + 1, end)
      }
    }
  }

  _firstIndex(0, arr.length - 1)

  return index
}

49. 含有重复元素的数组中返回特定元素的最后出现的位置


/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function lastIndex(arr, target){
  if (arr.length === 0) return -1
  let index = -1

  const _lastIndex = (start, end) => {
    if (start <= end) {
      const midd = Math.floor((start + end) / 2)
      if (arr[midd] === target) {
        if (index === -1) {
          index = midd
        } else {
          index = Math.max(index, midd)
        }
        _lastIndex(midd + 1, end)
      } else if (arr[midd] > target) {
        _lastIndex(start, midd - 1)
      } else {
        _lastIndex(midd + 1, end)
      }
    }
  }

  _lastIndex(0, arr.length - 1)

  return index
}

50. 含有重复元素的数组中返回特定元素之前的元素

/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function firstIndex(arr, target){
  if (arr.length === 0) return -1
  let index = -1

  const _firstIndex = (start, end) => {
    if (start <= end) {
      const midd = Math.floor((start + end) / 2)
      if (arr[midd] === target) {
        if (index === -1) {
          index = midd
        } else {
          index = Math.min(index, midd)
        }
        _firstIndex(start, midd - 1)
      } else if (arr[midd] > target) {
        _firstIndex(start, midd - 1)
      } else {
        _firstIndex(midd + 1, end)
      }
    }
  }

  _firstIndex(0, arr.length - 1)

  return index
}

/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function elementBefore(arr, target){
  if (arr.length === 0) return undefined

  const _firstIndex = firstIndex(arr, target)

  if (_firstIndex === 0 || _firstIndex === -1) return undefined

  return arr[_firstIndex - 1]
}

51. 含有重复元素的数组中返回特定元素的之后的元素

/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function lastIndex(arr, target){
  if (arr.length === 0) return -1
  let index = -1

  const _lastIndex = (start, end) => {
    if (start <= end) {
      const midd = Math.floor((start + end) / 2)
      if (arr[midd] === target) {
        if (index === -1) {
          index = midd
        } else {
          index = Math.max(index, midd)
        }
        _lastIndex(midd + 1, end)
      } else if (arr[midd] > target) {
        _lastIndex(start, midd - 1)
      } else {
        _lastIndex(midd + 1, end)
      }
    }
  }

  _lastIndex(0, arr.length - 1)

  return index
}


/**
 * @param {number[]} arr - ascending array with duplicates
 * @param {number} target
 * @return {number}
 */
function elementAfter(arr, target){
  if (arr.length === 0) return undefined

  const _lastIndex = lastIndex(arr, target)

  if (_lastIndex === arr.length - 1 || _lastIndex === -1) return undefined

  return arr[_lastIndex + 1]
}

53. 实现middleware

class Middleware {

  constructor () {
    this.callbacks = []
    this.handleErrors = []
    this.handleNextError = null
  }

  // 洋葱模型
  _compose(funcs, type) {
    if (funcs.length === 0) {
      return arg => arg 
    }
    if (funcs.length === 1) {
      return funcs[0]
    }
    if (type === 'callback') {
      return funcs.reduce((a, b) => {
        return (req, next) => {
          return a(req, (err) => {
            if (err) {
              this.handleNextError(err)
            } else {
              b(req, next)
            }
          })
        }
      })
    } else if (type === 'handleError') {
      return funcs.reduce((a, b) => {
        return (err, req, next) => {
          return a(err, req, (err) => {
            b(err, req, next)
          })
        }
      })
    }
  }

  /**
   * @param {MiddlewareFunc} func
   */
  use(func) {
    if (func.length === 2) {
      this.callbacks.push(func)
    } else {
      this.handleErrors.push(func)
    }
  }

  /**
   * @param {Request} req
   */
  start(req) {
    const callback = this._compose(this.callbacks, 'callback')
    const handleError = this._compose(this.handleErrors, 'handleError')
    this.handleNextError = (err) => {
      handleError(err, req, () => {})
    }
    try {
      callback(req, (err) => {
        if (err) {
          this.handleNextError(err)
        }
      })
    } catch (err) {
      handleError(err, req, () => {})
    }
  }
}

const middleware = new Middleware()

middleware.use((req, next) => {
   console.log(1)
   next()
   console.log(2)
})

// since error occurs, this is skipped
middleware.use((req, next) => {
  console.log(3)
  next(new Error())
  console.log(4)
})


// since error occurs, this is called
middleware.use((error, req, next) => {
   console.log(error)
   console.log(req)
})

middleware.start({})

class Middleware {

  constructor () {
    this.callbacks = []
    this.handleErrors = []
  }

  /**
   * @param {MiddlewareFunc} func
   */
  use(func) {
    if (func.length === 2) {
      this.callbacks.push(func)
    } else {
      this.handleErrors.push(func)
    }
  }

  /**
   * @param {Request} req
   */
  start(req) {
    let callbackIndex = 0
    let handleErrorIndex = 0
    const next = (error) => {
      const args = [req, next]
      let func
      if (error) {
        func = this.handleErrors[handleErrorIndex]
        args.unshift(error)
        handleErrorIndex += 1
      } else {
        func = this.callbacks[callbackIndex]
        callbackIndex += 1
      }

      try {
        func && func(...args)
      } catch (err) {
        next(err)
      }
    }

    next()
  }
}

🐜蚂蚁金服面试真题: 实现compose

function f(next) {console.log(1);next();console.log(2);}
function g(next) {console.log(3);next();console.log(4);}
function h(next) {console.log(5);next();console.log(6);}


// ((next) => {
//   ((next) => {
//     return f(() => {
//       g(next)
//     })
//   })(() => { h(next) })
// })(() => {
//   console.log('ok')
// })

// 最终需要实现的目标
// f(() => {
//   g(() => {
//     h(() => {
//     })
//   })
// })

// 实现compose
function compose(...funcs) {
  r
  return function () {
    return funcs.reduce((a,b) => {
      return (next) => {
        return a(() => {
          b(next)
        })
      }
    })(() => {})
  }
}

// 1,3,5,6,4,2
// 第一次返回(next) => f(() => g(next)）
// 第二次a: (next) => f(() => g(next)） b: h
// 第二次返回 (next) => （(next) => f(() => g(next))(() => h(next))
// (next) =>f(() => g(() => h(next)))
compose(f,g,h)()

53. 用es5实现`extends`

const myExtends = (SuperType, SubType) => {
  function Child (...args) {
    SuperType.call(this, ...args)
    SubType.call(this, ...args)
    // 这里Child其实本质还是SubType，还是这里要修改下
    Object.setPrototypeOf(this, SubType.prototype)
  }

  SubType.prototype = Object.create(SuperType.prototype)
  Child.prototype = Object.create(SubType.prototype)

  SubType.prototype.constructor = SubType
  Child.prototype.constructor = Child

  // 模拟es6额外的继承链
  Object.setPrototypeOf(Child, SuperType)

  return Child
}

55. HTML字符串中高亮关键字

// 获取所有关键词的排列组合
/**
 * @param {string} html
 * @param {string[]} keywords
 */
function highlightKeywords(html, keywords) {
  const combination = []
  let arr = html.split(' ')

  const getCombination = (head, arr) => {
    for (let i = 0; i < arr.length; i += 1) {
      const temp = [...arr]
      temp.splice(i, 1)
      const name = `${head}${arr[i]}`
      combination.push(name)
      getCombination(name, temp)
    }
  }

  getCombination('', keywords)

  arr = arr.map((item) => {
    if (combination.includes(item)) {
      return `<em>${item}</em>`
    } else if (keywords.some((keyword) => item.includes(keyword))) {
      for (let i = 0; i < keywords.length; i += 1) {
        const keyword = keywords[i]
        if (item.includes(keyword)) {
          const reg = new RegExp(keyword, 'g')
          item = item.replace(reg, `<em>${keyword}</em>`)
          break
        }
      }
      return item
    } else {
      return item
    }
  })

  return arr.join(' ')
};

// 单纯的使用正则

58. 返回DOM tree的高度


/**
 * @param {HTMLElement | null} tree
 * @return {number}
 */
function getHeight(tree) {
  if (!tree) return 0
  const result = []

  const bfs = (nodes) => {
    if (nodes.length) {
      let childs = []
      for (let i = 0; i < nodes.length; i += 1) {
        const children = nodes[i].children
        childs = [...childs, ...children]
      }
      result.push(childs)
      bfs(childs)
    }
  }

  bfs([tree])

  return result.length       
}

59. 实现browser history

class BrowserHistory {
  constructor(url) {
    this.history = [url || undefined]
    this.index = 0
  }

  visit(url) {
    if (this.index === this.history.length - 1) {
      this.history.push(url)
    } else {
      this.history = this.history.splice(0, this.index + 1)
      this.history.push(url)
    }
    this.index = this.history.length - 1
  }
  
  get current() {
    return this.history[this.index]
  }
  
  goBack() {
    this.index -= 1
    this.index = this.index < 0 ? 0 : this.index
    return this.history[this.index]
  }
  
  forward() {
    this.index += 1
    this.index = this.index > this.history.length - 1 ? this.history.length - 1 : this.index
    return this.history[this.index]
  }
}

60. 实现自己的`new`

/**
 * @param {Function} constructor
 * @param {any[]} args - argument passed to the constructor
 * `myNew(constructor, ...args)` should return the same as `new constructor(...args)`
 */
const myNew = (constructor, ...args) => {
  const obj = Object.create({})
  const returnValue = constructor.call(obj, ...args)
  Object.setPrototypeOf(obj, constructor.prototype)
  return returnValue || obj
}

61. 实现`Function.prototype.call`

Function.prototype.mycall = function(thisArg, ...args) {
  if (thisArg === undefined || thisArg === null) {
    thisArg = window
  }
  if (typeof thisArg === 'string') {
    thisArg = new String(thisArg)
  }
  if (typeof thisArg === 'number') {
    thisArg = new Number(thisArg)
  }
  if (typeof thisArg === 'boolean') {
    thisArg = new Boolean(thisArg)
  }
  const key = Symbol()
  thisArg[key] = this
  const result = thisArg[key](...args)
  delete thisArg[key]
  return result
}

手写apply

Function.prototype.myapply = function (thisArg, argsArray = []) {
  if (!Array.isArray(argsArray)) {
    throw new Error()
  }
  if (thisArg === undefined || thisArg === null) {
    thisArg = window
  }
  if (typeof thisArg === 'string') {
    thisArg = new String(thisArg)
  }
  if (typeof thisArg === 'number') {
    thisArg = new Number(thisArg)
  }
  if (typeof thisArg === 'boolean') {
    thisArg = new Boolean(thisArg)
  }
  const key = Symbol()
  thisArg[key] = this
  const result = thisArg[key](...argsArray)
  delete thisArg[key]
  return result
}

手写bind

Function.prototype.mybind = function (thisArg, ...initArgs) {
  if (thisArg === undefined || thisArg === null) {
    thisArg = window
  }
  if (typeof thisArg === 'string') {
    thisArg = new String(thisArg)
  }
  if (typeof thisArg === 'number') {
    thisArg = new Number(thisArg)
  }
  if (typeof thisArg === 'boolean') {
    thisArg = new Boolean(thisArg)
  }
  const that = this
  return function (...args) {
    const key = Symbol()
    thisArg[key] = that
    const result = thisArg[key](...initArgs, ...args)
    delete thisArg[key]
    return result
  }
}

63. 手写`_.cloneDeep()`

// 规避循环引用 Avoid circular references
const hash = new WeakMap()

function isObject(value) {
  return value != null && (typeof value === "object" || typeof value === "function")
}

function getSymbolKeys(value) {
  let keys = Object.getOwnPropertySymbols(value)
  keys = keys.filter((key) => value.propertyIsEnumerable(key))
  return keys
}

function getAllKeys(value) {
  let keys = Object.keys(value)
  keys = [...keys, ...getSymbolKeys(value)]
  return keys
}

function cloneDeep(data) {
  let result = null

  if (!isObject(data)) {
    return data
  }

  const isArray = Array.isArray(data)

  if (isArray) {
    result = []
  } else {
    result = Object.create(Object.getPrototypeOf(data))
  }

  if (hash.has(data)) {
    return hash.get(data)
  } else {
    hash.set(data, result)
  }

  const keys = getAllKeys(data)

  for (let i = 0; i < keys.length; i += 1) {
    const key = keys[i]
    const val = data[key]
    result[key] = cloneDeep(val)
  }

  return result
}

64. Promise reject的时候自动retry

/**
 * @param {() => Promise<any>} fetcher
 * @param {number} maximumRetryCount
 * @return {Promise<any>}
 */
function fetchWithAutoRetry(fetcher, maximumRetryCount) {
  return new Promise(async (resolve, reject) => {
    let error
    for (let i = 0; i <= maximumRetryCount; i += 1) {
      await fetcher().then((res) => {
        resolve(res)
      }).catch((err) => {
        error = err
      })
    }
    reject(error)
  })
}

65. 添加千位分隔符

/**
 * @param {number} num
 * @return {string}
 */
function addComma(num) {
  num = `${num}`
  const reg = /\B(?=(\d{3})+$)/g
  const decimalPlaces = num.split('.')[1]
  let integerBits = num.split('.')[0]
  integerBits = integerBits.replace(reg, ',')

  if (decimalPlaces) {
    return `${integerBits}.${decimalPlaces}`
  }
  return `${integerBits}`
  
}

66. 去掉数组中的重复元素

/**
 * @param {any[]} arr
 */
function deduplicate(arr) {
  const map = new Map()
  for (let i = 0; i < arr.length; i += 1) {
    if (map.has(arr[i])) {
      arr.splice(i, 1)
      i -= 1
    } else {
      map.set(arr[i], true)
    }
  }
  return arr
}

69. 实现`_.isEqual()`

const getSymbolKeys = (obj) => {
  let symbolKeys = Object.getOwnPropertySymbols(obj)
  symbolKeys = [...symbolKeys].filter((key) => {
    return obj.propertyIsEnumerable(key)
  })
  return symbolKeys
}

const getAllKeys = (obj) => {
  let keys = Object.keys(obj)
  keys = [...keys, ...getSymbolKeys(obj)]
  return keys
}

function isObject(value) {
  return value != null && (typeof value === "object" || typeof value === "function")
}

const hash = new WeakMap()

/**
 * @param {any} a
 * @param {any} b
 * @return {boolean}
 */
function isEqual(a, b) {
  let result = true

  const equal = (a, b) => {
    if (!isObject(a) || !isObject(b)) {
      result = (a === b)
      return 
    }

    if (hash.has(a) && hash.has(b)) {
      return true
    }

    const aKeys = getAllKeys(a)
    const bKeys = getAllKeys(b)

    if (aKeys.length !== bKeys.length) {
      result = false
      return
    }

    hash.set(a, true)
    hash.set(b, true)

    for (let i = 0; i < aKeys.length; i += 1) {
      const key = aKeys[i]
      const aValue = a[key]
      const bValue = b[key]
      if (!isObject(aValue) || !isObject(bValue)) {
        if (aValue !== bValue) {
          result = false
          break
        }
      } else {
        isEqual(aValue, bValue)
      }
    }
  }

  equal(a, b)
  
  return result
}

79. snake_case转换为camelCase

/**
 * @param {string} str
 * @return {string}
 */
function snakeToCamel(str) {
  const reg = /([^_])_([^_])/g
  return str.replaceAll(reg, (_, p1, p2) => `${p1}${p2.toUpperCase()}`)
}

camelCase转换为snake_case

/*
 * @param {string} str
 * @return {string}
 */
function camelToSnake(str) {
  const reg = /\B(\w)([A-Z])\B/g
  return str.replaceAll(reg, (_, p1, p2) => `${p1}_${p2.toLowerCase()}`)
}

81. 合并已排序的数组


/**
 * @param {number[][]} arrList
 * non-descending integer array
 * @return {number[]} 
 */
function merge(arrList) {
  let result = []
  let offset = false

  const isMerge = () => {
    let count = 0
    for (let i = 0; i < arrList.length; i += 1) {
      if (arrList[i].length > 0) count += 1
      if (count >= 2) break
    }
    return count >= 2 ? true : false
  }

  offset = isMerge()

  while (offset) {
    let index = 0
    let min = Number.MAX_VALUE
    for (let i = 0; i < arrList.length; i += 1) {
      if (arrList[i][0] <= min) {
        index = i
        min = arrList[i][0]
      }
    }
    result.push(...arrList[index].splice(0, 1))
    offset = isMerge()
  }

  for (let i = 0; i < arrList.length; i += 1) {
    result = [...result, ...arrList[i]]
  }


  return result 
}

85. 实现 `_.get()`

/**
 * @param {object} source
 * @param {string | string[]} path
 * @param {any} [defaultValue]
 * @return {any}
 */
function get(source, path, defaultValue = undefined) {
  if (typeof path === 'string') {
    path = path.split('.')
  }

  const newPath = []
  const reg = /^([A-Za-z]+)\[([0-9]+)]$/g
  let result = source

  for (let i = 0; i < path.length; i += 1) {
    if (path[i].includes('[')) {
      const arr = reg.exec(path[i])
      newPath.push(arr[1])
      newPath.push(arr[2])
    } else {
      newPath.push(path[i])
    }
  }

  for (let i = 0; i < newPath.length; i += 1) {
    if (result === undefined || result === null) {
      result = defaultValue
      break
    }
    result = result[newPath[i]]
  }

  console.log(result)

  return result ? result : defaultValue
}

87. 返回最长的不重复子字符串

/**
 * @param {string} str
 * @return {string}
 */
function longestUniqueSubstr(str) {
  const hash = {}
  let result = ''
  let current = ''
  let start = 0
  let end = 0
  
  while (start <= end && end <= str.length - 1) {
    if (hash[str[end]] === undefined) {
      hash[str[end]] = end
    } else {
      start = hash[str[end]] + 1
      hash[str[end]] = end
      Object.keys(hash).forEach((key) => {
        if (hash[key] < start) delete hash[key]
      })
    }
    current = str.slice(start, end + 1)
    if (result.length < current.length) {
      result = current
    }
    end += 1
  }
  return result
}

89. 返回DOM tree中”右边“的元素


/**
 * @param {HTMLElement} root
 * @param {HTMLElement} target
 * @return {HTMLElemnt | null}
 */
function nextRightSibling(root, target) {
  if (!root) return null
  let result = null

  const bfs = (nodes) => {
    if (nodes.length) {
      let childs = []
      for (let i = 0; i < nodes.length; i += 1) {
        const children = nodes[i].children
        childs = [...childs, ...children]
      }
      for (let i = 0; i < childs.length; i += 1) {
        if (childs[i] === target) {
          if (i === childs.length - 1) {
            result = null
            return
          } else {
            result = childs[i + 1]
            return
          }
        }
      }
      bfs(childs)
    }
  }

  bfs([root])

  return result   
}

90. 实现`instanceof`

/**
 * @param {any} obj
 * @param {target} target
 * @return {boolean}
 */
function myInstanceOf(obj, target) {
  if (typeof obj !== 'object' || obj === null) {
    return false
  }
  const proto = Object.getPrototypeOf(obj)
  if (proto === null) {
    return false
  }
  if (proto === target.prototype) {
    return true
  } else {
    return myInstanceOf(proto, target)
  }
}

91. 反转二叉树

/**
 * @param {Node} node
 * @returns {Node}
 */
function invert(node) {
  if (!node) return null

  const traverside = (node) => {
    if (node) {
      const left = node.left
      const right = node.right
      node.right = left
      node.left = right
      traverside(left)
      traverside(right)
    }
  }

  traverside(node)

  return node
}

92. Promise节流

/**
 * @param {() => Promise<any>} func
 * @param {number} max
 * @return {Promise}
 */
function throttlePromises(funcs, max){
  return new Promise(function (resolve, reject) {
    const result = []
    const len = funcs.length
    let jobs = 0
    let count = 0

    const handleJobs = () => {
      while (jobs < max && funcs.length) {
        let promise = funcs.shift()()
        let index = len - funcs.length - 1
        promise = promise instanceof Promise ? promise : Promise.resolve(promise)
        promise.then((res) => {
          result[index] = res
          count += 1
          jobs -= 1
          if (count === len) {
            resolve(result)
          }
          handleJobs()
        }).catch((err) => {
          reject(err)
        })
        jobs += 1
      }
    }
    handleJobs()
  })
}

94. 实现`Object.create`

/**
 * @param {any} proto
 * @return {object}
 */
function myObjectCreate(proto) {
  if (!(proto instanceof Object)) {
    throw new Error()
  }
  const obj = {}
  obj.__proto__ = proto
  return obj
}

/**
 * @param {any} proto
 * @return {object}
 */
function myObjectCreate(proto) {
  if (!(proto instanceof Object)) {
    throw new Error()
  }
  function Constructor () {}
  Constructor.prototype = proto
  return new Constructor();
}

// Error
// ES6的class语法无法修改类的Constructor.prototype
// https://stackoverflow.com/questions/37680766/how-to-change-classs-prototype-in-es6
/**
 * @param {any} proto
 * @return {object}
 */
function myObjectCreate(proto) {
  if (!(proto instanceof Object)) {
    throw new Error()
  }
  class Constructor {}
  Constructor.prototype = proto
  return new Constructor();
}

95. 实现 String.prototype.trim()

/**
 * @param {string} str
 * @return {string}
 */
function trim(str) {
  return str.replace(/^\s+|\s+$/g, '')
}

97. 压缩字符串

/**
 * @param {string} str
 * @return {string}
 */
function compress(str) {
  let s = str[0]
  let p = str[0]
  let n = 1

  for (let i = 1; i < str.length; i += 1) {
    if (str[i] === p) {
      n += 1
      if (n > 2) {
        let arr = s.split('')
        arr.pop()
        s = `${arr.join('')}${n}`
      } else {
        s = `${s}${n}`
      }
    } else {
      p = str[i]
      n = 1
      s = `${s}${str[i]}`
    }
  }

  return s
}

99. 在HTML字符串中抽出所有的`<a/>`

/**
 * @param {string} str
 * @return {string[]}
 */
function extract(str) {
  const result = []
  // (\s[^>]*)?, 如果有[^>]，就必须有空格
  // [^>]也包括了空格
  const reg = /<a(\s[^>]*)?>.*?<\s*\/\s*a>/g
  let controller = true

  while (controller) {
    const match = reg.exec(str)
    if (match) {
      result.push(match[0])
    } else {
      controller = false
    }
  }

  return result
}

100. 检测链表中是否有环

/**
 * @param {Node} head
 * @return {boolean}
 */
function hasCircle(head) {
  let fast = head
  let slow = head
  let offset = true
  let result = false

  while (offset && fast && slow) {
    fast = fast?.next
    slow = slow?.next?.next
    if (fast === slow) {
      result = true
      offset = false
    }
  }

  return result
}

102. 验证括号字符串

class Stack {
  constructor () {
    this.stack = []
    Object.defineProperties(this, {
      size: {
        set () {
          throw new Error()
        },
        get () {
          return this.stack.length
        }
      }
    })
  }

  push (n) {
    this.stack.push(n)
  }

  pop () {
    return this.stack.pop()
  }

  peek () {
    return this.stack[this.stack.length - 1]
  }
}

/**
 * @param {string} str
 * @return {boolean} 
 */
function validate(str) {
  const stack = new Stack()
  for (let i = 0; i < str.length; i += 1) {
    if (stack.size === 0) {
      stack.push(str[i])
    } else {
      const top = stack.peek()
      if (top === '[' && str[i] === ']') {
        stack.pop()
        continue
      } else if (top === '{' && str[i] === '}') {
        stack.pop()
        continue
      } else if (top === '(' && str[i] === ')') {
        stack.pop()
        continue
      } else {
        stack.push(str[i])
      }
    }
  }
  return stack.size === 0 ? true : false
}

103. 实现Math.sqrt()

/**
 * @param {any} x
 * @return {number}
 */
function mySqrt(x) {
  if (!(typeof x === "number" && !isNaN(x) && x >= 0)) return NaN
  if (x === 0) return 0
  if (x === 1) return 1

  let result = null

  const sqrt = (start, end) => {
    if (start <= end) {
      const midd = Math.floor((end + start) / 2)
      if (midd ** 2 < x) {
        result = midd
        sqrt(midd + 1, end)
      } else if (midd ** 2 > x) {
        sqrt(start, midd - 1)
      } else {
        result = midd
      }
    }
  }

  sqrt(0, x)

  return result
}

104. 按层遍历DOM树


/**
 * @param {HTMLElement | null} root
 * @return {HTMLElement[]}
 */
// bfs
function flatten(root) {
  if (!root) return []

  const result = [root]

  const bfs = (nodes) => {
    if (nodes.length) {
      const children = []
      for (let i = 0; i < nodes.length; i += 1) {
        const child = nodes[i].children
        children.push(...child)
      }
      result.push(...children)
      bfs(children)
    } 
  }

  bfs([root])

  return result
}

105. 找到第一个重复的字符

/**
 * @param {string} str
 * @return {string | null}
 */
function firstDuplicate(str) {
  let result = null
  const hash = {}
  for (let i = 0; i < str.length; i += 1) {
    const key = str[i]
    if (!hash[key]) {
      hash[key] = true
    } else {
      result = key
      break
    }
  }
  return result
}

106. 找到和为0的两个数


/**
 * @param {number[]} arr
 * @return {number[]}
 */
function findTwo(arr) {
  const hash = {}
  let result = null
  for (let i = 0; i < arr.length; i += 1) {
    if (!hash[arr[i]]) hash[arr[i]] = [i]
    else hash[arr[i]].push(i)
  } 

  for (let i = 0; i < arr.length; i += 1) {
    const a = arr[i]
    const index = hash[a].shift()
    const b = 0 - a
    if (hash[b] && hash[b].length > 0) {
      result = [index, hash[b][0]]
      break
    } else {
      hash[a].unshift(index)
    }
  }

  return result
}

107. 找到最大的差

/**
 * @param {number[]} arr
 * @return {number}
 */
function largestDiff(arr) {
  if (arr.length === 0) return 0
  if (arr.length === 1) return 0

  let min = Infinity
  let max = -Infinity

  for (let i = 0; i < arr.length; i += 1) {
    min = Math.min(arr[i], min)
    max = Math.max(arr[i], max)
  }

  return Math.abs(min - max)
}

108. 用队列(Queue)实现栈(Stack)

/* you can use this Queue which is bundled together with your code
class Queue {
  enqueue(element) {
    // add new element to the queue
  }
  peek() { 
    // return the head element
  }
  dequeue() { 
    // remove head element from the queue
  }
  size() { 
    // return the queue size
  }
}
*/

// you need to complete the following Stack, using only Queue
class Stack {
  constructor () {
    this.queue = new Queue()
    this.stack = new Queue()
  }

  push(element) {
    while (this.stack.size()) {
      this.queue.enqueue(this.stack.dequeue())
    }
    this.stack.enqueue(element)
    while (this.queue.size()) {
      this.stack.enqueue(this.queue.dequeue())
    }
  }

  peek() { 
    return this.stack.peek()
  }

  pop() { 
    return this.stack.dequeue()
  }

  size() { 
    return this.stack.size()
  }
}

109. 实现`Math.pow()`

/**
 * @param {number} base
 * @param {number} power - integer
 * @return {number}
 */
function pow(base, power){
  const isNegativeIndex = power < 0
  const isNegative = base < 0
  
  const _pow = (base, power) => {
    if (power === 0) return 1
    if (power === 1) return base
    let result = power % 2 === 0 ?
      pow(base * base, power / 2)
      :
      pow(base * base, (power - 1) / 2) * base
    return result
  }

  let result = _pow(Math.abs(base), Math.abs(power))

  result = isNegativeIndex ? 1 / result : result
  result = isNegative ? result * -1 : result

  return result
}

116. 实现Object.is()

/**
 * @param {any} a
 * @param {any} b
 * @return {boolean}
 */
function is(a, b) {
  if (typeof a === 'number' && typeof b === 'number') {
    if (isNaN(a) && isNaN(b)) {
      return true
    } else if (a === 0 && b === 0) {
      return 1/a === 1/b
    }
  }
  return a === b
}

119. 创建一个tokenizer


/**
 * @param {string} str
 * @return {Generator}
 */
function* tokenize(str) {
  let temp = str.replace(/\s/g, '')
  let num = ''
  for (let i = 0; i < temp.length; i += 1) {
    const s = temp[i]
    switch (s) {
      case '+':
      case '-':
      case '*':
      case '/':
      case '(':
      case ')':
        if (num !== '') {
          yield num
          num = ''
        }
        yield s
        break
      default:
        num += s
        break
    }
  }

  if (num !== '') {
    yield num
  }
}

122. 实现 memoizeOne()

const compare = (oldKey, newKey) => {
  if (Array.isArray(oldKey) && Array.isArray(newKey) && oldKey.length === newKey.length) {
    return oldKey.every((key, i) => key === newKey[i])
  }
  return oldKey === newKey
}

/**
 * @param {Function} func
 * @param {(args: any[], newArgs: any[]) => boolean} [isEqual]
 * @returns {any}
 */
function memoizeOne(func, isEqual = compare) {
  let oldKey = Symbol()
  let catchResult = null
  let that = null
  let isCall = false
  
  return function (...params) {
    if (that !== this) {
      oldKey = Symbol()
      that = this
    } 

    if (isCall && isEqual(oldKey, [...params])) {
      return catchResult
    }

    isCall = true
    oldKey = [...params]
    const result = func.apply(this, [...params])
    catchResult = result
    return result
  }
}

123. 实现 Promise.prototype.finally()

/**
 * @param {Promise<any>} promise
 * @param {() => void} onFinally
 * @returns {Promise<any>}
 */
function myFinally(promise, onFinally) {
  return promise.then((res) => {
    return Promise.resolve(onFinally()).then(() => {
      return res
    })
  }).catch((err) => {
    return Promise.resolve(onFinally()).then(() => {
      throw err
    })
  })
}

125. 实现classNames()

function getKeys (obj) {
  return [...Object.keys(obj)]
}

function getType (data) {
  const type = Object.prototype.toString.call(data);
  return /^\[object\s+([A-Za-z]+)\]$/.exec(type)[1].toLocaleLowerCase()
}

function trim(str) {
  return str.replace(/^\s+|\s+$/g, '')
}

function isNumberTypeOrStringNumber (data) {
  const type = getType(data)
  if (type === 'number' || type === 'string') {
    return true
  }
  return false
}

/**
 * @param {any[]} args
 * @returns {string}
 */
function classNames(...args) {
  let result = ''

  const _classNames = (value) => {
    const type = getType(value)

    if (type === 'number') {
      result += ` ${value}`
      return
    }

    if (type === 'string') {
      result += ` ${value}`
      return
    }

    if (type === 'map' || type === 'object') {
      const keys = getKeys(value)
      for (let i = 0; i < keys.length; i += 1) {
        const key = keys[i]
        const val = value[key]
        if (val) {
          if (isNumberTypeOrStringNumber(key)) {
            result += ` ${key}`
          } else {
            _classNames(val)
          }
        }
      }
      return
    }

    if (type === 'set' || type === 'array') {
      value.forEach((val) => {
        if (val) {
          if (isNumberTypeOrStringNumber(val)) {
            result += ` ${val}`
          } else {
            _classNames(val)
          }
        }
      })
    }
  }

  for (let i = 0; i < args.length; i += 1) {
    _classNames(args[i])
  }

  return trim(result)
}

130. 创建LazyMan()

// interface Laziness {
//   sleep: (time: number) => Laziness
//   sleepFirst: (time: number) => Laziness
//   eat: (food: string) => Laziness
// }

function sleep (time) {
  return new Promise(function (resolve) {
    setTimeout(() => {
      resolve()
    }, time)
  })
}

/**
 * @param {string} name
 * @param {(log: string) => void} logFn
 * @returns {Laziness}
 */
function LazyMan(name, logFn) {

  const queue = [() => {
    logFn(`Hi, I'm ${name}.`)
  }]
  let update = false

  const handleQueue = function () {
    update = true
    if (update) {
      Promise.resolve().then(async function () {
        update = false
        for (let i = 0; i < queue.length; i += 1) {
          await queue[i]()
        }
      })
    }
  }

  const laziness = {
    sleep (time) {
      queue.push(async () => {
        await sleep(time * 1000)
        logFn(time > 1 ? `Wake up after ${time} seconds.` : `Wake up after ${time} second.`)
      })
      if (!update) {
        handleQueue()
      }
      return laziness
    },
    sleepFirst (time) {
      queue.unshift(async () => {
        await sleep(time * 1000)
        logFn(time > 1 ? `Wake up after ${time} seconds.` : `Wake up after ${time} second.`)
      })
      if (!update) {
        handleQueue()
      }
      return laziness
    },
    eat (food) {
      queue.push(() => {
        logFn(`Eat ${food}.`)
      })
      if (!update) {
        handleQueue()
      }
      return laziness
    }
  }

  if (!update) {
    handleQueue()
  }

  return laziness
}

131. 实现_.chunk()

/** 
 * @param {any[]} items
 * @param {number} size
 * @returns {any[][]}
 */
function chunk(items, size) {
  if (size === 0 || items.length === 0) {
    return []
  }

  const result = []

  while (items.length) {
    result.push(items.splice(0, size))
  }
  
  return result
}

137. 垂直遍历二叉树

/**
 * @param {Node} root
 * @returns {number[]}
 */
function traverse(root) {
  let result = []
  const hash = {}
  const helper = (node, offset, fnode, depth) => {
    if (node) {
      result.push({
        value: node.value,
        offset,
        fnode,
        depth
      })
      helper(node.left, offset - 1, node, depth + 1)
      helper(node.right, offset + 1, node, depth + 1)
    }
  }
  
  helper(root, 0, null, 0)

  result.sort((a,b) => {
    if (a.offset !== b.offset) {
      return a.offset - b.offset
    } else if (a.depth !== b.depth) {
      return a.depth - b.depth
    } else {
      return 0
    }
  })
  // 构建hash, 方便查找父级的索引
  result.forEach((item, i) => {
    hash[item.value] = {
      ...item,
      i,
    }
  })
  console.log(hash)
  console.log(result)
  result.sort((a,b) => {
    if (a.offset !== b.offset) {
      return a.offset - b.offset
    } else if (a.depth !== b.depth) {
      return a.depth - b.depth
    } else {
      // 根节点特殊处理
      if (!a.fnode) {
        return -1
      } else if (!b.fnode) {
        return 1
      } else {
        const aFnodeIndex = hash[a.fnode.value].i
        const bFnodeIndex = hash[b.fnode.value].i
        return aFnodeIndex - bFnodeIndex
      }
    }
  })
  
  return result.map(item => item.value)
}

138. 已排序数组的交集

使用hash的方法：

/**
 * @param {number[]} arr1 - integers
 * @param {number[]} arr2 - integers
 * @returns {number[]}
 */
function intersect(arr1, arr2) {
  const hash = {}
  const result = []
  
  for (let i = 0; i < arr1.length; i += 1) {
    const key = arr1[i]
    if (!hash[key]) {
      hash[key] = 1
    } else {
      hash[key] += 1
    }
  }

  for (let i = 0; i < arr2.length; i += 1) {
    const key = arr2[i]
    if (hash[key]) {
      hash[key] -= 1
      result.push(key)
    }
  }

  return result
}

使用双指针:

/**
 * @param {number[]} arr1 - integers
 * @param {number[]} arr2 - integers
 * @returns {number[]}
 */
function intersect(arr1, arr2) {
  const result = []
  let pointer1 = 0
  let pointer2 = 0
  
  while (pointer1 < arr1.length && pointer2 < arr2.length) {
    if (arr1[pointer1] === arr2[pointer2]) {
      result.push(arr1[pointer1])
      pointer1 += 1
      pointer2 += 1
    } else if (arr1[pointer1] > arr2[pointer2]) {
      pointer2 += 1
    } else if (arr1[pointer1] < arr2[pointer2]) {
      pointer1 += 1
    }
  }
  
  return result
}

139. 实现_.partial()

function merge(params1, params2) {
  for (let i = 0; i < params1.length; i += 1) {
    if (params2.length) {
      if (typeof params1[i] === 'symbol') {
        const first = params2.shift()
        params1[i] = first
      }
    } else {
        break
    }
  }
  return [...params1, ...params2]
}


/**
 * @param {Function} func
 * @param {any[]} args
 * @returns {Function}
 */
function partial(func, ...initArgs) {
  return function (...args) {
    let params = merge([...initArgs], [...args])
    params = params.map((arg) => {
      if (typeof arg === 'symbol') {
        return undefined
      } else {
        return arg
      }
    })
    return func.call(this, ...params)
  }
}

partial.placeholder = Symbol()

145. 最多重复出现的字符

function count(str: string): string | string[] {
  const map = {}
  let result = []
  for (let i = 0; i < str.length; i += 1) {
    if (!map[str[i]]) {
      map[str[i]] = 1
    } else {
      map[str[i]] += 1
    }
  }
  const max = Math.max(...Object.values(map))
  for (let key in map) {
    if (map[key] === max) {
      result.push(key)
    }
  }

  return result.length > 1 ? result : result[0]
}

146. 实现Array.prototype.reduce()

Array.prototype.myReduce = function (callback, initialValue) {
  const argsLength = arguments.length

  if (argsLength === 1 && this.length === 0) {
    throw new Error()
  }
  
  let index = argsLength === 1 ? 1 : 0
  let resultValue = argsLength === 1 ? this[0] : initialValue

  for (let i = index; i < this.length; i += 1) {
    resultValue = callback(resultValue, this[i], i, this)
  }

  return resultValue
}

147. 取石头

function canWinStonePicking(n) {
  if (n === 0) return null
  if (n === 1) return 'B'
  if (n === 2) return 'A'

  const firstMove = new Map([
    [1, false],
    [2, true]
  ])
  const backhand = new Map([
    [1, true],
    [2, false]
  ])

  for (let i = 3; i <= n; i += 1) {
    firstMove.set(i, (backhand.get(i-2) || backhand.get(i-1)))
    backhand.set(i, (firstMove.get(i-2) && firstMove.get(i-1)))
  }

  return firstMove.get(n) ? 'A' : backhand.get(n) ? 'B' : null
}

148. 创建一个counter对象

function createCounter() {
  const obj = { _count: 0 }
  return Object.defineProperties(obj, {
    count: {
      get () {
        const r = obj._count
        obj._count += 1
        return r
      }
    }
  })
}

149. interpolation

function t(translation, data = {}) {
  return translation.replace(/{{(.*?)}}/g, (_, key) => data[key] || '')
}

151. 实现Array.prototype.map()

Array.prototype.myMap = function(callback, thisArg) {
  const result = []
  const that = thisArg || this
  this.forEach(function (item, i) {
    result[i] = callback.call(that, item, i, that)
  })
  return result
}

152. 找到最大的前k个元素

// 分治
/*
 * @param {number[]} arr
 * @param {number} k
 * @returns {number[]}
 */
function topK(arr, k) {
  if (!arr.length) return []

  let result = []
  let i = 1

  const divideAndConquer = function (arr, base, k) {
    console.log(arr)
    if (arr.length === 1) {
      return arr[0]
    }

    const max = []
    const min = []
    let maxLen = 0
    const referenceValue = arr[0]

    for (let i = 1; i < arr.length; i += 1) {
      if (arr[i] > referenceValue) {
        max.push(arr[i])
      } else {
        min.push(arr[i])
      }
    }
    max.push(referenceValue)

    maxLen = max.length + base

    if (maxLen >= k && max.length) {
      return divideAndConquer(max, base, k)
    } else if (maxLen < k && min.length) {
      return divideAndConquer(min, maxLen, k)
    }
  }

  while (i <= k && i <= arr.length) {
    result.push(divideAndConquer(arr, 0, i))
    i += 1
  }

  return result
}

153. uglify CSS class names

/**
 * @returns {string}
 */
function getUniqueClassName() {
  const letterSet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',]
  const letterSetLength = letterSet.length
  let prefix = ''
  let className = letterSet[getUniqueClassName.index]
  // 最后的边界情况
  if (getUniqueClassName.prefix.length === letterSetLength) {
    return ''
  }
  for (let i = 0; i < getUniqueClassName.prefix.length; i += 1) {
    prefix += letterSet[getUniqueClassName.prefix[i]]
  }
  getUniqueClassName.index += 1
  if (getUniqueClassName.index === letterSetLength) {
    getUniqueClassName.index = 0
    if (!getUniqueClassName.prefix.length) {
      getUniqueClassName.prefix.push(0)
    } else {
      if (getUniqueClassName.prefix.every(item => item === letterSetLength - 1)) {
        getUniqueClassName.prefix = getUniqueClassName.prefix.map(() => 0)
        getUniqueClassName.prefix.push(0)
      } else {
        for (let i = getUniqueClassName.prefix.length - 1; i >= 0; i -= 1) {
          if (getUniqueClassName.prefix[i] < letterSetLength - 1) {
            getUniqueClassName.prefix[i] += 1
            break
          }
        }
      }
    }
  }
  return prefix + className;
}

getUniqueClassName.reset = function () {
  getUniqueClassName.prefix = []
  getUniqueClassName.index = 0
}

// 前缀的长度
getUniqueClassName.prefix = []
getUniqueClassName.index = 0

154. 简单实现双向绑定Two-way binding


/**
 * @param {{value: string}} state
 * @param {HTMLInputElement} element
 */
function model(state, element) {
  element.value = state.value;
  Object.defineProperty(state, 'value', {
    get: () => element.value,
    set: (value) => element.value = value,
  })
}

155. 请实现一个count函数

function count() {
  count.count += 1
  return count.count
}

count.reset = function () {
  count.count = 0
}

count.count = 0

156. 请实现`_.set()`

function set(obj, path, value) {
  // 字符串路径和数组路径统一变为数组路径
  if (typeof path === 'string') {
    path = path.split('.')
  }

  const newPath = []
  const reg = /^([A-Za-z]+)\[([0-9]+)]$/g
  let current = obj
  let laskKey = ''

  const isNumberKey = function (key) {
    // 是否是数字的key
    const isNumberKey = !isNaN(Number(key))
    // 是否为无效的数字key，1 有效的，01 无效的
    return isNumberKey && (Number(key) + '' === key) 
  }

  for (let i = 0; i < path.length; i += 1) {
    if (path[i].includes('[')) {
      const arr = reg.exec(path[i])
      newPath.push(arr[1])
      newPath.push(arr[2])
    } else {
      newPath.push(path[i])
    }
  }

  laskKey = newPath[newPath.length - 1]

  for (let i = 0; i < newPath.length - 1; i += 1) {
    const key = newPath[i]
    const nextKey = newPath[i + 1]
    const v = current[key]
    if (v instanceof Object) {
      current = v
    } else {
      if (isNumberKey(nextKey)) {
        current[key] = []
      } else {
        current[key] = {}
      }
      current = current[key]
    }
  }

  current[laskKey] = value

  return obj
}

157. semver 比较

/**
 * @param {string} v1
 * @param {string} v2
 * @returns 0 | 1 | -1
 */
function compare(v1, v2) {
  let result = 0
  const reg = /([0-9]+)\.([0-9]+)\.([0-9]+)/
  const v1Arr = []
  const v2Arr = []
  for (let i = 1; i <= 3; i += 1) {
    v1Arr.push(Number(reg.exec(v1)[i]))
    v2Arr.push(Number(reg.exec(v2)[i]))
  }
  for (let i = 0; i < v1Arr.length; i += 1) {
    if (v1Arr[i] > v2Arr[i]) {
      result = 1
      break
    } else if (v1Arr[i] < v2Arr[i]) {
      result = -1
      break
    }
  }
  return result
}

158. 返回DOM tree中”左边“的元素

/**
 * @param {Element} root
 * @param {Element} target
 * @return {Elemnt | null}
 */
function previousLeftSibling(root, target) {
  let result = null

  const bfs = (nodes) => {
    if (nodes.length) {
      let childrenArr = []
      let index = null
      for (let i = 0; i < nodes.length; i += 1) {
        const children = [...nodes[i].children]
        childrenArr = [...childrenArr, ...children]
      }
      for (let i = 0; i < childrenArr.length; i += 1) {
        if (childrenArr[i] === target) {
          index = i - 1
          if (index !== -1) {
            result = childrenArr[index]
          }
        }
      }
      if (index === null) {
        bfs(childrenArr)
      }
    }
  }

  bfs([root])

  return result
}

159. 实现promisify()

/**
 * @param {(...args) => void} func
 * @returns {(...args) => Promise<any}
 */
function promisify(func) {
  return function(...args) {
    const that = this
    return new Promise(function(resolve, reject) {
      func.apply(that, [...args, function (error, data) {
        if (error) {
          reject(error)
        } else {
          resolve(data)
        }
      }])
    })
  }
}

162. 请找到未重复出现的整数

/**
 * @param {number[]} arr
 * @returns number
 */
function findSingle(arr) {
  let result = arr[0]
  for (let i = 1; i < arr.length; i += 1) {
    result = result ^ arr[i]
  }
  return result
}

「面试题」100+道JS代码实现题

前言

1. 实现curry

2. 实现支持placeholder的curry()

3. 实现Array.prototype.flat()

4. 手写throttle()

6. 手写debounce()

8. 手写shuffle()随机打乱一个数组（实现一个洗牌算法）

9. 解密消息

10. 找出第一个不良版本

11. 什么是Composition?实现pipe()

13. 利用栈(Stack)创建队列(Queue)

14. 实现memo()

15. 实现类似jQuery的DOM wrapper

16. 实现一个Event Emitter

17. 实现一个DOM element store

18. 优化一个function

19. 相同结构的DOM tree上面寻找对应的节点

20. 检测 data type

21. 手写JSON.stringify()

22. 手写JSON.parse()

23. 实现一个sum()方法

24. 用JavaScript手写一个Priority Queue

25. 更新数组的顺序

26. 实现Object.assign()

27. 实现completeAssign()

28. 实现clearAllTimeout()

29. 实现async helper - sequence()

30. 实现async helper - parallel()

31. 实现async helper - race()

32. 实现Promise.all()

33. 实现Promise.allSettled()

34. 实现Promise.any()

35. 实现Promise.race()

37. 手写Binary Search (unique)

38. 实现jest.spyOn()

39. 手写range()

40. 手写冒泡排序

41. 手写归并排序

42. 手写插入排序

43. 手写快速排序

44. 手写选择排序

45. 在未排序的数组中找到第K大的元素

46. 实现_.once()

47. 反转链表

48. 含有重复元素的数组中返回特定元素的首次出现的位置

49. 含有重复元素的数组中返回特定元素的最后出现的位置

50. 含有重复元素的数组中返回特定元素之前的元素

51. 含有重复元素的数组中返回特定元素的之后的元素

53. 实现middleware

🐜蚂蚁金服面试真题: 实现compose

53. 用es5实现extends

55. HTML字符串中高亮关键字

58. 返回DOM tree的高度

59. 实现browser history

60. 实现自己的new

61. 实现Function.prototype.call

手写apply

手写bind

63. 手写_.cloneDeep()

64. Promise reject的时候自动retry

65. 添加千位分隔符

66. 去掉数组中的重复元素

69. 实现_.isEqual()

79. snake_case转换为camelCase

camelCase转换为snake_case

81. 合并已排序的数组

85. 实现 _.get()

87. 返回最长的不重复子字符串

89. 返回DOM tree中”右边“的元素

90. 实现instanceof

91. 反转二叉树

92. Promise节流

94. 实现Object.create

95. 实现 String.prototype.trim()

97. 压缩字符串

99. 在HTML字符串中抽出所有的<a/>

100. 检测链表中是否有环

102. 验证括号字符串

103. 实现Math.sqrt()

14. 实现`memo()`

29. 实现async helper - `sequence()`

30. 实现async helper - `parallel()`

31. 实现async helper - `race()`

32. 实现`Promise.all()`

33. 实现`Promise.allSettled()`

34. 实现`Promise.any()`

35. 实现`Promise.race()`

38. 实现`jest.spyOn()`

46. 实现`_.once()`

53. 用es5实现`extends`

60. 实现自己的`new`

61. 实现`Function.prototype.call`

63. 手写`_.cloneDeep()`

69. 实现`_.isEqual()`

85. 实现 `_.get()`

90. 实现`instanceof`

94. 实现`Object.create`

99. 在HTML字符串中抽出所有的`<a/>`

109. 实现`Math.pow()`

156. 请实现`_.set()`