关于javascript:js手写题梳理汇总

1、实现原生 AJAX 封装

const ajax = {get(url, fn) {const xhr = new XMLHttpRequest()
    xhr.open('GET', url, true)
    xhr.onreadystatechange = function() {if(xhr.readyState === 4) {fn(xhr.responseText)
      }
    }
    xhr.send()},
  post(url, fn, data) {const xhr = new XMLHttpRequest()
    xhr.open('POST', url, true)
    xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded')
    xhr.onreadystatechange = function() {if(xhr.readyState === 4) {fn(xhr.responseText)
      }
    }
    xhr.send(data)
  }
}

2、实现 new 过程

function myNew(fn, ...args) {const obj = {}

    obj.__proto__ = fn.prototype

    fn.apply(obj, args)

    return obj
}

3、打乱一个数组

// 办法 1
const shuffle = (arr) => {return arr.sort(() => {return Math.random() > 0.5 ? 1 : -1
  })
}

// 办法 2
const shuffle = (arr) => { 
    let i = arr.length
    while (i) {let j = Math.floor(Math.random() * i--)
        [arr[j], arr[i]] = [arr[i], arr[j]]
    } 
}

4、防抖函数

function debounce(fn, delay = 200) {
  let timer
  return function(...args) {if (timer) {clearTimeout(timer)
      timer = null
    }
    timer = setTimeout(() => {fn.apply(this, args)
      clearTimeout(timer)
      timer = null
    }, delay)
  }
}

5、节流函数

function throttle(fn, delay = 200) {
  let flag = true
  return function(...args) {if(!flag) {return}
    flag = false
    let timer = setTimeout(() => {fn.apply(this, args)
      flag = true
      clearTimeout(timer)
    }, delay)
  }
}

6、数组去重

// 办法 1
const quchong = (arr) => {return [...new Set(arr)]
}

// 办法 2
const quchong = (arr) => {const res = []
  arr.reduce((pre, next) => {if (!pre.has(next)) {pre.set(next, 1)
      res.push(next)
    }
    return pre
  }, new Map())

  return res
}

7、setTimeout 实现 setInterval

const mySetInterval = (fn, delay) => {
  let timer = null
  const interval = () => {fn()
    timer = setTimeout(interval, delay)
  }
  timer = setTimeout(interval, delay)

  return {cancel: () => {clearTimeout(timer)
    }
  }
}

8、setInterval 实现 setTimeout

const mySetTimeout = (fn, delay) => {const timer = setInterval(() => {fn()
    clearInterval(timer)
  }, delay)
}

9、compose 函数

function fn1(x) {return x + 1;}

function fn2(x) {return x + 2;}

function fn3(x) {return x + 3;}

function fn4(x) {return x + 4;}
const compose = (...fns) => {if (fns.length === 0) return (num) => num
  if (fns.length === 1) return fns[0]
  return fns.reduce((pre, next) => {return (num) => {return pre(next(num))
    }
  })
}
const a = compose(fn1, fn2, fn3, fn4)
console.log(a(1)); // 1+2+3+4=11

10、curring 函数

const add = (a, b, c) => a + b + c;
const currying = (fn, ...args) => {let allArgs = [...args]
  const num = fn.length
  const res = (...args2) => {allArgs = [...allArgs, ...args2]
    if (allArgs.length === num) {return fn(...allArgs)
    } else {return res}
  }
  return res
}
const a = currying(add, 1);
console.log(a(2)(3)) // 1 + 2 + 3=6

11、LRU 算法

class LRUCache {constructor(size) {
    this.size = size
    this.cache = new Map()}
  get(key) {const hasKey = this.cache.has(key)
    if (!hasKey) {return -1} else {const val = this.cache.get(key)
      this.cache.delete(key)
      this.cache.set(key, val)
      return val
    }
  }
  put(key, value) {const hasKey = this.cache.has(key)
    if (hasKey) {this.cache.delete(key)
    }
    this.cache.set(key, value)
    if (this.cache.size > this.size) {this.cache.delete(this.cache.keys().next().value)
    }
  }
}

12、公布订阅模式

class EventEmitter {constructor() {this.cache = []
  }

  on(name, fn) {const tasks = this.cache[name]
    if (tasks) {tasks.push(fn)
    } else {this.cache[name] = [fn]
    }
  }

  off(name, fn) {if (!name) {this.cache = []
      return
    }
    const tasks = this.cache[name]
    if (tasks) {if (!fn) {this.cache[name] = []}
      const index = tasks.findIndex(item => item === fn)
      if (index >= 0) {tasks.splice(index, 1)
      }
    }
  }

  emit(name, ...args) {
    // 复制一份。避免回调里持续 on，导致死循环
    const tasks = this.cache[name].slice()
    if (tasks) {for (let task of tasks) {task(...args)
      }
    }
  }

  once(name, cb) {const fn = (...args) => {cb(...args)
      this.off(name, fn)
    }
    this.on(name, fn)
  }
}

13、DOM 转 对象

const dom2tree = (node) => {const obj = {}
  obj.tag = node.tagName
  obj.children = []
  node.childNodes.forEach(child => obj.children.push(dom2tree(child)))
  return obj
}

14、对象 转 DOM

function _render(vnode) {if (typeof vnode === 'number') {vnode = String(vnode)
  }

  if (typeof vnode === 'string') {return document.createTextNode(vnode)
  }

  const dom = document.createElement(vnode.tag)
  if (vnode.attrs) {Object.keys(attrs).forEach(key => {const attr = artts[key]
      dom.setAttribute(key, attr)
    })
  }
  vnode.children.forEach(child => dom.appenChild(_render(child)))

  return dom
}

15、判断对象环援用

const cycleDetector = (obj) => {const arr = [obj]
  let flag = false

  const cycle = (o) => {const values = Object.values(o)
    for (let value of values) {if (typeof value === 'object' && value !== null) {if (arr.includes(value)) {
          flag = true
          return
        }
        arr.push(value)
        cycle(value)
      }
    }
  }

  cycle(obj)

  return flag
}

16、计算对象的层数

const loopGetLevel = (obj) => {
  let num = 1

  const computed = (obj, level) => {
    level = level || 0
    if (typeof obj === 'object' && obj !== null) {Object.values(obj).forEach(v => {if (typeof v === 'object' && v !== null) {computed(v, level + 1)
        } else {num = level + 1 > num ? level + 1 : num}
      })
    } else {num = level > num ? level : num}
  }
  computed(obj)

  return num
}

17、对象的扁平化

const flatten = obj => {if (!isObject(obj)) return
  const res = {}
  const dfs = (cur, prefix) => {if (isObject(cur)) {if (Array.isArray(cur)) {for(let i in cur) {dfs(cur[i], `${prefix}[${i}]`)
        }
      } else {for(let i in cur) {dfs(cur[i], `${prefix}${prefix ? '.' : ''}${i}`)
        }
      }
    } else {res[prefix] = cur
    }
  }
  dfs(obj, '')

  return res
}

18、(a == 1 && a == 2 && a == 3)

// 第一种办法
var a = {
  i: 1,
  toString: function () {return a.i++;}
}
console.log(a == 1 && a == 2 && a == 3) // true

// 第二种办法
var a = [1, 2, 3];
a.join = a.shift;
console.log(a == 1 && a == 2 && a == 3); // true

// 第三种办法
var val = 0;
Object.defineProperty(window, 'a', {get: function () {return ++val;}
});
console.log(a == 1 && a == 2 && a == 3) // true

19、Promise 并发器
题目形容:

JS 实现一个带并发限度的异步调度器 Scheduler，保障同时运行的工作最多有两个

addTask(1000,"1");
addTask(500,"2");
addTask(300,"3");
addTask(400,"4");
的输入程序是：2 3 1 4

整个的残缺执行流程：一开始 1、2 两个工作开始执行
500ms 时，2 工作执行结束，输入 2，工作 3 开始执行
800ms 时，3 工作执行结束，输入 3，工作 4 开始执行
1000ms 时，1 工作执行结束，输入 1，此时只剩下 4 工作在执行
1200ms 时，4 工作执行结束，输入 4 

实现

class Scheduler {constructor(limit) {
    this.limit = limit
    this.queue = []
    this.count = 0
  }
  add(time, str) {const request = () => {
      return new Promise(resolve => {setTimeout(() => {console.log(str)
          resolve()}, time)
      })
    }
    this.queue.push(request)
  }
  taskStart() {for(let i = 0; i < this.limit; i++) {this.request()
    }
  }

  request() {if (!this.queue.length || this.count > this.limit) {return}
    this.count++
    this.queue.shift()().then(() => {
      this.count--
      this.request()})
  }
}

20、lazyMan 函数
要求：

实现一个 LazyMan，能够依照以下形式调用:
LazyMan(“Hank”)输入:
Hi! This is Hank!

LazyMan(“Hank”).sleep(10).eat(“dinner”)输入
Hi! This is Hank!
// 期待 10 秒..
Wake up after 10
Eat dinner~

LazyMan(“Hank”).eat(“dinner”).eat(“supper”)输入
Hi This is Hank!
Eat dinner~
Eat supper~

LazyMan(“Hank”).eat(“supper”).sleepFirst(5)输入
// 期待 5 秒
Wake up after 5
Hi This is Hank!
Eat supper

解题：

class _lazyMan {constructor(name) {const fn = () => {console.log(`Hi! This is ${name}`)
      this.next()}
    this.tasks = []
    this.tasks.push(fn)
    setTimeout(() => {this.next()
    })
  }
  next() {const task = this.tasks.shift()
    task && task()}
  sleep(delay) {this.sleepWrapper(delay)
    return this
  }
  sleepFirst(delay) {this.sleepWrapper(delay, true)
    return this
  }
  sleepWrapper(time, first) {const fn = () => {setTimeout(() => {console.log(`Wake up after ${time}`)
        this.next()}, time * 1000)
    }
    if (first) {this.tasks.unshift(fn)
    } else {this.tasks.push(fn)
    }
  }
  eat(food) {const fn = () => {console.log(`Eat ${food}`)
      this.next()}
    this.tasks.push(fn)
    return this
  }

}

const LazyMan = (name) => {return new _lazyMan(name)
}

21、add 函数
要求：实现一个 add 办法 使计算结果可能满足如下预期：

add(1)(2)(3)()=6
add(1,2,3)(4)()=10
解题：

function add(...args1) {let allArgs = [...args1]

  const fn = (...args2) => {allArgs = [...args1, ...args2]

    return fn
  }

  fn.toString = function() {return allArgs.reduce((pre, next) => {return pre + next})
  }

  return fn
}

22、深拷贝

const tagMap = {mapTag: '[object Map]',
  setTag: '[object Set]',
  arrayTag: '[object Array]',
  objectTag: '[object Object]',
  symbolTag: '[object Symbol]',
  regexpTag: '[object RegExp]'
}


const checkType = (target) => {return Object.prototype.toString.call(target)
}

const checkTemp = (target) => {
  const c = target.constructor
  return new c()}

const cloneSymbol = (target) => {return Object(Symbol.prototype.valueOf.call(target))
}

const cloneReg = (target) => {
  const reFlags = /\w*$/
  const result = new target.constructor(target.source, reFlags.exec(target))
  result.lastIndex = target.lastIndex
  return result
}

const deepClone = (target, map = new Map()) => {const type = checkType(target)

  if (!Object.values(tagMap).includes(type)) {return target}

  if (type === tagMap.symbolTag) {return cloneSymbol(target)
  }
  if (type === tagMap.regexpTag) {return cloneReg(target)
  }

    const temp = checkTemp(target)

  if (map.get(target)) {return map.get(target)
  }

  map.set(target, temp)

  if (type === tagMap.setTag) {
    target.forEach(value => {temp.add(deepClone(value, map))
    })
  }

  if (type === tagMap.mapTag) {target.forEach((value, key) => {temp.set(key, deepClone(value, map))
    })
  }

  for (const key in target) {temp[key] = deepClone(target[key], map)
  }
  return temp
}

23、计算 LocalStorage 总容量

let str = '0123456789'
let temp = ''
// 先做一个 10KB 的字符串
while (str.length !== 10240) {str = str + '0123456789'}

// 先清空
localStorage.clear()

const computedTotal = () => {return new Promise((resolve) => {
    // 一直往 LocalStorage 中累积存储 10KB
    const timer = setInterval(() => {
      try {localStorage.setItem('temp', temp)
      } catch {
        // 报错阐明超出最大存储
        resolve(temp.length / 1024 - 10)
        clearInterval(timer)
        // 统计完记得清空
        localStorage.clear()}
      temp += str
    }, 0)
  })
}

(async () => {const total = await computedTotal()
  console.log(` 最大容量 ${total}KB`)
})()

24、实现 async/await

const toAsync = (fn) => {return function() {const gen = fn.apply(this. arguments)

    return new Promise((resolve, reject) => {function go(key, arg) {
        let res
        try {res = gen[key](arg)
        } catch(e) {return reject(e)
        }

        const {value, done} = res
        if (done) {return resolve(value)
        } else {return Promise.resolve(value).then(val => {go('next', val)
          }).catch(err => {go('throw', err)
          })
        }
      }
      go('next')
    })
  }
}

25、forEach

Array.prototype.sx_forEach = (cb) => {for (let i = 0; i < this.length; i++) {cb && cb(this[i], i, this)
    }
}

26、map

Array.prototype.sx_map = (cb) => {const res = []
    for (let i = 0; i < this.length; i++) {res[i] = cb && cb(this[i], i, this)
    }
    return res
}

27、filter

Array.prototype.sx_filter = function (cb) {const res = []
    for (let i = 0; i < this.length; i++) {cb && cb(this[i], i, this) && (res.push(this[i]))
    }
    return res
}

28、every

Array.prototype.sx_every = function (cb) {for (let i = 0; i < this.length; i++) {if (!cb && cb(this[i], i, this)) {return false}
    }
    return true
}

29、some

Array.prototype.sx_some = function (cb) {for (let i = 0; i < this.length; i++) {if (cb && cb(this[i], i, this)) {return true}
    }
    return false
}

30、reduce

Array.prototype.sx_reduce = function (cb, ...args) {
    let pre, start = 0
    if (args.length) {pre = args[0]
    } else {pre = this[0]
        start = 1
    }
    for (let i = start; i < this.length; i++) {pre = cb(pre, this[i], i, this)
    }
    return pre
}

31、findIndex

Array.prototype.sx_findIndex = function (cb) {for (let i = 0; i < this.length; i++) {if (cb && cb(this[i], i, this)) {return i}
    }
    return -1
}

32、find

Array.prototype.sx_find = function (cb) {for (let i = 0; i < this.length; i++) {if (cb && cb(this[i], i, this)) {return this[i]
        }
    }
    return undefined
}

33、fill

Array.prototype.sx_fill = function (value, start = 0, end) {
    end = end || this.length
    for (let i = start; i < end; i++) {this[i] = value
    }
    return this
}

34、include

Array.prototype.sx_include = function (value, start = 0) {const isnan = Number.isNaN(value)
    for (let i = start; i < this.length; i++) {if (this[i] === value || (isnan && Number.isNaN(this[i]))) {return true}
    }
    return false
}

35、join

Array.prototype.sx_join = function (str = ',') {
    let resStr = ''
    for (let i = 0; i < this.length; i++) {const item = this[i]
        resStr = i === 0 ? item : `${resStr}${str}${item}`
    }
    return resStr
}

36、flat

Array.prototype.sx_flat = function (num = Infinity) {
    let arr = this, i = 0
    while (arr.some(item => Array.isArray(item))) {arr = [].concat(...arr)
        i++
        if (i >= num) break
    }
    return arr
}

37、splice

Array.prototype.sx_splice = function (start, length, ...values) {if (length === 0) return []
    length = start + length > this.length - 1 ? this.length - start : length
    console.log(length)
    const res = [], tempArr = [...this]
    for (let i = start; i < start + values.length; i++) {this[i] = values[i - start]
    }
    this.length = start + values.length
    if (values.length < length) {
        const cha = length - values.length
        for (let i = start + values.length; i < tempArr.length; i++) {this[i] = tempArr[i + cha]
        }
        this.length = this.length - cha
    }
    if (values.length > length) {for (let i = start + length; i < tempArr.length; i++) {this.push(tempArr[i])
        }
    }
    for (let i = start; i < start + length; i++) {res.push(tempArr[i])
    }
    return res
}

38、entries

Object.prototype.sx_entries = function (obj) {const res = []
    for (let key in obj) {obj.hasOwnProperty(key) && (res.push([key, obj[key]]))
    }
    return res
}

39、fromEntries

Object.prototype.sx_fromEntries = function (arr) {const obj = {}
    for (let item of arr) {const [key, value] = item
        obj[key] = item[value]
    }
    return obj
}

40、keys

Object.prototype.sx_keys = function (obj) {const res = []
    for (let key in obj) {obj.hasOwnProperty(key) && res.push(key)
    }
    return res
}

41、values

Object.prototype.sx_values = function (obj) {const res = []
    for (let key in obj) {obj.hasOwnProperty(key) && res.push(obj[key])
    }
    return res
}

42、instanceOf

const instanceOf = function (parent, children) {
    const fp = parent.prototype
    let cp = children.__proto__
    while (cp) {if (fp === cp) {return true}
        cp = cp.__proto__
    }
    return false
}

43、is

Object.prototype.sx_is = function (x, y) {if (x === y) {
        // 避免 +0 和 -0
        return x !== 0 && 1 / x === 1 / y
    }

    // 避免 NaN
    return x !== x && y !== y
}

44、assign

Object.prototype.sx_assign = function (target, ...args) {if (target === null || target === undefined) {throw new TypeError('Cannot convert undefined or null to object')
    }

    target = Object(target)

    for (let obj of args) {for (let key in obj) {obj.hasOwnProperty(key) && (target[key] = obj[key])
        }
    }

    return target
}

45、all

Promise.sx_all = (promises) => {return new Promise((resolve, reject) => {const result = []
        let count = 0
        for (let i = 0; i < promises.length; i++) {const promise = Promise.resolve(promises[i])
            promise.then(res => {result[i] = res
                count++
                if (count === promises.length) {resolve(result)
                }
            }).catch(err => {reject(err)
            })
        }
    })
}

46、race

Promise.sx_race = (promises) => {return new Promise((resolve, reject) => {for (let i = 0; i < promises.length; i++) {const promise = Promise.resolve(promises[i])
            promise.then(res => {resolve(res)
            }).catch(err => {reject(err)
            })
        }
    })
}

47、allSettled

Promise.sx_allSettled = (promises) => {return new Promise((resolve) => {const result = []
        let count = 0
        const addData = (status, value, i) => {result[i] = {
                status,
                value
            }
            count++
            if (count === promises.length) {resolve(result)
            }
        }
        for (let i = 0; i < promises.length; i++) {const promise = Promise.resolve(promises[i])
            promise.then(res => {addData('fulfilled', res, i)
            }).catch(err => {addData('rejected', err, i)
            })
        }
    })
}

48、any

Promise.sx_any = (promises) => {return new Promise((resolve, reject) => {
        let count = 0
        for (let i = 0; i < promises.length; i++) {const promise = Promise.resolve(promises[i])
            promise.then(res => {resolve(res)
            }).catch(err => {
                count++
                if (count === promises.length) {reject('全错！！！')
                }
            })
        }
    })
}

49、finally

Promise.prototype.sx_finally = function (fn) {return this.then((res) => {fn()
    return res
  }).catch((err) => {fn()
    return err
  })
}

50、call

Function.prototype.sx_call = function(obj, ...args) {
  obj = obj || window
  const fn = Symbol()
  obj[fn] = this
  const res = obj[fn](...args)
  delete obj[fn]
  return res
}

51、apply

Function.prototype.sx_apply = function(obj, args) {
  obj = obj || window
  const fn = Symbol()
  obj[fn] = this
  const res = obj[fn](...args)
  delete obj[fn]
  return res
}

52、bind

Function.prototype.sx_bind = function(obj, ...args) {
  obj = obj || window
  const fn = Symbol()
  obj[fn] = this
  const _this = this

  const res = function(...innerArgs) {console.log(this, _this)
    if (this instanceof _this) {this[fn] = _this
      this[fn](...[...args, ...innerArgs])
      delete this[fn]
    } else {obj[fn](...[...args, ...innerArgs])
      delete obj[fn]
    }
  }
  res.prototype = Object.create(this.prototype)
  return res
}

53、slice

String.prototype.sx_slice = function (start = 0, end) {
    start = start < 0 ? this.length + start : start
    end = !end && end !== 0 ? this.length : end

    if (start >= end) return ''let str =''
    for (let i = start; i < end; i++) {str += this[i]
    }

    return str
}

54、substr

String.prototype.sx_substr = function (start = 0, length) {if (length < 0) return ''

    start = start < 0 ? this.length + start : start
    length = (!length && length !== 0) || length > this.length - start ? this.length : start + length

    let str = ''
    for (let i = start; i < length; i++) {str += this[i]
    }
    return str
}

55、substring

String.prototype.sx_sunstring = function (start = 0, end) {
    start = start < 0 ? this.length + start : start
    end = !end && end !== 0 ? this.length : end

    if (start >= end) [start, end] = [end, start]
    let str = ''
    for (let i = start; i < end; i++) {str += this[i]
    }

    return str
}