js原理题的实现

参考微信
1.实现一个call函数

//将改变this指向的方法挂到目标this上执行并且返回
    Function.prototype.mycall = function (context) {

        if(typeof this !== 'function')
            throw  new TypeError("not function");
        context = context || window;
        context.fn = this;
        //获取参数
        //从后面获取
        let args = [...arguments].slice(1);
        let result = context.fn(...args);
        delete  context.fn;
        return result;
    }

2.实现一个apply函数

Function.prototype.myapply = function (context) {
       if(typeof this !== 'function')
           throw new TypeError("not function");
       context = context || window;
       context.fn = this;
       let result;
       if(arguments[1]){
           
          result =  context.fn(arguments[1]);
       }else
           result =  context.fn();
       delete context.fn;
       return result;
   }

3.实现bind函数，思路与call相似，但是返回的是函数

Function.prototype.mybind = function (context) {
       if(typeof this !== 'function')
           throw  new TypeError("not function");
       let _this = this;
       let arg = [...arguments].slice(1);
       return function F() {
           //处理函数使用new的情况
           if (this instanceof F) {
               return new _this(...arg, ...arguments)
           } else {
               return _this.apply(context, arg.concat(...arguments))
           }
       }

   };

实现instancceof方法

//instanceof 右边变量的原型位于左边变量的原型上 左边变量一直递归查找
    function instanceOf(left,right) {
        let leftValue = left.__proto__;
        let rightValue = right.prototype;
        while (true){
            if(left == null)
                return false;
           if(leftValue == rightValue)
               return true;
        leftValue = leftValue.__proto__;
        }

    }

实现object.create方法

//object.create的原理
   //思路 将传入的对象作为原型
   function create(obj) {
       function F() {

       }
       F.prototype = obj;
       return new F();
   }

new的本质

//new 的本质 创建一个新对象并且将其隐式原型指向构造函数
    function myNew(fun) {
        return function () {
            let obj = {
                __proto__:fun.prototype
            }
            fun.call(obj,...arguments);
            return obj;
        }
    }
    function person(name,age) {
        this.name = name;
        this.age = age;
    }
    let obj = myNew(person)('chen',18);

实现一个promise

 //实现一个promise
    class Promise{
        constructor(fn) {
            this.state = 'pending';
            this.value = undefined;
            this.reason = undefined;
            let resolve = value => {
                if (this.state == 'pending') {
                    this.state = 'fullfilled';
                    this.value = value;
                }
            }
            let reject = value => {
                if (this.state == 'pending') {
                    this.state = 'rejected';
                    this.reason = value;
                }
            }
            try {
                fn(resolve, reject);
            } catch (e) {
                reject(e);
            }
        }
then(onFulfilled,onRejected){
               switch (this.state) {
                   case 'fullfilled':
                       onFulfilled();
                       break;
                   case 'rejected':
                       onRejected();
                       break;
               }
            }



    }

实现浅拷贝

// 1. ...实现
let copy1 = {...{x:1}}

// 2. Object.assign实现

let copy2 = Object.assign({}, {x:1})

实现基本的深拷贝

// 1. JOSN.stringify()/JSON.parse()
let obj = {a: 1, b: {x: 3}}
JSON.parse(JSON.stringify(obj))

// 2. 递归拷贝
function deepClone(obj) {
  let copy = obj instanceof Array ? [] : {}
  for (let i in obj) {
    if (obj.hasOwnProperty(i)) {
      copy[i] = typeof obj[i] === 'object' ? deepClone(obj[i]) : obj[i]
    }
  }
  return copy
}

使用setTimeout模拟setInterval

setTimeout (function () {
  // do something
  setTimeout (arguments.callee, 500)
}, 500)

js实现继承方法

// 借用构造函数继承实例属性
function Child () {
  Parent.call(this)
}
// 寄生继承原型属性
(function () {
  let Super = function () {}
  Super.prototype = Parent.prototype
  Child.prototype = new Super()
})()

实现一个eventBus

// 组件通信，一个触发与监听的过程
class EventEmitter {
  constructor () {
    // 存储事件
    this.events = this.events || new Map()
  }
  // 监听事件
  addListener (type, fn) {
    if (!this.events.get(type)) {
      this.events.set(type, fn)
    }
  }
  // 触发事件
  emit (type) {
    let handle = this.events.get(type)
    handle.apply(this, [...arguments].slice(1))
  }
}

// 测试
let emitter = new EventEmitter()
// 监听事件
emitter.addListener('ages', age => {
  console.log(age)
})
// 触发事件
emitter.emit('ages', 18)  // 18

实现双向数据绑定

let obj = {}
let input = document.getElementById('input')
let span = document.getElementById('span')
// 数据劫持
Object.defineProperty(obj, 'text', {
  configurable: true,
  enumerable: true,
  get() {
    console.log('获取数据了')
  },
  set(newVal) {
    console.log('数据更新了')
    input.value = newVal
    span.innerHTML = newVal
  }
})
// 输入监听
input.addEventListener('keyup', function(e) {
  obj.text = e.target.value
})

实现路由

// hash路由
class Route{
  constructor(){
    // 路由存储对象
    this.routes = {}
    // 当前hash
    this.currentHash = ''
    // 绑定this，避免监听时this指向改变
    this.freshRoute = this.freshRoute.bind(this)
    // 监听
    window.addEventListener('load', this.freshRoute, false)
    window.addEventListener('hashchange', this.freshRoute, false)
  }
  // 存储
  storeRoute (path, cb) {
    this.routes[path] = cb || function () {}
  }
  // 更新
  freshRoute () {
    this.currentHash = location.hash.slice(1) || '/'
    this.routes[this.currentHash]()
  }   
}

实现懒加载

let imgs =  document.querySelectorAll('img')
// 可视区高度
let clientHeight = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight
function lazyLoad () {
  // 滚动卷去的高度
  let scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
  for (let i = 0; i < imgs.length; i ++) {
    // 图片在可视区冒出的高度
    let x = clientHeight + scrollTop - imgs[i].offsetTop
    // 图片在可视区内
    if (x > 0 && x < clientHeight+imgs[i].height) {
      imgs[i].src = imgs[i].getAttribute('data')
    } 
  }      
}
// addEventListener('scroll', lazyLoad) or setInterval(lazyLoad, 1000)

rem实现原理

// 原始配置
function setRem () {
  let doc = document.documentElement
  let width = doc.getBoundingClientRect().width
  let rem = width / 75
  doc.style.fontSize = rem + 'px'
}
// 监听窗口变化
addEventListener("resize", setRem)

手写ajax

// 1. 简单流程

// 实例化
let xhr = new XMLHttpRequest()
// 初始化
xhr.open(method, url, async)
// 发送请求
xhr.send(data)
// 设置状态变化回调处理请求结果
xhr.onreadystatechange = () => {
  if (xhr.readyStatus === 4 && xhr.status === 200) {
    console.log(xhr.responseText)
  }
}

// 2. 基于promise实现 

function ajax (options) {
  // 请求地址
  const url = options.url
  // 请求方法
  const method = options.method.toLocaleLowerCase() || 'get'
  // 默认为异步true
  const async = options.async
  // 请求参数
  const data = options.data
  // 实例化
  const xhr = new XMLHttpRequest()
  // 请求超时
  if (options.timeout && options.timeout > 0) {
    xhr.timeout = options.timeout
  }
  // 返回一个Promise实例
  return new Promise ((resolve, reject) => {
    xhr.ontimeout = () => reject && reject('请求超时')
    // 监听状态变化回调
    xhr.onreadystatechange = () => {
      if (xhr.readyState == 4) {
        // 200-300 之间表示请求成功，304资源未变，取缓存
        if (xhr.status >= 200 && xhr.status < 300 || xhr.status == 304) {
          resolve && resolve(xhr.responseText)
        } else {
          reject && reject()
        }
      }
    }
    // 错误回调
    xhr.onerror = err => reject && reject(err)
    let paramArr = []
    let encodeData
    // 处理请求参数
    if (data instanceof Object) {
      for (let key in data) {
        // 参数拼接需要通过 encodeURIComponent 进行编码
        paramArr.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]))
      }
      encodeData = paramArr.join('&')
    }
    // get请求拼接参数
    if (method === 'get') {
      // 检测url中是否已存在 ? 及其位置
      const index = url.indexOf('?')
      if (index === -1) url += '?'
      else if (index !== url.length -1) url += '&'
      // 拼接url
      url += encodeData
    }
    // 初始化
    xhr.open(method, url, async)
    // 发送请求
    if (method === 'get') xhr.send(null)
    else {
      // post 方式需要设置请求头
      xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded;charset=UTF-8')
      xhr.send(encodeData)
    }
  })
}

实现拖拽

window.onload = function () {
  // drag处于绝对定位状态
  let drag = document.getElementById('box')
  drag.onmousedown = function(e) {
    var e = e || window.event
    // 鼠标与拖拽元素边界的距离 = 鼠标与可视区边界的距离 - 拖拽元素与边界的距离
    let diffX = e.clientX - drag.offsetLeft
    let diffY = e.clientY - drag.offsetTop
    drag.onmousemove = function (e) {
      // 拖拽元素移动的距离 = 鼠标与可视区边界的距离 - 鼠标与拖拽元素边界的距离
      let left = e.clientX - diffX
      let top = e.clientY - diffY
      // 避免拖拽出可视区
      if (left < 0) {
        left = 0
      } else if (left > window.innerWidth - drag.offsetWidth) {
        left = window.innerWidth - drag.offsetWidth
      }
      if (top < 0) {
        top = 0
      } else if (top > window.innerHeight - drag.offsetHeight) {
        top = window.innerHeight - drag.offsetHeight
      }
      drag.style.left = left + 'px'
      drag.style.top = top + 'px'
    }
    drag.onmouseup = function (e) {
      this.onmousemove = null
      this.onmouseup = null
    }
  }
}

实现防抖

// 思路:在规定时间内未触发第二次，则执行
function debounce (fn, delay) {
  // 利用闭包保存定时器
  let timer = null
  return function () {
    let context = this
    let arg = arguments
    // 在规定时间内再次触发会先清除定时器后再重设定时器
    clearTimeout(timer)
    timer = setTimeout(function () {
      fn.apply(context, arg)
    }, delay)
  }
}

function fn () {
  console.log('防抖')
}
addEventListener('scroll', debounce(fn, 1000))

实现节流

// 思路：在规定时间内只触发一次
function throttle (fn, delay) {
  // 利用闭包保存时间
  let prev = Date.now()
  return function () {
    let context = this
    let arg = arguments
    let now = Date.now()
    if (now - prev >= delay) {
      fn.apply(context, arg)
      prev = Date.now()
    }
  }
}

function fn () {
  console.log('节流')
}
addEventListener('scroll', throttle(fn, 1000))

只能说tql