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前端面试常见的手写功能

1. 防抖

function debounce(func, ms = 1000) {
  let timer;
  return function (...args) {
    if (timer) {
      clearTimeout(timer)
    }
    timer = setTimeout(() => {
      func.apply(this, args)
    }, ms)
  }
}

// 测试
const task = () => { console.log('run task') }
const debounceTask = debounce(task, 1000)
window.addEventListener('scroll', debounceTask)
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2. 节流

function throttle(func, ms = 1000) {
  let canRun = true
  return function (...args) {
    if (!canRun) return
    canRun = false
    setTimeout(() => {
      func.apply(this, args)
      canRun = true
    }, ms)
  }
}

// 测试
const task = () => { console.log('run task') }
const throttleTask = throttle(task, 1000)
window.addEventListener('scroll', throttleTask)
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3. new

function myNew(Func, ...args) {
  const instance = {};
  if (Func.prototype) {
    Object.setPrototypeOf(instance, Func.prototype)
  }
  const res = Func.apply(instance, args)
  if (typeof res === "function" || (typeof res === "object" && res !== null)) {
    return res
  }
  return instance
}

// 测试
function Person(name) {
  this.name = name
}
Person.prototype.sayName = function() {
  console.log(`My name is ${this.name}`)
}
const me = myNew(Person, 'Jack')
me.sayName()
console.log(me)
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4. bind

Function.prototype.myBind = function (context = globalThis) {
  const fn = this
  const args = Array.from(arguments).slice(1)
  const newFunc = function () {
    const newArgs = args.concat(...arguments)
    if (this instanceof newFunc) {
      // 通过 new 调用,绑定 this 为实例对象
      fn.apply(this, newArgs)
    } else {
      // 通过普通函数形式调用,绑定 context
      fn.apply(context, newArgs)
    }
  }
  // 支持 new 调用方式
  newFunc.prototype = Object.create(fn.prototype)
  return newFunc
}

// 测试
const me = { name: 'Jack' }
const other = { name: 'Jackson' }
function say() {
  console.log(`My name is ${this.name || 'default'}`);
}
const meSay = say.myBind(me)
meSay()
const otherSay = say.myBind(other)
otherSay()
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5. call

Function.prototype.myCall = function (context = globalThis) {
  // 关键步骤,在 context 上调用方法,触发 this 绑定为 context,使用 Symbol 防止原有属性的覆盖
  const key = Symbol('key')
  context[key] = this
  // es5 可通过 for 遍历 arguments 得到参数数组
  const args = [...arguments].slice(1)
  const res = context[key](...args)
  delete context[key]
  return res
};

// 测试
const me = { name: 'Jack' }
function say() {
  console.log(`My name is ${this.name || 'default'}`);
}
say.myCall(me)
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6. apply

Function.prototype.myApply = function (context = globalThis) {
  // 关键步骤,在 context 上调用方法,触发 this 绑定为 context,使用 Symbol 防止原有属性的覆盖
  const key = Symbol('key')
  context[key] = this
  let res
  if (arguments[1]) {
    res = context[key](...arguments[1])
  } else {
    res = context[key]()
  }
  delete context[key]
  return res
}

// 测试
const me = { name: 'Jack' }
function say() {
  console.log(`My name is ${this.name || 'default'}`);
}
say.myApply(me)
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7. deepCopy

function deepCopy(obj, cache = new WeakMap()) {
  if (!obj instanceof Object) return obj
  // 防止循环引用
  if (cache.get(obj)) return cache.get(obj)
  // 支持函数
  if (obj instanceof Function) {
    return function () {
      return obj.apply(this, arguments)
    }
  }
  // 支持日期
  if (obj instanceof Date) return new Date(obj)
  // 支持正则对象
  if (obj instanceof RegExp) return new RegExp(obj.source, obj.flags)
  // 还可以增加其他对象,比如:Map, Set等,根据情况判断增加即可,面试点到为止就可以了

  // 数组是 key 为数字素银的特殊对象
  const res = Array.isArray(obj) ? [] : {}
  // 缓存 copy 的对象,用于处理循环引用的情况
  cache.set(obj, res)

  Object.keys(obj).forEach((key) => {
    if (obj[key] instanceof Object) {
      res[key] = deepCopy(obj[key], cache)
    } else {
      res[key] = obj[key]
    }
  });
  return res
}

// 测试
const source = {
  name: 'Jack',
  meta: {
    age: 12,
    birth: new Date('1997-10-10'),
    ary: [1, 2, { a: 1 }],
    say() {
      console.log('Hello');
    }
  }
}
source.source = source
const newObj = deepCopy(source)
console.log(newObj.meta.ary[2] === source.meta.ary[2]); // false
console.log(newObj.meta.birth === source.meta.birth); // false
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8. 事件总线 | 发布订阅模式

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

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

  off(name, fn) {
    const tasks = this.cache[name]
    if (tasks) {
      const index = tasks.findIndex((f) => f === fn || f.callback === fn)
      if (index >= 0) {
        tasks.splice(index, 1)
      }
    }
  }

  emit(name) {
    if (this.cache[name]) {
      // 创建副本,如果回调函数内继续注册相同事件,会造成死循环
      const tasks = this.cache[name].slice()
      for (let fn of tasks) {
        fn();
      }
    }
  }

  emit(name, once = false) {
    if (this.cache[name]) {
      // 创建副本,如果回调函数内继续注册相同事件,会造成死循环
      const tasks = this.cache[name].slice()
      for (let fn of tasks) {
        fn();
      }
      if (once) {
        delete this.cache[name]
      }
    }
  }
}

// 测试
const eventBus = new EventEmitter()
const task1 = () => { console.log('task1'); }
const task2 = () => { console.log('task2'); }
eventBus.on('task', task1)
eventBus.on('task', task2)

setTimeout(() => {
  eventBus.emit('task')
}, 1000)
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9. 柯里化:只传递给函数一部分参数来调用它,让它返回一个函数去处理剩下的参数

function curry(func) {
  return function curried(...args) {
    // 关键知识点:function.length 用来获取函数的形参个数
    // 补充:arguments.length 获取的是实参个数
    if (args.length >= func.length) {
      return func.apply(this, args)
    }
    return function (...args2) {
      return curried.apply(this, args.concat(args2))
    }
  }
}

// 测试
function sum (a, b, c) {
  return a + b + c
}
const curriedSum = curry(sum)
console.log(curriedSum(1, 2, 3))
console.log(curriedSum(1)(2,3))
console.log(curriedSum(1)(2)(3))
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10. es5 实现继承

function create(proto) {
  function F() {}
  F.prototype = proto;
  return new F();
}

// Parent
function Parent(name) {
  this.name = name
}

Parent.prototype.sayName = function () {
  console.log(this.name)
};

// Child
function Child(age, name) {
  Parent.call(this, name)
  this.age = age
}
Child.prototype = create(Parent.prototype)
Child.prototype.constructor = Child

Child.prototype.sayAge = function () {
  console.log(this.age)
}

// 测试
const child = new Child(18, 'Jack')
child.sayName()
child.sayAge()
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11. instanceof

function isInstanceOf(instance, klass) {
  let proto = instance.__proto__
  let prototype = klass.prototype
  while (true) {
    if (proto === null) return false
    if (proto === prototype) return true
    proto = proto.__proto__
  }
}

// 测试
class Parent {}
class Child extends Parent {}
const child = new Child()
console.log(isInstanceOf(child, Parent), isInstanceOf(child, Child), isInstanceOf(child, Array));
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12. 异步并发数限制

/**
 * 关键点
 * 1. new promise 一经创建,立即执行
 * 2. 使用 Promise.resolve().then 可以把任务加到微任务队列,防止立即执行迭代方法
 * 3. 微任务处理过程中,产生的新的微任务,会在同一事件循环内,追加到微任务队列里
 * 4. 使用 race 在某个任务完成时,继续添加任务,保持任务按照最大并发数进行执行
 * 5. 任务完成后,需要从 doingTasks 中移出
 */
function limit(count, array, iterateFunc) {
  const tasks = []
  const doingTasks = []
  let i = 0
  const enqueue = () => {
    if (i === array.length) {
      return Promise.resolve()
    }
    const task = Promise.resolve().then(() => iterateFunc(array[i++]))
    tasks.push(task)
    const doing = task.then(() => doingTasks.splice(doingTasks.indexOf(doing), 1))
    doingTasks.push(doing)
    const res = doingTasks.length >= count ? Promise.race(doingTasks) : Promise.resolve()
    return res.then(enqueue)
  };
  return enqueue().then(() => Promise.all(tasks))
}

// test
const timeout = i => new Promise(resolve => setTimeout(() => resolve(i), i))
limit(2, [1000, 1000, 1000, 1000], timeout).then((res) => {
  console.log(res)
})
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13. 异步串行 | 异步并行

// 字节面试题,实现一个异步加法
function asyncAdd(a, b, callback) {
  setTimeout(function () {
    callback(null, a + b);
  }, 500);
}

// 解决方案
// 1. promisify
const promiseAdd = (a, b) => new Promise((resolve, reject) => {
  asyncAdd(a, b, (err, res) => {
    if (err) {
      reject(err)
    } else {
      resolve(res)
    }
  })
})

// 2. 串行处理
async function serialSum(...args) {
  return args.reduce((task, now) => task.then(res => promiseAdd(res, now)), Promise.resolve(0))
}

// 3. 并行处理
async function parallelSum(...args) {
  if (args.length === 1) return args[0]
  const tasks = []
  for (let i = 0; i < args.length; i += 2) {
    tasks.push(promiseAdd(args[i], args[i + 1] || 0))
  }
  const results = await Promise.all(tasks)
  return parallelSum(...results)
}

// 测试
(async () => {
  console.log('Running...');
  const res1 = await serialSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12)
  console.log(res1)
  const res2 = await parallelSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12)
  console.log(res2)
  console.log('Done');
})()
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14. vue reactive

// Dep module
class Dep {
  static stack = []
  static target = null
  deps = null
  
  constructor() {
    this.deps = new Set()
  }

  depend() {
    if (Dep.target) {
      this.deps.add(Dep.target)
    }
  }

  notify() {
    this.deps.forEach(w => w.update())
  }

  static pushTarget(t) {
    if (this.target) {
      this.stack.push(this.target)
    }
    this.target = t
  }

  static popTarget() {
    this.target = this.stack.pop()
  }
}

// reactive
function reactive(o) {
  if (o && typeof o === 'object') {
    Object.keys(o).forEach(k => {
      defineReactive(o, k, o[k])
    })
  }
  return o
}

function defineReactive(obj, k, val) {
  let dep = new Dep()
  Object.defineProperty(obj, k, {
    get() {
      dep.depend()
      return val
    },
    set(newVal) {
      val = newVal
      dep.notify()
    }
  })
  if (val && typeof val === 'object') {
    reactive(val)
  }
}

// watcher
class Watcher {
  constructor(effect) {
    this.effect = effect
    this.update()
  }

  update() {
    Dep.pushTarget(this)
    this.value = this.effect()
    Dep.popTarget()
    return this.value
  }
}

// 测试代码
const data = reactive({
  msg: 'aaa'
})

new Watcher(() => {
  console.log('===> effect', data.msg);
})

setTimeout(() => {
  data.msg = 'hello'
}, 1000)
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15. promise

// 建议阅读 [Promises/A+ 标准](https://promisesaplus.com/)
class MyPromise {
  constructor(func) {
    this.status = 'pending'
    this.value = null
    this.resolvedTasks = []
    this.rejectedTasks = []
    this._resolve = this._resolve.bind(this)
    this._reject = this._reject.bind(this)
    try {
      func(this._resolve, this._reject)
    } catch (error) {
      this._reject(error)
    }
  }

  _resolve(value) {
    setTimeout(() => {
      this.status = 'fulfilled'
      this.value = value
      this.resolvedTasks.forEach(t => t(value))
    })
  }

  _reject(reason) {
    setTimeout(() => {
      this.status = 'reject'
      this.value = reason
      this.rejectedTasks.forEach(t => t(reason))
    })
  }

  then(onFulfilled, onRejected) {
    return new MyPromise((resolve, reject) => {
      this.resolvedTasks.push((value) => {
        try {
          const res = onFulfilled(value)
          if (res instanceof MyPromise) {
            res.then(resolve, reject)
          } else {
            resolve(res)
          }
        } catch (error) {
          reject(error)
        }
      })
      this.rejectedTasks.push((value) => {
        try {
          const res = onRejected(value)
          if (res instanceof MyPromise) {
            res.then(resolve, reject)
          } else {
            reject(res)
          }
        } catch (error) {
          reject(error)
        }
      })
    })
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }
}

// 测试
new MyPromise((resolve) => {
  setTimeout(() => {
    resolve(1);
  }, 500);
}).then((res) => {
    console.log(res);
    return new MyPromise((resolve) => {
      setTimeout(() => {
        resolve(2);
      }, 500);
    });
  }).then((res) => {
    console.log(res);
    throw new Error('a error')
  }).catch((err) => {
    console.log('==>', err);
  })
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16. 数组扁平化

// 方案 1
function recursionFlat(ary = []) {
  const res = []
  ary.forEach(item => {
    if (Array.isArray(item)) {
      res.push(...recursionFlat(item))
    } else {
      res.push(item)
    }
  })
  return res
}
// 方案 2
function reduceFlat(ary = []) {
  return ary.reduce((res, item) => res.concat(Array.isArray(item) ? reduceFlat(item) : item), [])
}

// 测试
const source = [1, 2, [3, 4, [5, 6]], '7']
console.log(recursionFlat(source))
console.log(reduceFlat(source))
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17. 对象扁平化

function objectFlat(obj = {}) {
  const res = {}
  function flat(item, preKey = '') {
    Object.entries(item).forEach(([key, val]) => {
      const newKey = preKey ? `${preKey}.${key}` : key
      if (val && typeof val === 'object') {
        flat(val, newKey)
      } else {
        res[newKey] = val
      }
    })
  }
  flat(obj)
  return res
}

// 测试
const source = { a: { b: { c: 1, d: 2 }, e: 3 }, f: { g: 2 } }
console.log(objectFlat(source));
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18. 图片懒加载

// <img src="default.png" data-src="https://xxxx/real.png">
function isVisible(el) {
  const position = el.getBoundingClientRect()
  const windowHeight = document.documentElement.clientHeight
  // 顶部边缘可见
  const topVisible = position.top > 0 && position.top < windowHeight;
  // 底部边缘可见
  const bottomVisible = position.bottom < windowHeight && position.bottom > 0;
  return topVisible || bottomVisible;
}

function imageLazyLoad() {
  const images = document.querySelectorAll('img')
  for (let img of images) {
    const realSrc = img.dataset.src
    if (!realSrc) continue
    if (isVisible(img)) {
      img.src = realSrc
      img.dataset.src = ''
    }
  }
}

// 测试
window.addEventListener('load', imageLazyLoad)
window.addEventListener('scroll', imageLazyLoad)
// or
window.addEventListener('scroll', throttle(imageLazyLoad, 1000))
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欢迎一起补充 ~ Github 地址

最近用心总结了一篇《系统的准备前端面试》,文章故意没有写明很多具体的知识细节,而是从整体上分析了面试的各个环节与重点,希望能够给面试新手一些指导,不过由于"我理解不了"的原因,这篇文章一直无法上推荐,到目前为止还没有人能看到,如果感兴趣的话,可以点击查看,我自认为该文章是能作为面试新手的一份行动指南的,希望能帮到需要的人。