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) 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) 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) 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.bind(me)
meSay()
const otherSay = say.bind(other)
otherSay() 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
let args = [].slice.call(arguments, 1)
let 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) 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) 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 () {
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]); 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) 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)) 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() 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)) 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)
}) 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');
})() 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) 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);
}) 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)) 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)); 18. 图片懒加载
// 
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)) 
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)) 原作者姓名：iboying
原出处：掘金
原文链接： https://juejin.im/post/6873513007037546510
发布于 09-20