hacker0limbo

const factorial = n => {
    if (n == 0 || n == 1) {
        return 1
    }
    return n * factorial(n-1)
}

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const currentTime = () => {
    const d = new Date()
    const year = d.getYear()
    const month = d.getMonth() + 1
    const date = d.getDate()
    const hours = d.getHours()
    const minutes = d.getMinutes()
    const seconds = d.getSeconds()

    const timeString = `${year}/${month}/${date} ${hours}:${minutes}:${seconds}`

hacker0limbo

import React, { useReducer, useContext, createContext } from 'react'
import ReactDOM from 'react-dom'

const initialState = {
  count: 0
}

const CounterContext = React.createContext()

const counterReducer = (state, action) => {

hacker0limbo

import React, { useReducer, useRef } from 'react'
import ReactDOM from 'react-dom'

const Counter = () => {
  const [count, dispatch] = useReducer((state, action) => {
    switch(action) {
      case 'INCREMENT':
        return state + 1
      case 'DECREMENT':
        return state - 1

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const useFormInput = initialValue => {
  const [value, setValue] = useState(initialValue)

  const handleChange = e => {
    setValue(e.target.value)
  }

  return {
    value,
    onChange: handleChange

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cookie 和 session 总结

直接使用cookie也是可以追踪用户的, 但是不安全, 所以比较好的做法为, 在第一次登录以后, 往客户端发送一个 cookie, cookie 里面的 value 设置为session_id, 发送请求到服务器的时候, 服务器有一个 session 使用[session_id: username]来验证发送过来的cookie

这样的好处是:

• 服务端可以使用 session 来跟踪用户
• 客户端用户登录一次以后由于被设置了 cookie, 下次可以免去登录

具体流程为:

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mongodb

安装

使用homebrew安装, 安装以后需要配置环境变量, 在.bash_profile和.zshrc里面加上export PATH=/usr/local/Cellar/mongodb/4.0.3_1/bin:$PATH, 然后source使文件生效. 同时需要在 root 目录下新建数据目录, 如下:

mkdir -p /data/db

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/*
使用 react-redux 实现的计数器
*/

import React from 'react'
import ReactDOM from 'react-dom'
import { connect, Provider } from 'react-redux'
import { createStore } from 'redux'

const mapStateToProps = state => {

hacker0limbo

import React, { useState } from 'react'
import ReactDOM from 'react-dom'

const text = 'To see the answers, we need to take a step back. The goal of this article isn’t to give you a list of bullet point recipes. It’s to help you truly “grok” useEffect. There won’t be much to learn. In fact, we’ll spend most of our time unlearning.'

const RevealText = props => {
  const { text, maxLength } = props
  const [hidden, setHidden] = useState(true)

  if (text.length <= maxLength) {

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重点:

• 可以返回一个对象, 或者一个函数
• 闭包是一个有状态（不消失的私有数据）的函数
• 闭包是一个有记忆的函数
• 闭包相当于一个只有一个方法的紧凑对象

示例 1, 非闭包:

// 无论调用多少次 fakeCounter(), 永远返回的是 0 
const fakeCounter = function() {