Live Note

Remain optimistic

Posted on Edited on

教育部关于同意复旦大学章程部分条款修改的批复 教政法函〔2019〕18 号

重点修改如下：

序言第一段

“国人自主创办”改为“中国人自主创办”。

序言第二段

删去“学校的办学理念是其校歌所传颂的……思想自由……强调以学术精神滋养师生，坚守价值，治学严谨，为学有恒。学校始终秉持团结、服务与牺牲的精神，强调学校、师生的社会责任和国家使命。”增加“敦行爱国奉献……海纳百川、追求卓越。学校倡导‘文明、健康、团结、奋发’的校风和‘刻苦、严谨、求实、创新’的学风，强调坚持理想、探究真理、正谊明道、守护文明。”

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codePointAt()

JavaScript-16 的格式储存，每个字符固定为两个字节。需要四个字节储存的字符，JavaScript 会认为那是两个字符。
ES6 提供了 codePointAt 方法，能够正确处理四个字节储存的字符，返回一个字符的码点。

String.fromCodePoint()

识别 UTF-32 字符。

includes()、startsWith()、endsWith()

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var s = "hello world"
s.startWith("hello") //true
s.endsWith("d") //true
s.includes("o") //true

s.startsWith("world", 6) //true;
s.includes("hello", 6) //false

repeat()

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"x".repeat(3) //'xxx'
"na".repeat(0) //''

"na".repeat(2.9) //取整 -> 'nana'

"na".repeat(Infinity) //Error
"na".repeat(-1) //Error

"na".repeat(-0.9) //''
"na".repeat(NaN) //''
"na".repeat("3") //'nanana'
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属性名表达式

JavaScript 定义属性有两种方法：

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obj.foo = true
obj["a" + "bc"] = 123

ES6 允许字面量定义对象时使用第二种方法：

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let propKey = "foo"
let obj = {
  [propKey]: true,
  ["a" + "bc"]: 123,
  ["he" + "llo"]() {
    return "hi"
  },
}
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尾调用

Tail Call 是函数式编程的一个重要概念，就是指某个函数的最后一步是调用另一个函数。

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function f(x) {
  return g(x)
}

尾调用不一定出现在函数尾部，只要是最后一步操作即可。

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function f(x) {
  if (x > 0) return t(x)
  return m(x)
}

尾调用优化

函数调用会在内存中形成一个 call frame，保存调用位置和内部变量等信息。所有的 call frame 形成一个 call stack。
尾调用由于是函数的最后一步操作，所以不需要保留外层函数的 call frame，这就叫 Tail Call Optimization，即只保留内层函数的调用帧。

尾递归

递归非常耗内存，因为需要同时保存多个 call frame，很容易发生 stack overflow。但对于尾递归来说，由于只存在一个 call frame，所以不会发生溢出。

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function Fibonacci(n) {
  if (n <= 1) return 1
  return Fibonacci(n - 1) + Fibonacci(n - 2)
}
console.log(Fibonacci(10)) //89
console.log(Fibonacci(100)) //overflow
console.log(Fibonacci(500)) //overflow
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工厂模式

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let Basketball = function () {
  this.name = 'basketball'
}
Basketball.prototype = {
  getMember: function () {
    console.log(5)
  },
  getBallSize: function () {
    console.log('big')
  },
}

let FootBall = function () {
  this.name = 'football'
}
FootBall.prototype = {
  getMember: function () {
    console.log(11)
  },
  getBallSize: function () {
    console.log('big')
  },
}

let Tennis = function () {
  this.name = 'tennis'
}
Tennis.prototype = {
  getMember: function () {
    console.log(1)
  },
  getBallSize: function () {
    console.log('small')
  },
}

let SportFactory = function (name) {
  switch (name) {
    case 'basketball':
      return new Basketball()
    case 'football':
      return new FootBall()
    case 'tennis':
      return new Tennis()
  }
}

let footBall = new SportFactory('football')
footBall.getMember()
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// Product base attribute
interface Product {
  operation(): string
}

class ConcreteProduct1 implements Product {
  public operation(): string {
    return 'This is Product1'
  }
}
class ConcreteProduct2 implements Product {
  public operation(): string {
    return 'This is Product2'
  }
}

abstract class Creator {
  public abstract factoryMethod(): Product

  public operation(): string {
    const product = this.factoryMethod()

    return product.operation()
  }
}

class ConcreteCreator1 extends Creator {
  public factoryMethod(): Product {
    return new ConcreteProduct1()
  }
}
class ConcreteCreator2 extends Creator {
  public factoryMethod(): Product {
    return new ConcreteProduct2()
  }
}

// usage
function clientCode(creator: Creator) {
  console.log(creator.operation())
}

clientCode(new ConcreteCreator1())
clientCode(new ConcreteCreator2())

安全工厂

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let Factory = function (type, content) {
  if (this instanceof Factory) {
    // check if is instance Factory
    return new this[type](content)
  } else {
    return new Factory(type, content)
  }
}
Factory.prototype = {
  Java: function (content) {
    console.log(content)
  },
  JavaScript: function (content) {
    console.log(content)
  },
}

let data = [
  {
    type: 'Java',
    content: 'This is Java',
  },
  {
    type: 'JavaScript',
    content: 'This is JavaScript',
  },
]

// for test
for (let i = 0; i < data.length; i++) Factory(data[i].type, data[i].content)

抽象工厂

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/**
 * to extend superType
 * @param {*} subType
 * @param {*} superType
 */
let VehicleFactory = (subType, superType) => {
  if (typeof VehicleFactory[superType] === 'function') {
    function F() {}
    F.prototype = new VehicleFactory[superType]()
    subType.constuctor = subType
    subType.prototype = new F()
  } else {
    throw new Error("don' not have this abstract class")
  }
}

// create base class
VehicleFactory.Car = function () {
  this.type = 'car'
}
VehicleFactory.Car.prototype = {
  getPrice: function () {
    return new Error('abstract')
  },
  getSpeed: function () {
    return new Error('abstract')
  },
}

let BMW = function (price, speed) {
  this.price = price
  this.speed = speed
}
VehicleFactory(BMW, 'Car') // interface
BMW.prototype = {
  getPrice: function () {
    return this.price
  },
  getSpeed: function () {
    return this.speed
  },
}

// for test
let myBmw = new BMW(10000, 100)
console.log(myBmw.getPrice())
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// product interface
interface AbstractProductA {
  functionA(): string
}

interface AbstractProductB {
  /**
   * base operation
   * @return {string}
   */
  functionA(): string

  /**
   * inject some operation
   * @param {AbstractProductA} product
   * @return {string}
   */
  functionB(product: AbstractProductA): string
}

// concrete production
class ConcreteProductA1 implements AbstractProductA {
  functionA(): string {
    return 'This is functionA from ProductA1'
  }
}
class ConcreteProductA2 implements AbstractProductA {
  functionA(): string {
    return 'This is functionA from ProductA2'
  }
}
class ConcreteProductB1 implements AbstractProductB {
  functionA(): string {
    return 'This is functionA from ProductB1'
  }

  functionB(product: AbstractProductA): string {
    return 'Function B2: ' + product.functionA()
  }
}
class ConcreteProductB2 implements AbstractProductB {
  functionA(): string {
    return 'This is functionA from ProductB2'
  }

  functionB(product: AbstractProductA): string {
    return 'Function B2: ' + product.functionA()
  }
}

// factory interface
interface AbstractFactory {
  createProductA(): AbstractProductA
  createProductB(): AbstractProductB
}

// concrete factory
class ConcreteFactory1 implements AbstractFactory {
  createProductA(): AbstractProductA {
    return new ConcreteProductA1()
  }

  createProductB(): AbstractProductB {
    return new ConcreteProductB1()
  }
}
class ConcreteFactory2 implements AbstractFactory {
  createProductA(): AbstractProductA {
    return new ConcreteProductA2()
  }

  createProductB(): AbstractProductB {
    return new ConcreteProductB2()
  }
}

// usage
/**
 * use with different facotry.
 * @param {AbstractFactory} factory
 */
function clientCode(factory: AbstractFactory) {
  const productA = factory.createProductA()
  const productB = factory.createProductB()

  console.log(productB.functionA())
  console.log(productB.functionB(productA))
}

clientCode(new ConcreteFactory1())
clientCode(new ConcreteFactory2())