Live Note

Remain optimistic

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Reducer

Reducer 將多個 input fold 成一個 output.

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const add = (a, b) => a + b
const multiply = (a, b) => a * b
const concatString = (a, b) => a + b
const concatArray = (a, b) => [...a, ...b]

Transducer

Transducer 做的事情大致相同,但是與普通的 reducer 不同的是,它可以通過多個 function 組合而成.而普通的 reducer 不能組合,因爲他們接受兩個參數,但是只返回一個值,所以不能將這次的結果傳入下一個 reducer:

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// reducer
f: (a, c) => a
g: (a, c) => a

// transducer
f: (reducer) => reducer
g: (reducer) => reducer
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川普不对伊朗动武 美股涨 金价回落 油价跳水

伊朗袭击美军驻伊拉克基地后，美国总统川普表示将升级经济制裁取代付诸武力。市场乐观情绪上扬，美股上涨，金价回落，油价则大跳水。

川普 1 月 8 日在白宫发表谈话，伊朗袭击并未伤害到任何美国人，称伊朗有所退让，暗示不会对伊朗动武，伊朗外交部长也称该国不寻求升级事态，这些消息都缓解了投资人对美伊局势的不安。

8 日美国股市上涨，道琼工业指数收涨 161.41 点或 0.56%，报 28745.09 点。标普 500 指数收涨 15.87 点或 0.49%，报 3253.05 点。那斯达克指数收涨 60.66 点或 0.67%，报 9129.24 点。费城半导体指数上涨 0.31 点或 0.02%，报 1867.59 点。

那斯达克指数创收盘新高，道琼工业指数、标普 500 指数、费城半导体指数徘回历史次高位。

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Classes

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class Greeter {
  greeting: string

  constructor(message: string) {
    this.greeting = message
  }

  greet(): string {
    return "Hello, " + this.greeting
  }
}

Private

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class Animal {
  constructor(private name: string) {}
}

class Horse extends Animal {
  constructor() {
    super("Horse")
  }
}

class Human {
  constructor(private name: string) {}
}

let animal = new Animal("Lucy")
let horse = new Horse()
let human = new Human("Jack")
animal = horse // success
animal = human // failed
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Numeric enums

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enum Direction {
  UP,
  DOWN,
  LEFT,
  RIGHT,
}

function move(length: number, direction: Direction): void {
  console.log(`move: ${direction} ${length}meters.`)
}

move(10, Direction.UP)

String enums

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enum Direction {
  UP = "UP",
  DOWN = "DOWN",
  LEFT = "LEFT",
  RIGHT = "RIGHT",
}
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Function Types

Typing the function

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function foo(x: number, y: string): string {
  return x + y
}

let fo = function (x: number, y: string): string {
  return x + y
}

Writing the function type

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let foo: (x: number, y: string) => string = function (
  x: number,
  y: string
): string {
  return x + y
}

Inferring types

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let foo: (x: number, y: string) => string = function (x, y) {
  return x + y
}
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