TypeScript代码示例

一：类型映射(Mapped Types)

const data = {
  value: 123,
  text: "text",
  subData: {
    value: false
  }
};
type Data = typeof data;
// type Data = {
// value: number;
// text: string;
// subData: {
//   value: boolean;
// }

const data = ["text 1", "text 2"] as const;
type Data = typeof data[number]; // "text 1" | "text 2"

const locales = [
  {
    locale: "se",
    language: "Swedish"
  },
  {
    locale: "en",
    language: "English"
  }
] as const;
type Locale = typeof locales[number]["locale"]; // "se" | "en"

const currencySymbols = {
  GBP: "£",
  USD: "$",
  EUR: "€"
};
type CurrencySymbol = keyof typeof currencySymbols; // "GBP" | "USD" | "EUR"

二：Keyof与泛型（Generics）

interface HasPhoneNumber {
  name: string;
  phone: number;
}

interface HasEmail {
  name: string;
  email: string;
}

interface CommunicationMethods {
  email: HasEmail;
  phone: HasPhoneNumber;
  fax: { fax: number };
}

function contact<K extends keyof CommunicationMethods>(
  method: K,
  contact: CommunicationMethods[K] // turning key into value - a mapped type
) {
  //...
}
contact("email", { name: "foo", email: "mike@example.com" });
contact("phone", { name: "foo", phone: 3213332222 });
contact("fax", { fax: 1231 });

三：const断言
数字转为数字字面量类型

// Type '10'
let x = 10 as const;

四：高阶函数 (Higher Order Function)

function logDuration<T extends (...args: any[]) => any>(func: T) {
  const funcName = func.name;

  // Return a new function that tracks how long the original took
  return (...args: Parameters<T>): ReturnType<T> => {
    console.time(funcName);
    const results = func(...args);
    console.timeEnd(funcName);
    return results;
  };
}

function addNumbers(a: number, b: number): number {
  return a + b;
}
// Hover over is `addNumbersWithLogging: (a: number, b: number) => number`
const addNumbersWithLogging = logDuration(addNumbers);

addNumbersWithLogging(5, 3);

五：工厂
工厂模式的存在是为了解耦(废话，任何设计模式都为了解耦)调用者和被调用模块间的关系。比如当下系统使用的是sql server，为了未来切换其他db server时不至于每个调用的地方都修改，就要使用工厂来创建sql server模块。
然后，以个人理解，ioc可以有效地替换工厂。 不论如何，先记录一下代码的实例。

class Hero {
  constructor(public point: [number, number]) {}
}

const entities = [];

const entityFactory = <
  T extends {
    new (...args: any[]): any;
  }
>(
  classToCreate: T,
  numberOf: number,
  ...args: ConstructorParameters<T>
): InstanceType<T>[] =>
  [...Array(numberOf)].map(() => new classToCreate(...args));

entities.push(...entityFactory(Hero, 10, [12, 10]));

六：never的使用
利用never与其他任何类型都不兼容的特性(除了never外，没有值可以赋给never，包括any)，以控制代码在编译时就报错。

interface Square {
  kind: "square";
  size: number;
}
interface Rectangle {
  kind: "rectangle";
  width: number;
  height: number;
}
interface Circle {
  kind: "circle";
  radius: number;
}
interface Triangle {
  kind: "triangle";
  whatever: number;
}

type Shape = Square | Rectangle | Circle | Triangle;

function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}

function area(s: Shape) {
  switch (s.kind) {
    case "square":
      return s.size * s.size;
    case "rectangle":
      return s.height * s.width;
    case "circle":
      return Math.PI * s.radius ** 2;
    default:
      return assertNever(s); // Error
    // Argument of type 'Triangle' not assignable to param of type 'never'
  }
}

七：断言(Assert)

function assert(condition: any, msg?: string): asserts condition {
  if (!condition) {
    throw new AssertionError(msg);
  }
}

function yell(str) {
  assert(typeof str === "string");

  return str.toUppercase();
  //         ~~~~~~~~~~~
  // error: Property 'toUppercase' does not exist on type 'string'.
  //        Did you mean 'toUpperCase'?
}

参考：
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html
https://github.com/David-Else/modern-typescript-with-examples-cheat-sheet