【TypeScript 演化史 — 第八章】字面量类型扩展 和 无类型导入

const stringLiteral = "https"; // Type "https"
const numericLiteral = 42;     // Type 42
const booleanLiteral = true;   // Type true

let widenedStringLiteral = stringLiteral;   // Type string
let widenedNumericLiteral = numericLiteral; // Type number
let widenedBooleanLiteral = booleanLiteral; // Type boolean

enum FlexDirection {
  Row,
  Column
}

const enumLiteral = FlexDirection.Row; //  FlexDirection.Row 类型
let widenedEnumLiteral = enumLiteral; // FlexDirection 类型

const stringLiteral: "https" = "https"; // 类型 "https" (非扩展)
const numericLiteral: 42 = 42; // 类型 42 (非扩展)

let widenedStringLiteral = stringLiteral; // 类型 "https" (非扩展)
let widenedNumericLiteral = numericLiteral; // 类型 42 (非扩展)

const http = "http"; // Type "http" (可扩展)
const https = "https"; // Type "https" (可扩展)

const protocols = [http, https]; // Type string[]

const first = protocols[0]; // Type string
const second = protocols[1]; // Type string

const http: "http" = "http"; // Type "http" (非扩展)
const https: "https" = "https"; // Type "https" (非扩展

const protocols = [http, https]; // Type ("http" | "https")[]

const first = protocols[0]; // Type "http" | "https"
const second = protocols[1]; // Type "http" | "https"

const http = "http"; // Type "http" (可扩展)
const https = "https"; // Type "https" (可扩展)

const protocols: ["http", "https"] = [http, https]; // Type ["http", "https"]

const first = protocols[0]; // Type "http" (非扩展)
const second = protocols[1]; // Type "https" (非扩展)

// Succeeds if `node_modules/asdf/index.js` exists
import { x } from "asdf";

let x;      // 隐式 'any'
let y = []; // 隐式 'any[]'

let z: any; // 显式 'any'.

let x;

// 你仍然可以给'x'赋值任何你需要的任何值。
x = () => 42;

// 在刚赋值后，TypeScript 2.1 知道'x'的类型是'() => number'。
let y = x();

// 感谢，现在它会告诉你，你不能添加一个数字到一个函数！
console.log(x + y);
//          ~~~~~
// 错误！运算符 '+' 不能应用于类型`() => number`和'number'。

// TypeScript仍然允许你给'x'赋值你需要的任何值。
x = "Hello world!";

// 并且现在它也知道'x'是'string'类型的！
x.toLowerCase();

function f1() {
    let x = [];
    x.push(5);
    x[1] = "hello";
    x.unshift(true);
    return x;  // (string | number | boolean)[]
}

function f2() {
    let x = null;
    if (cond()) {
        x = [];
        while (cond()) {
            x.push("hello");
        }
    }
    return x;  // string[] | null
}

function f3() {
    let x = [];  // 错误：当变量'x'类型无法确定时，它隐式具有'any[]'类型。
    x.push(5);
    function g() {
        x;    // 错误：变量'x'隐式具有'any【】'类型。
    }
}

function isValidPasswordLength(
  password: string,
  min: number,
  max?: number
) {
  return password.length >= min && password.length <= max;
}

isValidPasswordLength("open sesame", 6, 128); // true
isValidPasswordLength("open sesame", 6, 8); // false

isValidPasswordLength("open sesame", 6); // false

function isValidPasswordLength(
  password: string,
  min: number,
  max?: number
) {
  return password.length >= min  && password.length <= max; // Error: 对象可能为“未定义”.
}

function isValidPasswordLength(
  password: string,
  min: number,
  max: number = Number.MAX_VALUE
) {
  return password.length >= min && password.length <= max;
}

type Constructor<T = {}> = new (..args: any[]) => T;

function Timestamped<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    timestamp = Date.now()
  }
}

type Constructor<T = {}> = new (..args: any[]) => T;

function Timestamped<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    timestamp = Date.now();
  };
}

function Timestamped<TBase extends Constructor>(Base: TBase) {
  return class Timestamped extends Base {
    timestamp = Date.now();
  };
}

class User {
  name: string;

  constructor(name: string) {
    this.name = name;
  }
}

// 通过将`"Timestamped"`混合到"User"中创建一个新类
const TimestampedUser = Timestamped(User);

// 实例化新的 "TimestampedUser" 类
const user = new TimestampedUser("前端小智")

// 现在，咱们可以同时从User 类中访问属性
// 也可以从 Timestamped 类中访问属性
console.log(user.name);
console.log(user.timestamp);

function Tagged<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    tag: string | null;

    constructor(...args: any[]) {
      super(...args);
      this.tag = null;
    }
  };
}

// 通过 User 作为混合 Tagged 来创建一个新类
const TaggedUser = Tagged(User);

// 实例化 "TaggedUser" 类
const user = new TaggedUser("John Doe");

// 现在，可以从 User 类访问属性和 Tagged 中的属性

user.name = "Jane Doe";
user.tag = "janedoe";

fucntion Activatable<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    isActivated = false;

    activate() {
      this.isActivated = true;
    }

    deactivate() {
      this.isActivated = false;
    }
  }
}

const ActivatableUser = Activatable(User);

// 实例化新的"ActivatableUser"类
const user = new ActivatableUser("John Doe");

//初始化，isActivated 的值为 false
console.log(user.isActivated);

user.activate();

console.log(user.isActivated); // true

const SpecialUser = Activatable(Tagged(Timestamped(User)));
const user = new SpecialUser("John Doe");