let value: any = "hello";
value = 42;           // OK
value = true;         // OK
value.toUpperCase();  // OK（運(yùn)行時(shí)可能出錯(cuò)）

let data: any = { name: "Alice" };
console.log(data.age.toFixed());  // ? 編譯通過(guò)，運(yùn)行時(shí)報(bào)錯(cuò)（Cannot read property 'toFixed' of undefined）

let dangerous: any = "text";
let safe: number = dangerous;   // 不報(bào)錯(cuò)，但 safe 實(shí)際是 string，后續(xù)使用可能出問(wèn)題

let value: unknown = "hello";
value = 42;           // OK
value = true;         // OK

// 直接使用會(huì)報(bào)錯(cuò)
value.toUpperCase();  // ? 類型 unknown 沒(méi)有 toUpperCase 方法

// 必須先收窄類型
if (typeof value === "string") {
    console.log(value.toUpperCase());  // OK
}

function safeParse(input: string): unknown {
    return JSON.parse(input);
}

const result = safeParse('{"name":"Alice"}');
// 不能直接訪問(wèn) result.name，需要收窄
if (typeof result === "object" && result !== null && "name" in result) {
    console.log(result.name);  // OK
}

function logMessage(msg: string): void {
    console.log(msg);
    // 沒(méi)有 return，或 return; 或 return undefined;
}

let unusable: void = undefined;
unusable = null;  // 僅在 strictNullChecks: false 時(shí)允許

function throwError(message: string): never {
    throw new Error(message);
}

function infiniteLoop(): never {
    while (true) {}
}

type Shape = "circle" | "square";

function area(shape: Shape) {
    switch (shape) {
        case "circle":
            return Math.PI * 1 ** 2;
        case "square":
            return 1 * 1;
        default:
            const _exhaustiveCheck: never = shape;  // 如果 shape 類型被擴(kuò)展，這里會(huì)報(bào)錯(cuò)
            return _exhaustiveCheck;
    }
}

// strictNullChecks: true
let name: string = "Alice";
name = null;      // ? 不能將 null 賦給 string
name = undefined; // ?

let age: number | null = 30;
age = null;       // OK

let maybeName: string | null = "Alice";
maybeName = null;   // OK

let maybeAge: number | undefined = 25;
maybeAge = undefined; // OK

interface User {
    address?: {
        city?: string;
    };
}

let user: User = {};

// 傳統(tǒng)方式
let city = user.address && user.address.city;  // undefined

// 可選鏈
let city2 = user.address?.city;  // undefined

let result = obj.method?.();   // 如果 method 不存在，返回 undefined
let item = arr?.[0];           // 如果 arr 是 null/undefined，返回 undefined

let value1 = null ?? "default";   // "default"
let value2 = undefined ?? "default"; // "default"
let value3 = 0 ?? "default";      // 0（不是 null/undefined，保留原值）
let value4 = "" ?? "default";     // ""（同上）

// 對(duì)比 ||
let value5 = 0 || "default";      // "default"（|| 會(huì)將 0 視為假值）

let element = document.getElementById("app")!;  // 斷言該元素一定存在
element.innerHTML = "Hello";

// 也可以用于屬性訪問(wèn)
interface User {
    name?: string;
}
let user: User = { name: "Alice" };
let name = user.name!.toUpperCase();  // 斷言 name 存在

let data: any = fetchData();
let result = data.user.name;  // 無(wú)報(bào)錯(cuò)，但可能運(yùn)行時(shí)崩潰

function fn(): void {
    return undefined;  // OK
}
function fn2(): void {
    return null;       // ? 嚴(yán)格模式下不能返回 null
}

let maybeNum: number | null = Math.random() > 0.5 ? 42 : null;
let definitelyNum = maybeNum!;  // 如果為 null，運(yùn)行時(shí)錯(cuò)誤

let val = obj?.prop ?? "default";
// 如果 obj 不存在，obj?.prop 返回 undefined，觸發(fā) ??
// 如果 obj.prop 存在但值為 null，也會(huì)觸發(fā) ??

// 模擬 API 響應(yīng)
interface ApiResponse<T> {
    data?: T;
    error?: string;
}

// 處理不確定的響應(yīng)
async function fetchUser(): Promise<ApiResponse<{ name: string }>> {
    // 模擬隨機(jī)成功或失敗
    const success = Math.random() > 0.5;
    if (success) {
        return { data: { name: "Alice" } };
    } else {
        return { error: "Network error" };
    }
}

async function handleUser() {
    const response = await fetchUser();
    
    // 使用 unknown 安全處理
    const unknownData: unknown = response.data;
    
    // 類型收窄
    if (unknownData && typeof unknownData === "object" && "name" in unknownData) {
        console.log(unknownData.name);  // 類型推斷為 string
    } else {
        console.log(response.error ?? "Unknown error");
    }
    
    // 可選鏈與空值合并示例
    const userName = response.data?.name ?? "Guest";
    console.log(`User: ${userName}`);
}

// never 示例：類型守衛(wèi)窮盡檢查
type Status = "pending" | "success" | "error";

function getStatusMessage(status: Status): string {
    switch (status) {
        case "pending":
            return "Loading...";
        case "success":
            return "Success!";
        case "error":
            return "Failed.";
        default:
            const _exhaustive: never = status;  // 如果 Status 增加新值，這里會(huì)報(bào)錯(cuò)
            return _exhaustive;
    }
}