基础类型与类型注解

let count: number = 0;
const label: string = "总计";
 
function add(a: number, b: number): number {
  return a + b;
}

let s: string = null;        // 错误：不能将 null 分配给 string
let t: string | null = null; // 正确：显式声明可能为空

const title: string = "TypeScript";
const version: number = 5.6;
const stable: boolean = true;
const empty: null = null;
const missing: undefined = undefined;
const key: symbol = Symbol("userId");
const huge: bigint = 9007199254740993n;
 
console.log(title + " v" + version + " stable=" + stable);
console.log("null 的 typeof 是 " + typeof empty);
console.log("undefined 的 typeof 是 " + typeof missing);
console.log("symbol 描述: " + String(key.description));
console.log("bigint: " + huge.toString());
 
// strictNullChecks 下必须显式声明可空
let nickname: string | null = null;
console.log("nickname = " + String(nickname));
nickname = "Ada";
console.log("nickname = " + nickname);

let x: any = 42;
x.foo.bar.baz();   // 编译通过，运行时炸
x = "现在是字符串";

function describe(value: unknown): string {
  if (typeof value === "string") {
    return "字符串，长度 " + value.length;
  }
  if (typeof value === "number") {
    return "数字，两倍是 " + value * 2;
  }
  if (Array.isArray(value)) {
    return "数组，元素个数 " + value.length;
  }
  if (value === null) {
    return "空值";
  }
  return "其他类型: " + typeof value;
}
 
console.log(describe("hello"));
console.log(describe(21));
console.log(describe([1, 2, 3]));
console.log(describe(null));
console.log(describe(true));
 
// 下面这行会被类型检查拦下，因为 unknown 未收窄不能访问属性
// function bad(v: unknown) { return v.length; }

function log(msg: string): void {
  console.log(msg);
}

type Shape = "circle" | "square" | "triangle";
 
function fail(x: never): never {
  throw new Error("未处理的分支: " + String(x));
}
 
function sides(shape: Shape): number {
  switch (shape) {
    case "circle":
      return 0;
    case "square":
      return 4;
    case "triangle":
      return 3;
    default:
      // 到这里 shape 的类型已被收窄为 never
      // 如果将来给 Shape 加了新成员却忘了处理，这里会立刻报类型错误
      return fail(shape);
  }
}
 
console.log("circle -> " + sides("circle"));
console.log("square -> " + sides("square"));
console.log("triangle -> " + sides("triangle"));
 
function loopForever(): never {
  throw new Error("这个函数永远不会正常返回");
}
try {
  loopForever();
} catch (e) {
  console.log("捕获到: " + (e instanceof Error ? e.message : "未知"));
}

let n = 42;             // 推断为 number
const s = "hi";         // 推断为字面量类型 "hi"
let arr = [1, 2, 3];    // 推断为 number[]

const names = ["ada", "alan"];
names.forEach((n) => console.log(n.toUpperCase()));
// n 被上下文推断为 string，不需要写 (n: string)

let mutable = "hi";        // string
const fixed = "hi";        // 字面量类型 "hi"
const nums = [1, 2, 3];    // number[]
const mixed = [1, "two"];  // (string | number)[]
 
mutable = "bye";
console.log(mutable + " / " + fixed);
console.log("nums 求和 = " + nums.reduce((a, b) => a + b, 0));
console.log("mixed 长度 = " + mixed.length);
 
// 上下文推断：这里的 item 自动是 string
const words = ["alpha", "beta", "gamma"];
words.forEach((item, index) => {
  console.log(index + ": " + item.toUpperCase());
});
 
// 返回类型也能推断，但导出 API 建议显式写出
function double(n: number) {
  return n * 2;               // 推断返回 number
}
console.log("double(21) = " + double(21));

const el = document.getElementById("app") as HTMLDivElement;
const data = JSON.parse(text) as { id: number };

const el = document.getElementById("app")!;   // 去掉 null