TypeScript 字典：索引签名、Record 与 Map

TypeScript 没有单独的字典或哈希表类。字典要么是用索引签名 { [key: string]: number } 标注类型的普通对象（同一个类型也可以写作 Record<string, number>），要么是 Map<string, number>。三者都按键存储值；区别在于键的类型、缺失的键如何标注类型，以及如何序列化。

// 1. Index signature on an object
const ages: { [name: string]: number } = { ada: 36 };
ages["grace"] = 85;
ages.alan = 41;

// 2. Record: the same type, shorter to write
const prices: Record<string, number> = { tea: 2.5 };
prices["coffee"] = 3.2;

// 3. Map: a real hash map class
const stock = new Map<string, number>();
stock.set("apples", 3);

console.log(ages);            // { ada: 36, grace: 85, alan: 41 }
console.log(prices.coffee);   // 3.2
console.log(stock.get("apples")); // 3

对于字符串键和 JSON 形式的数据，通常选用 Record<string, T> 类型的对象。对于非字符串键，或者条目经常增删的情况，使用 Map。

索引签名

索引签名 [key: KeyType]: ValueType 的意思是“这种类型的任何键都对应这种类型的值”。键名（key、name、userId）只起说明作用。键的类型可以是 string、number、symbol、模板字面量模式，或者它们的联合类型。

type User = { name: string; admin: boolean };

// The key name documents what the key is
const usersById: { [userId: string]: User } = {
    u1: { name: "Ada", admin: true },
};
usersById["u2"] = { name: "Grace", admin: false };

// Index signature plus named properties: named ones must fit the signature
type Scores = {
    total: number;
    [player: string]: number;
};
const scores: Scores = { total: 12, ada: 7, alan: 5 };

// Every value is checked against the value type
// @ts-expect-error: Type 'string' is not assignable to type 'number'.
scores.grace = "nine";

console.log(Object.keys(usersById), scores.total);

JavaScript 会把数字键转换成字符串，所以 { [id: number]: string } 对象在运行时的键仍然是字符串：Object.keys({ 1: "one" }) 的结果是 [ '1' ]。数字索引签名只是限制了你在 TypeScript 中能用什么来索引它。

Record<K, V>

Record<string, V> 是 { [key: string]: V } 的简写。如果用字面量键的联合类型代替 string，它就成了一个必须包含所有键的固定字典：

// Open dictionary: any string key
const counts: Record<string, number> = {};
for (const word of ["a", "b", "a"]) {
    counts[word] = (counts[word] ?? 0) + 1;
}
console.log(counts); // { a: 2, b: 1 }

// Closed dictionary: exactly these keys, all required
type Env = "dev" | "staging" | "prod";
const urls: Record<Env, string> = {
    dev: "http://localhost:3000",
    staging: "https://staging.example.com",
    prod: "https://example.com",
};
console.log(urls.prod);

// Some of the keys: wrap in Partial
const overrides: Partial<Record<Env, number>> = { prod: 5 };
console.log(overrides.dev ?? "default"); // default

在 urls 中漏掉 staging 会报编译错误 TS2741（Property 'staging' is missing...），这让带联合类型键的 Record 成为一张经过检查的查找表。Partial 则会让每个值变成 number | undefined。

把 Map 当作哈希表

Map 接受任何类型的键，保留插入顺序，有 size 属性，而且如实地为缺失的键标注类型：get 返回 V | undefined。

type Point = { x: number; y: number };

// Object keys work: compared by identity
const home: Point = { x: 0, y: 0 };
const labels = new Map<Point, string>([[home, "home"]]);
console.log(labels.get(home)); // home

// A string key built from the contents, for lookup by value
const grid = new Map<string, string>();
const key = (p: Point) => `${p.x},${p.y}`;
grid.set(key({ x: 2, y: 3 }), "tree");
console.log(grid.get(key({ x: 2, y: 3 }))); // tree
console.log(grid.get(key({ x: 9, y: 9 })) ?? "empty"); // empty
console.log(grid.size); // 1

判断键是否存在

有好几种检查方式，它们的含义并不完全相同：

const dict: Record<string, number> = { zero: 0, one: 1 };

console.log(Object.hasOwn(dict, "zero"));   // true: own property
console.log("zero" in dict);                // true
console.log("toString" in dict);            // true: inherited from Object.prototype!
console.log(Object.hasOwn(dict, "toString")); // false

console.log(dict["zero"] !== undefined);    // true
console.log(dict["zero"] ? "truthy" : "falsy"); // falsy: 0 is falsy, the key exists

const value = dict["two"];
if (value !== undefined) {
    console.log(value.toFixed(1)); // not reached
} else {
    console.log("no key 'two'");
}

继承键的问题正是为什么用户提供的 "constructor" 或 "__proto__" 这类键会让普通对象当字典变得有风险。Map 没有这样的键。

缺失键的类型问题

对于索引签名或 Record<string, T>，读取任何键得到的类型都是 T，即使这个键并不存在。编译器会允许你在一个运行时为 undefined 的值上调用方法：

const colors: Record<string, string> = { sky: "blue" };

const grass = colors["grass"]; // type: string, value: undefined
console.log(typeof grass);     // undefined

try {
    console.log(grass.toUpperCase()); // compiles, then throws
} catch (e) {
    if (e instanceof Error) console.log(e.message);
}

编译选项 noUncheckedIndexedAccess 可以解决这个问题：开启后，colors["grass"] 的类型是 string | undefined，在你做检查之前，调用 toUpperCase 会报编译错误。它不属于 strict，所以必须在 tsconfig.json 中单独开启；与它并列的其他开关请参阅严格模式。Map 没有这个漏洞，因为 get 的返回类型总是包含 undefined。

添加、删除和遍历

const inventory: Record<string, number> = { apples: 3, pears: 0 };

inventory["kiwis"] = 12;   // add or update
delete inventory["pears"]; // remove: allowed on index signatures

for (const [item, qty] of Object.entries(inventory)) { // [string, number]
    console.log(item, qty);
}

console.log(Object.keys(inventory).length); // 2 (the size)

// Map equivalents
const m = new Map(Object.entries(inventory)); // Map<string, number>
m.delete("apples");
console.log(m.size, [...m.keys()]); // 1 [ 'kiwis' ]

delete 可以用于索引签名属性。如果用在对象类型中必需的具名属性上，会报编译错误 TS2790：The operand of a 'delete' operator must be optional.

该用哪一种