TypeScript Dictionary: Index Signatures, Record and Map

TypeScript has no separate dictionary or hashmap class. A dictionary is either a plain object typed with an index signature, { [key: string]: number }, the same type written Record<string, number>, or a Map<string, number>. All three store values by key; they differ in key types, how missing keys are typed, and how they serialize.

// 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

For string keys and JSON-shaped data, an object with Record<string, T> is the usual choice. For non-string keys, or entries added and removed all the time, use a Map.

Index Signatures

An index signature, [key: KeyType]: ValueType, says "any key of this type maps to a value of this type". The key name (key, name, userId) is only documentation. Key types can be string, number, symbol, template literal patterns, or unions of these.

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);

Number keys are converted to strings by JavaScript, so a { [id: number]: string } object still has string keys at runtime: Object.keys({ 1: "one" }) is [ '1' ]. The number index signature only restricts how you may index it in TypeScript.

Record<K, V>

Record<string, V> is shorthand for { [key: string]: V }. With a union of literal keys instead of string, it becomes a fixed dictionary that must contain every key:

// 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

Leaving staging out of urls is compile error TS2741 (Property 'staging' is missing...), which makes Record with union keys a checked lookup table. Partial makes each value number | undefined.

Map as a Hash Map

A Map accepts keys of any type, keeps insertion order, has a size, and types missing keys honestly: get returns 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

Checking if a Key Exists

Several checks exist, and they do not all mean the same thing:

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'");
}

The inherited-key problem is why user-provided keys like "constructor" or "__proto__" make plain objects risky as dictionaries. A Map has no such keys.

The Missing-Key Type Problem

On an index signature or Record<string, T>, reading any key has type T, even a key that does not exist. The compiler lets you call methods on a value that is undefined at runtime:

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);
}

The compiler option noUncheckedIndexedAccess fixes this: with it, colors["grass"] has type string | undefined and the toUpperCase call is a compile error until you check it. It is not part of strict, so it must be turned on separately in tsconfig.json; see strict mode for the other flags it sits beside. A Map has no such gap, since get always includes undefined.

Adding, Removing and Iterating

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 works on index-signature properties. On a required named property of an object type it is compile error TS2790, The operand of a 'delete' operator must be optional.

Which One to Use