TypeScript Map: Map<K, V>, get, set, has and Iterate

A TypeScript Map is JavaScript's built-in Map with typed keys and values: Map<string, number> maps strings to numbers. Create one with new Map<K, V>(), then use set, get, has and delete. get returns V | undefined, because the key may be missing.

const ages = new Map<string, number>();

ages.set("ada", 36);
ages.set("grace", 85).set("alan", 41); // set returns the map, so calls chain

console.log(ages.get("ada"));    // 36
console.log(ages.get("linus"));  // undefined
console.log(ages.has("grace"));  // true
console.log(ages.size);          // 3

ages.delete("alan");
console.log(ages); // Map(2) { 'ada' => 36, 'grace' => 85 }

// @ts-expect-error: Argument of type 'string' is not assignable to parameter of type 'number'.
ages.set("bob", "forty");

The type arguments are what make a Map useful in TypeScript: every set is checked and every get returns the value type. The last line is a compile error (TS2345); @ts-expect-error keeps the block running.

This page is about the Map collection. If you searched for array.map(), it is covered in the last section.

Creating a Map

The types come from type arguments, from initial entries, or from an annotation. Initial entries are an array of [key, value] tuples, or anything else that yields them.

// 1. Type arguments, empty map
const scores = new Map<string, number[]>();

// 2. Inferred from entries: Map<string, number>
const prices = new Map([
    ["tea", 2.5],
    ["coffee", 3.2],
]);

// 3. From an object: Map<string, boolean>
const flags = new Map(Object.entries({ darkMode: true, beta: false }));

// 4. Keys and values of any type
type User = { id: number; name: string };
const lastSeen = new Map<User, Date>();
lastSeen.set({ id: 1, name: "Ada" }, new Date("2024-03-15T10:30:00Z"));

console.log(scores.size, prices.get("tea"), flags.get("beta"), lastSeen.size);

Two inference traps:

• new Map() with no type arguments and no entries is Map<any, any>. Nothing you put in or take out is checked. Always write new Map<K, V>().
• Entries with different value types do not infer a union. new Map([["a", 1], ["b", "x"]]) is error TS2769 (No overload matches this call). Write the type: new Map<string, number | string>([...]).

get Returns V | undefined

A Map cannot promise that a key exists, so get is typed V | undefined. Under strict, you must deal with the undefined before using the value as a V:

// This does not compile: get may return undefined.
const stock = new Map<string, number>([["apples", 3]]);

const apples: number = stock.get("apples");
console.log(apples);

index.ts(4,7): error TS2322: Type 'number | undefined' is not assignable to type 'number'.
  Type 'undefined' is not assignable to type 'number'.

The fixes, from most to least common:

const stock = new Map<string, number>([["apples", 3]]);

// 1. A default value with ??
const pears = stock.get("pears") ?? 0;
console.log(pears); // 0

// 2. Store the result and check it
const apples = stock.get("apples");
if (apples !== undefined) {
    console.log(apples * 2); // 6: apples is number here
}

// 3. Update a counter in one line
stock.set("apples", (stock.get("apples") ?? 0) + 1);
console.log(stock.get("apples")); // 4

// has() does not narrow a later get():
if (stock.has("apples")) {
    const n = stock.get("apples"); // still number | undefined
    console.log(n);
}

TypeScript does not remember that has returned true when you later call get. Checking the result of get directly is both shorter and safe. A non-null assertion, stock.get("apples")!, silences the error but gives no protection if the key is missing.

Map Methods and Their Types

Iterating a Map

A Map iterates in insertion order. for...of over the map yields [key, value] tuples, typed [K, V].

const prices = new Map<string, number>([
    ["tea", 2.5],
    ["coffee", 3.2],
    ["juice", 4],
]);

for (const [item, price] of prices) {   // item: string, price: number
    console.log(`${item}: ${price.toFixed(2)}`);
}

prices.forEach((price, item) => {       // value first, then key
    console.log(item.toUpperCase(), price);
});

const items = [...prices.keys()];       // string[]
const total = [...prices.values()].reduce((a, b) => a + b, 0);
console.log(items, total);              // [ 'tea', 'coffee', 'juice' ] 9.7

// Sorted by value: sort the entries, build a new Map
const byPrice = new Map([...prices].sort((a, b) => b[1] - a[1]));
console.log([...byPrice.keys()]);       // [ 'juice', 'coffee', 'tea' ]

Setting an existing key updates the value but keeps its original position in the order.

Object Keys and Counting

Any value can be a key, including objects and arrays. Keys are compared like ===: two objects with the same contents are different keys. A Map is also the standard way to count or group items.

const a = { id: 1 };
const b = { id: 1 };
const visits = new Map<object, number>([[a, 5]]);
console.log(visits.get(a), visits.get(b)); // 5 undefined: b is another object

// Counting words
const words = "the cat and the hat and the bat".split(" ");
const counts = new Map<string, number>();
for (const w of words) {
    counts.set(w, (counts.get(w) ?? 0) + 1);
}
console.log(counts); // Map(5) { 'the' => 3, 'cat' => 1, 'and' => 2, 'hat' => 1, 'bat' => 1 }

// Grouping into arrays
const byLength = new Map<number, string[]>();
for (const w of new Set(words)) {
    const group = byLength.get(w.length) ?? [];
    group.push(w);
    byLength.set(w.length, group);
}
console.log(byLength); // Map(1) { 3 => [ 'the', 'cat', 'and', 'hat', 'bat' ] }

To key by an object's contents, derive a string or number key instead, such as user.id or `${x},${y}`.

Map vs Object vs Record

Rule of thumb: a Map for a collection whose keys are data (user ids, words, cache entries) and change at runtime; an object type or Record for a fixed set of known keys and for anything that goes to or from JSON. The dictionary page compares index signatures, Record and Map for string-keyed lookups.

Converting Maps to Objects and JSON

A Map's entries are not properties, so JSON.stringify does not see them. Convert through Object.fromEntries and Object.entries:

const settings = new Map<string, string | number>([
    ["theme", "dark"],
    ["fontSize", 14],
]);

console.log(JSON.stringify(settings)); // {}

const obj = Object.fromEntries(settings); // { [k: string]: string | number }
const json = JSON.stringify(obj);
console.log(json); // {"theme":"dark","fontSize":14}

// And back again
const restored = new Map(Object.entries(JSON.parse(json) as Record<string, string | number>));
console.log(restored.get("theme")); // dark

JSON.parse returns any, so the as Record<...> states what the data is expected to be. It is not a runtime check; validate untrusted JSON before trusting that type.

Typing array.map()

Many searches for "typescript map" mean the array method, which transforms each element and returns a new array. Its type is inferred from the callback, so annotations are rarely needed:

type User = { id: number; first: string; last: string };
const users: User[] = [
    { id: 1, first: "Ada", last: "Lovelace" },
    { id: 2, first: "Alan", last: "Turing" },
];

const names = users.map((u) => `${u.first} ${u.last}`); // string[]
const ids = users.map((u) => u.id);                     // number[]

// An explicit result type, checked against what the callback returns
type Option = { value: number; label: string };
const options = users.map((u): Option => ({ value: u.id, label: u.first }));

// With the index
const numbered = users.map((u, i) => `${i + 1}. ${u.first}`);

console.log(names, ids);
console.log(options);
console.log(numbered);

Annotating the callback's return type ((u): Option => ...) is the clearest way to state the result type: a missing or misspelled property in the returned object is then a compile error at the callback.