How to Initialize a Map in TypeScript (With Examples)

To initialize a Map in TypeScript with data, pass an array of [key, value] pairs to the constructor and put the types in angle brackets:

const prices = new Map<string, number>([["SKU-1001", 19.99]]);

That gives a typed Map<string, number>, and TypeScript rejects later set() calls with the wrong types.

In this tutorial, I’ll show you how to initialize a Map in TypeScript five ways, using a small SKU price list I compiled with TypeScript 7.0.2 and Node.js v22.22.2 in strict mode.

How do you initialize an empty Map in TypeScript?

Write the types in the generic, then add entries with set(), which returns the Map so calls chain:

const prices = new Map<string, number>();
prices.set("SKU-1001", 19.99).set("SKU-1002", 4.5);

console.log("size:", prices.size);
console.log("SKU-1002:", prices.get("SKU-1002"));
console.log("SKU-9999:", prices.get("SKU-9999"));

Output:

size: 2
SKU-1002: 4.5
SKU-9999: undefined

The size is 2, and a missing key gives undefined. Skip the generic and you’ll get Map<any, any>. For the bigger picture, see key value pairs in TypeScript.

Initialize a Map with values from an array of pairs

Same price list, now passed straight into the constructor:

const prices = new Map<string, number>([
  ["SKU-1001", 19.99],
  ["SKU-1002", 4.5],
  ["SKU-1003", 129],
]);

for (const [sku, price] of prices) {
  console.log(sku, `$${price.toFixed(2)}`);
}

Output:

SKU-1001 $19.99
SKU-1002 $4.50
SKU-1003 $129.00

Leave out the generic and TypeScript infers the types from the pairs. The for…of loop in TypeScript gives you each pair as a destructured [key, value].

Got an array of objects instead? You can create a Map from an array in TypeScript too.

Initialize a Map from an object with Object.entries()

The constructor won’t take a plain object, but Object.entries() turns one into pairs. Here the price list starts as a Record:

const priceList: Record<string, number> = {
  "SKU-1001": 19.99,
  "SKU-1002": 4.5,
  "SKU-1003": 129,
};

const prices = new Map(Object.entries(priceList));
console.log(prices);

const back = Object.fromEntries(prices);
console.log(back);

Output:

Map(3) { 'SKU-1001' => 19.99, 'SKU-1002' => 4.5, 'SKU-1003' => 129 }
{ 'SKU-1001': 19.99, 'SKU-1002': 4.5, 'SKU-1003': 129 }

Object.fromEntries() goes the other way, which is handy for JSON. If you’re unsure which shape to keep, compare Record vs Map in TypeScript.

Initialize a Map with object values or object keys

Values can be any type, so an interface works well. Keys can be objects too:

interface Product { name: string; price: number; inStock: boolean }

const catalog = new Map<string, Product>([
  ["SKU-1001", { name: "Desk lamp", price: 19.99, inStock: true }],
  ["SKU-1002", { name: "Coffee mug", price: 4.5, inStock: false }],
]);
console.log(catalog.get("SKU-1001")?.name);

const megan = { name: "Megan Bowen" };
const visits = new Map<object, number>([[megan, 3]]);
console.log(visits.get(megan));
console.log(visits.get({ name: "Megan Bowen" }));

Output:

Desk lamp
3
undefined

Important: object keys match by reference, not by content.

The ?. guards get(), which can return undefined. To list the values later, convert the Map to an array.

Copy an existing Map in TypeScript

Pass one Map to the constructor and you’ll get a new Map with the same entries:

const prices = new Map<string, number>([["SKU-1001", 19.99]]);

const sale = new Map(prices);
sale.set("SKU-1001", 17.99);
console.log("original:", prices.get("SKU-1001"));
console.log("copy:    ", sale.get("SKU-1001"));

Output:

original: 19.99
copy:     17.99

The original keeps 19.99. The copy is shallow, so object values stay shared; see how to deep clone an object in TypeScript. Combining two Maps is a separate job covered in merging Maps in TypeScript.

ReadonlyMap drops set(), delete() and clear() from the type; use it when a Map shouldn’t change after setup.

Common errors when initializing a Map

Two mistakes come up most: passing an object to the constructor and writing to a ReadonlyMap:

const fromObject = new Map({ "SKU-1001": 19.99 });

const locked: ReadonlyMap<string, number> = new Map();
locked.set("SKU-1002", 4.5);

Output:

map_errors.ts(1,30): error TS2769: No overload matches this call.
  The last overload gave the following error.
    Object literal may only specify known properties, and '"SKU-1001"' does not exist in type 'readonly (readonly [unknown, unknown])[]'.
map_errors.ts(4,8): error TS2339: Property 'set' does not exist on type 'ReadonlyMap<string, number>'.

Fix the first with Object.entries(). For the second, edit a copy: new Map(locked).

Working with key-value data elsewhere? These pair well with Maps:

• TypeScript Record type explained
• Iterate over a Record in TypeScript
• Find a key by value in a TypeScript dictionary
• Convert an array to a dictionary in TypeScript

Frequently asked questions