TypeScript readonly: Properties, Readonly<T> and Arrays

readonly marks a property that can be set once, when the object is created, and never reassigned. Readonly<T> applies it to every property of a type, and readonly T[] does the same for arrays:

interface Point {
  readonly x: number;
  readonly y: number;
}

const origin: Point = { x: 0, y: 0 };
// @ts-expect-error: Cannot assign to 'x' because it is a read-only property
origin.x = 5;

const config: Readonly<{ host: string; port: number }> = { host: "localhost", port: 8080 };
const ports: readonly number[] = [80, 443];

console.log(config.host, ports.length); // localhost 2
console.log(origin);                    // { x: 5, y: 0 }

The last line shows the most important fact about readonly: it is checked by the compiler, not enforced at runtime. The assignment was a compile error (suppressed here with @ts-expect-error), yet the emitted JavaScript still ran it. Without the suppression, the file would not compile, and that is where readonly does its job.

readonly Properties

Put readonly before a property name in an interface, a type literal or a class. The property can be initialized but not reassigned:

class Invoice {
  readonly id: string;
  readonly createdAt = "2024-03-15";
  total = 0;

  constructor(id: string) {
    this.id = id; // allowed: the constructor may initialize readonly fields
  }

  add(amount: number) {
    this.total += amount; // fine: total is not readonly
    // this.id = "x" would be error TS2540: Cannot assign to 'id' because it is a read-only property.
  }
}

const inv = new Invoice("INV-7");
inv.add(19.5);
console.log(inv); // Invoice { id: 'INV-7', createdAt: '2024-03-15', total: 19.5 }

In a class, a readonly field can be assigned in its declaration or in the constructor, and nowhere else. The shortest form is a parameter property, constructor(readonly id: string) {}, which declares and assigns the field in one step. The classes page covers fields and constructors in general.

Readonly<T>: Every Property at Once

Readonly<T> is a utility type that marks every property of T as readonly. It is useful for values you pass around but should not change, such as application state:

interface State {
  user: string;
  cart: string[];
  total: number;
}

function addItem(state: Readonly<State>, item: string, price: number): State {
  // state.total += price would be error TS2540
  return { ...state, cart: [...state.cart, item], total: state.total + price };
}

const s0: State = { user: "ada", cart: [], total: 0 };
const s1 = addItem(s0, "book", 12);
const s2 = addItem(s1, "pen", 2);

console.log(s0.cart, s2); // [] { user: 'ada', cart: [ 'book', 'pen' ], total: 14 }

The function signature tells the reader that addItem returns a new state instead of changing the old one, and the compiler holds the function to it. Readonly<T> is defined as the mapped type { readonly [P in keyof T]: T[P] }.

Readonly Arrays: readonly T[] and ReadonlyArray<T>

readonly number[] and ReadonlyArray<number> are the same type. They remove every mutating method (push, pop, shift, splice, sort, reverse, fill...) and forbid index assignment. The non-mutating methods remain and return ordinary arrays:

const scores: readonly number[] = [42, 7, 19];

const doubled = scores.map((s) => s * 2);        // number[] (a new, mutable array)
const sorted = [...scores].sort((a, b) => a - b); // copy first, then sort the copy
const top = scores.slice(0, 2);

// @ts-expect-error: Property 'push' does not exist on type 'readonly number[]'
scores.push(1);

console.log(doubled, sorted, top); // [ 84, 14, 38 ] [ 7, 19, 42 ] [ 42, 7 ]
console.log(scores);               // [ 42, 7, 19, 1 ]  (push ran: types are erased)

Taking readonly T[] as a parameter is a promise to callers that you will not modify their array. The other direction is where people get stuck: a readonly array cannot be passed to a function that takes a plain T[], because that function might mutate it.

// This does not compile: a readonly array cannot go where a mutable one is expected
function sum(values: number[]): number {
  return values.reduce((a, b) => a + b, 0);
}

const prices: readonly number[] = [3, 4, 5];
console.log(sum(prices));

index.ts(7,17): error TS4104: The type 'readonly number[]' is 'readonly' and cannot be assigned to the mutable type 'number[]'.

The fix is to change sum to accept readonly number[], since it does not mutate anything. Functions that only read an array should always take the readonly type; then they accept both kinds. If you do not own the function, pass a copy: sum([...prices]).

ReadonlyMap and ReadonlySet

Maps and sets have readonly versions too. ReadonlyMap<K, V> has get, has, size, forEach and the iterators but no set, delete or clear; ReadonlySet<T> has no add, delete or clear:

const prices: ReadonlyMap<string, number> = new Map([
  ["tea", 2.5],
  ["coffee", 3.2],
]);
const admins: ReadonlySet<string> = new Set(["ada"]);

console.log(prices.get("tea"), prices.size, admins.has("ada")); // 2.5 2 true

// @ts-expect-error: Property 'set' does not exist on type 'ReadonlyMap<string, number>'
prices.set("juice", 4);

A class often keeps a private mutable Map and exposes it through a getter typed ReadonlyMap, so outside code can read the data but not change it through that reference.

readonly Is Shallow

readonly and Readonly<T> protect only the property itself, not the object or array it points to:

interface Team {
  readonly name: string;
  readonly members: string[];
  readonly lead: { name: string };
}

const team: Readonly<Team> = { name: "Core", members: ["ada"], lead: { name: "ada" } };

team.members.push("linus"); // allowed: the array itself is not readonly
team.lead.name = "linus";   // allowed: only the lead property is readonly
// team.members = [] would be error TS2540

console.log(team); // { name: 'Core', members: [ 'ada', 'linus' ], lead: { name: 'linus' } }

type DeepReadonly<T> = T extends (...args: any[]) => unknown ? T
  : T extends object ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
  : T;

const frozenTeam: DeepReadonly<Team> = team;
// @ts-expect-error: Property 'push' does not exist on type 'readonly string[]'
frozenTeam.members.push("grace");

DeepReadonly<T> applies itself to every nested object type, and since a mapped type over an array type produces a readonly array, members becomes readonly string[]. It is still a type-level promise, not runtime protection.

Compile-Time Only: Mutation Through Another Reference

A readonly type controls what one reference may do. Another reference to the same object, typed without readonly, can change it, and TypeScript even allows assigning a readonly type to a mutable one:

interface Settings {
  readonly theme: string;
}

const settings: Settings = { theme: "dark" };
const mutable: { theme: string } = settings; // allowed, no error
mutable.theme = "light";

console.log(settings.theme); // light

// Runtime protection needs Object.freeze
const locked = Object.freeze({ theme: "dark" }); // typed Readonly<{ theme: "dark" }>
try {
  (locked as { theme: string }).theme = "light";
} catch (e) {
  console.log((e as Error).name); // TypeError
}
console.log(locked.theme); // dark

The assignment mutable = settings compiles because TypeScript does not factor in readonly properties when it checks whether two object types are compatible; the TypeScript handbook states this directly, and notes that readonly properties can therefore change through aliasing. Readonly arrays are different: the TS4104 error above is exactly that check. Object.freeze really prevents changes at runtime: the emitted code runs in strict mode, where writing to a frozen property throws a TypeError. Like readonly, Object.freeze is shallow.

readonly, const, as const and Object.freeze

as const on a literal makes every property readonly at every depth and keeps literal types, which is often the easiest way to get a deeply readonly value:

const theme = { mode: "dark", sizes: [12, 14] } as const;
// { readonly mode: "dark"; readonly sizes: readonly [12, 14] }

const and readonly answer different questions: const stops the name from pointing somewhere else, readonly stops a property from changing. A const object's properties can still be reassigned unless they are readonly.