Object shapes

Readonly

Adds `readonly` to every property of an object, one level deep.

What it is

The standard homomorphic mapped type for a read-only view. It walks `keyof T`, adds the modifier to every property, and preserves the rest of the shape. The word “view” matters: the generated JavaScript contains no freeze and a mutable alias may still change the same object.

Examples

```
Readonly<{ a: string; b: number }>
```
→{ readonly a: string; readonly b: number; }
```
Readonly<{ nested: { value: number } }>
```
→{ readonly nested: { value: number; }; }
The outer reference is read-only; `nested.value` is not.
```
Readonly<[1, 2]>
```
→readonly [1, 2]

Each resolved type above was printed by TypeScript 6.0.3, not written by hand.

What it does not do

It is not deep. Nested objects keep their original modifiers, so a read-only property can still point at mutable data.
It does not call `Object.freeze` or survive at run time. It restricts assignments the compiler can see, not mutations performed elsewhere.

Takeaway

`Readonly<T>` is a compile-time read-only view of one level. Use recursion for depth and a run-time operation for actual freezing.