AI solution for TypeScript puzzle on BFE.dev
3. implement Readonly<T>

Here's the implementation of MyReadonly<T>:

type MyReadonly<T> = {
  readonly [K in keyof T]: T[K]
};

Explanation:

• MyReadonly<T> is a mapped type that iterates over all properties of T.
• For each property K, we declare a readonly property with the same name and type T[K].
• This effectively sets all properties of T to readonly.

We can use MyReadonly to create a readonly version of Foo:

type Foo = {
  a: string
}

const a: Foo = {
  a: 'BFE.dev',
}
a.a = 'bigfrontend.dev'
// OK

const b: MyReadonly<Foo> = {
  a: 'BFE.dev'
}
b.a = 'bigfrontend.dev'
// Error: Cannot assign to 'a' because it is a read-only property.

Now, a.a = 'bigfrontend.dev' is allowed because a is of type Foo, but b.a = 'bigfrontend.dev' is not allowed because b is of type MyReadonly<Foo>, which has readonly properties.