Readonly

introduction#

Implement the built-in Readonly<T> generic without using it.

Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.

For example

ts
interface Todo {
  title: string;
  description: string;
}
type MyReadonly<T> = any;
const todo: MyReadonly<Todo> = {
  title: 'Hey',
  description: 'foobar',
};
todo.title = 'Hello'; // Error: cannot reassign a readonly property
todo.description = 'barFoo'; // Error: cannot reassign a readonly property

ts
interface Todo {
  title: string;
  description: string;
}
type MyReadonly<T> = any;
const todo: MyReadonly<Todo> = {
  title: 'Hey',
  description: 'foobar',
};
todo.title = 'Hello'; // Error: cannot reassign a readonly property
todo.description = 'barFoo'; // Error: cannot reassign a readonly property

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start point#

ts
/* _____________ Your Code Here _____________ */
type MyReadonly<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [Expect<Equal<MyReadonly<Todo1>, Readonly<Todo1>>>];
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'. 
interface Todo1 {
  title: string;
  description: string;
  completed: boolean;
  meta: {
    author: string;
  };
}Try

ts
/* _____________ Your Code Here _____________ */
type MyReadonly<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [Expect<Equal<MyReadonly<Todo1>, Readonly<Todo1>>>];
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'. 
interface Todo1 {
  title: string;
  description: string;
  completed: boolean;
  meta: {
    author: string;
  };
}Try

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my solution#

Spoiler warning // Click to reveal answer

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

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

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