Get Readonly Keys

introduction#

Implement a generic GetReadonlyKeys<T> that returns a union of the readonly keys of an Object.

For example

ts
interface Todo {
  readonly title: string;
  readonly description: string;
  completed: boolean;
}
type Keys = GetReadonlyKeys<Todo>; // expected to be "title" | "description"

ts
interface Todo {
  readonly title: string;
  readonly description: string;
  completed: boolean;
}
type Keys = GetReadonlyKeys<Todo>; // expected to be "title" | "description"

View on GitHub

start point#

ts
/* _____________ Your Code Here _____________ */
type GetReadonlyKeys<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<'title', GetReadonlyKeys<Todo1>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<'title' | 'description', GetReadonlyKeys<Todo2>>>
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.];
 
interface Todo1 {
  readonly title: string;
  description: string;
  completed: boolean;
}
 
interface Todo2 {
  readonly title: string;
  readonly description: string;
  completed?: boolean;
}Try

ts
/* _____________ Your Code Here _____________ */
type GetReadonlyKeys<T> = any;
 
/* _____________ Test Cases _____________ */
type cases = [
  Expect<Equal<'title', GetReadonlyKeys<Todo1>>>,
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.  Expect<Equal<'title' | 'description', GetReadonlyKeys<Todo2>>>
Type 'false' does not satisfy the constraint 'true'.2344Type 'false' does not satisfy the constraint 'true'.];
 
interface Todo1 {
  readonly title: string;
  description: string;
  completed: boolean;
}
 
interface Todo2 {
  readonly title: string;
  readonly description: string;
  completed?: boolean;
}Try

take the challenge

my solutions#

Spoiler warning // Click to reveal answer

ts
type GetReadonlyKeys<T> = keyof {
  [K in keyof T as Equal<{ [_ in K]: T[K] }, { readonly [_ in K]: T[K] }> extends true
    ? K
    : never]: 1;
};Try

ts
type GetReadonlyKeys<T> = keyof {
  [K in keyof T as Equal<{ [_ in K]: T[K] }, { readonly [_ in K]: T[K] }> extends true
    ? K
    : never]: 1;
};Try

view more solutions