Mutable

Problem Link

Problem

Implement the generic Mutable<T> which makes all properties in T mutable (removes readonly).

interface Todo {
  readonly title: string
  readonly description: string
  readonly completed: boolean
}

type MutableTodo = Mutable<Todo>
// {
//   title: string
//   description: string
//   completed: boolean
// }

Solution

Approach 1: Mapped Type with -readonly Modifier

The most direct solution uses TypeScript's -readonly modifier in a mapped type to strip the readonly qualifier from every property:

type Mutable<T extends object> = {
  -readonly [K in keyof T]: T[K]
}

How it works:

• [K in keyof T] iterates over all keys of T.
• -readonly removes the readonly modifier from each property.
• T[K] preserves the original value type unchanged.

This is the idiomatic, one-liner solution.

Approach 2: Using as Remapping (explicit)

You can make the remapping more explicit with as:

type Mutable<T extends object> = {
  -readonly [K in keyof T as K]: T[K]
}

This is functionally identical to Approach 1 but shows the key remapping step explicitly. Useful when you also want to filter or rename keys.

Approach 3: Utility Type Composition

If you already have built-in Required in scope, you might be tempted to combine it, but Required only removes ? (optionality), not readonly. There's no built-in Mutable in TypeScript's standard library — that's exactly why this challenge exists!

For reference, the mirror type Readonly<T> from the standard library adds readonly:

// Standard library Readonly (adds readonly):
type Readonly<T> = {
  readonly [K in keyof T]: T[K]
}

// Our Mutable (removes readonly):
type Mutable<T extends object> = {
  -readonly [K in keyof T]: T[K]
}

Approach 4: Deep Mutable

The challenge only asks for shallow mutability, but in practice you often need recursion:

type DeepMutable<T> = T extends object
  ? { -readonly [K in keyof T]: DeepMutable<T[K]> }
  : T

This recursively removes readonly from nested objects as well.

Key Takeaways

• TypeScript supports + and - prefixes on mapped type modifiers (readonly and ?).
• -readonly removes the readonly modifier; +readonly (or just readonly) adds it.
• -? removes optionality (same as Required<T>); +? adds it.
• The standard library has no Mutable<T> — you need to write it yourself.

Modifier	Effect
readonly or +readonly	Make property readonly
-readonly	Remove readonly
? or +?	Make property optional
-?	Make property required