Deep partial

Learn the deep Partial pattern.

What is deep partial?

DeepPartial, unlike normal Partial, also makes all fields of nested objects optional. It is written by combining a mapped type and a conditional type: for each field, if it is an object, DerinPartial is applied recursively. This makes deeply nested configuration type-safe.

type DeepPartial<T> = {
  [K in keyof T]?: T[K] extends object
    ? DeepPartial<T[K]>
    : T[K];
};

Object check with conditional

In DeepPartial, the condition T[K] extends object checks whether the field is an object. If it is, DerinPartial<T[K]> is applied recursively; otherwise the value is made optional as-is. This way primitive fields are preserved while nested objects are processed deeply.

Exercise Write and use DeepPartial

Define a recursive mapped type named DerinPartial: <T> = { [K in keyof T]?: T[K] extends object ? DerinPartial<T[K]> : T[K] }. The Ayarlar interface should contain baslik: string and alt: { renk: string }. The p variable should be of type DerinPartial<Ayarlar> with value { baslik: "test", alt: { renk: "mavi" } }. Print p.alt.renk with console.log.

Show example solution

type DerinPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DerinPartial<T[K]> : T[K];
};
interface Ayarlar {
  baslik: string;
  alt: { renk: string; };
}
const p: DerinPartial<Ayarlar> = { baslik: "test", alt: { renk: "mavi" } };
console.log(p.alt.renk);

Test yourself

What is the difference between DeepPartial and normal Partial?
- Takes no type parameter
- Runs faster
- Makes only numbers optional
- Also makes nested object fields optional
Show answer

Also makes nested object fields optional — Partial only makes top-level fields optional; DeepPartial recursively descends into nested objects and makes all fields optional.

Recursive DerinPartial call

type DerinPartial<T> = {
  [K in keyof T]?: T[K] extends object ? ____<T[K]> : T[K];
};

Show answer

type DerinPartial<T> = {
  [K in keyof T]?: T[K] extends object ? DerinPartial<T[K]> : T[K];
};

DeepPartial concepts

Partial<T>

Only top level
DerinPartial<T>

Nested optional
keyof T

Keys of T
extends object

Object check

Next lesson: Summary - Recursive types

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies