What is a recursive type?

Learn self-referencing types.

What is a recursive type?

A recursive type is a type that references itself in its definition. Linked lists, trees, and nested structures are defined this way. A field points to a value of the same type; this allows modeling data of unlimited depth. The chain can be terminated by making the field optional (?).

type LinkedList<T> = {
  value: T;
  next?: LinkedList<T>;
};

Termination with optional fields

In recursive types, the self-referencing field is made optional (?); this lets the chain end at some point. For example, in next?: LinkedList<T>, if next is absent the list ends. This allows modeling finite data without falling into infinite recursion.

Exercise Create a linked list object

Define a recursive generic type named BagliListe: <T> = { deger: T; sonraki?: BagliListe<T> }. The liste variable should be of type BagliListe<number> with value { deger: 1, sonraki: { deger: 2 } }. Print liste.deger with console.log.

Show example solution

type BagliListe<T> = {
  deger: T;
  sonraki?: BagliListe<T>;
};
const liste: BagliListe<number> = { deger: 1, sonraki: { deger: 2 } };
console.log(liste.deger);

Test yourself

What is the defining feature of a recursive type?
- It contains only primitive types
- It changes at runtime
- It cannot take generic parameters
- It references itself in its definition
Show answer

It references itself in its definition — A recursive type references its own name in its definition; this allows modeling nested data of unlimited depth.

Self-reference

type BagliListe<T> = {
  deger: T;
  sonraki?: ____<T>;
};

Show answer

type BagliListe<T> = {
  deger: T;
  sonraki?: BagliListe<T>;
};

Recursive type concepts

deger

Node value
sonraki?

Optional next node
BagliListe<T>

Self-referencing type
T

Type of node value

Next lesson: Tree types

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