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Tree types

Learn to express tree structures as types.

What are tree types?

A tree type is defined by each node containing a value and an array of child nodes. The cocuklar: Dugum[] field references the same type again; this allows unlimited branching. File systems, categories, and organization structures are modeled this way.

type Node = {
  name: string;
  children: Node[];
};

Tree depth

In a tree type, since each node carries child nodes within itself, the data can be of unlimited depth. Thanks to cocuklar: Dugum[], each child node conforms to the same structure. At leaf nodes, cocuklar is an empty array and branching ends.

Practice Create a tree object

Define a recursive type named Dugum: { ad: string; cocuklar: Dugum[] }. The kok variable should be of type Dugum with value { ad: "kok", cocuklar: [{ ad: "cocuk1", cocuklar: [] }] }. Print kok.ad with console.log.

Starter code

index.ts
// Dugum adinda recursive type tanimla
// ad ve cocuklar alanlari olsun
// kok nesnesini bir cocukla tanimla
// kok.ad yazdir

Sample solution

index.ts
type Dugum = {
  ad: string;
  cocuklar: Dugum[];
};
const kok: Dugum = { ad: "kok", cocuklar: [{ ad: "cocuk1", cocuklar: [] }] };
console.log(kok.ad);

Expected output

kok

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 What is the cocuklar field of a leaf node in a tree type?

  • null
  • Itself again
  • Empty array [] Correct answer
  • undefined

Explanation: A leaf node has no children, so its cocuklar field is an empty array []; not null or undefined.

2 Children field type

Question 
type Dugum = {
  ad: string;
  cocuklar: ____[];
};
Answer 
type Dugum = {
  ad: string;
  cocuklar: Dugum[];
};

Explanation: In a tree type, the cocuklar field is of type Dugum[]; each child node conforms to the same structure.

3 Tree type concepts

Term Meaning
cocuklar: Dugum[] Child node array
ad: string Node name
yaprak Childless node
Dugum Self-referencing type

All lessons in Advanced

1 Advanced generics

  1. Variadic tuples
  2. Tuple inference
  3. Recursive generic types
  4. Summary - Advanced generics

2 Template literal types

  1. What is a template literal?
  2. Combination with union
  3. Generation with mapped
  4. Summary - Template literal types

3 Recursive types

  1. What is a recursive type?
  2. Tree types
  3. Deep partial
  4. Summary - Recursive types

4 Variance

  1. Covariance
  2. Contravariance
  3. Bivariance and strict
  4. Summary - Variance

5 Branded types

  1. What is a branded type?
  2. Brand definition
  3. Using brands
  4. Summary - Branded types

6 Phantom types

  1. What is a phantom type?
  2. State Encoding
  3. API with phantom
  4. Summary - Phantom types

7 Type narrowing

  1. typeof narrowing
  2. in operator
  3. instanceof
  4. Summary - Type narrowing

8 Discriminated unions

  1. What is a discriminated union?
  2. Literal discriminant
  3. Pattern matching
  4. Summary - Discriminated unions

9 Exhaustive check

  1. Exhaustive with never
  2. Switch exhaustive
  3. Compile-time safety
  4. Summary - Exhaustive check

10 Type guards

  1. User-defined type guard
  2. Assertion function
  3. Primitives guard
  4. Summary - Type guards

11 Assertion functions

  1. asserts is
  2. asserts defined
  3. Assertion vs throw
  4. Summary - Assertion functions

12 Strict null and definite assignment

  1. strictNullChecks deep
  2. Definite assignment
  3. Non-null assertion
  4. Summary - Strict null

13 Project: Type-safe compiler

  1. AST model
  2. Visitor design
  3. Type guards
  4. Exhaustive eval
  5. Integration

14 Advanced final

  1. Challenge 1: Type math
  2. Challenge 2: Variance
  3. Challenge 3: Branded + phantom
  4. Challenge 4: Mini compiler