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Summary - Recursive types

Write a JSON tree model with recursive types.

Summary - Recursive types

In this group you learned recursive types: self-referencing types model nested structures like linked lists and trees; DeepPartial makes nested objects deeply optional. A JSON tree model references itself in array and object forms alongside a union of primitive types.

type JsonValue =
  | string | number | boolean | null
  | JsonValue[]
  | { [k: string]: JsonValue };

Practice Write a JSON tree model

Define a recursive union type named JsonDeger: string | number | boolean | null | JsonDeger[] | { [anahtar: string]: JsonDeger }. The veri variable should be of type JsonDeger with value { ad: "test", yas: 30 }. Print veri["ad"] with console.log.

Starter code

index.ts
// JsonDeger adinda recursive union type tanimla
// primitif, dizi ve nesne turlerini birlestir
// veri nesnesini tanimla
// veri["ad"] yazdir

Sample solution

index.ts
type JsonDeger =
  | string
  | number
  | boolean
  | null
  | JsonDeger[]
  | { [anahtar: string]: JsonDeger };
const veri: JsonDeger = { ad: "test", yas: 30 };
console.log(veri["ad"]);

Expected output

test

Questions and answers

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

1 What should be the type of array elements in a JSON tree model?

  • Only string
  • JsonDeger (itself) Correct answer
  • any[]
  • Only number

Explanation: Elements of a JSON array can themselves be any JSON value; so the array type references itself as JsonDeger[].

2 JSON array self-reference

Question 
type JsonDeger =
  | string
  | number
  | boolean
  | null
  | ____[]
  | { [anahtar: string]: JsonDeger };
Answer 
type JsonDeger =
  | string
  | number
  | boolean
  | null
  | JsonDeger[]
  | { [anahtar: string]: JsonDeger };

Explanation: Elements of a JSON array are again of type JsonDeger; this allows arrays or objects inside arrays.

3 Recursive types summary

Term Meaning
JsonDeger[] Array element is itself
{ [k: string]: JsonDeger } Object values are itself
string | number Primitive options
DerinPartial Nested optional

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