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Summary - keyof and typeof

Write a safe settings manager with the operators.

Summary - keyof and typeof

keyof gives a type's keys, typeof gives a value's type. Combining them: derive a type from an object with typeof, take its keys with keyof, and write functions that fetch values safely via indexed access. This is ideal for structures like a settings manager.

const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}

Practice Settings manager

Assign the object { tema: "koyu", dil: "tr" } to a variable ayarlar. Define type AyarTip as typeof ayarlar. Define type AyarAnahtar as keyof AyarTip. Write function al(k: AyarAnahtar): AyarTip[AyarAnahtar] with body return ayarlar[k];. Print the result of al("tema") with console.log.

Starter code

index.ts
// ayarlar değişkenine { tema: "koyu", dil: "tr" } ata
// AyarTip türünü ayarlar'ın türü olarak tanımla
// AyarAnahtar türünü AyarTip'in anahtarları olarak tanımla
// al fonksiyonu AyarAnahtar parametre alsın
// dönüş türü AyarTip'in AyarAnahtar indeksi olsun
// gövde return ayarlar[k]; olsun
// tema anahtarının değerini al ile getir ve yazdır

Sample solution

index.ts
const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}
console.log(al("tema"));

Expected output

koyu

Questions and answers

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

1 Which order enables safe property access from a settings object?

  • indexed access, keyof, typeof
  • typeof for type, keyof for keys, indexed access for value Correct answer
  • keyof for keys, typeof for type, indexed access
  • typeof, indexed access, keyof

Explanation: First typeof derives a type from the object, then keyof takes its keys, and finally indexed access fetches the value safely. Any other order breaks the type-level inference.

2 Indexed access return type

Question 
const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): ____ {
  return ayarlar[k];
}
Answer 
const ayarlar = { tema: "koyu", dil: "tr" };
type AyarTip = typeof ayarlar;
type AyarAnahtar = keyof AyarTip;
function al(k: AyarAnahtar): AyarTip[AyarAnahtar] {
  return ayarlar[k];
}

Explanation: Since the return value is ayarlar[k] and k is AyarAnahtar, the return type must be AyarTip[AyarAnahtar]. Just AyarTip or string is too narrow/wide; AyarAnahtar is the key type.

3 Match the operators

Term Meaning
typeof ayarlar Derives a type from a value
keyof AyarTip Gives a type's keys
AyarTip[AyarAnahtar] Value type of a key
al("tema") Safe value fetch

All lessons in Intermediate

1 Classes

  1. Class basics
  2. Constructor and fields
  3. Methods
  4. Static members
  5. Summary - Classes

2 Access modifiers

  1. public
  2. private
  3. protected
  4. Parameter properties
  5. Summary - Access modifiers

3 Readonly properties

  1. readonly fields
  2. Readonly parameter
  3. Readonly vs const
  4. Summary - Readonly

4 Generics

  1. What is a generic?
  2. Generic functions
  3. Generic classes
  4. Multiple type parameters
  5. Summary - Generics

5 Generic constraints

  1. extends constraint
  2. keyof constraint
  3. Default type parameter
  4. Summary - Generic constraints

6 Utility types

  1. Partial
  2. Required and Pick
  3. Omit and Record
  4. Summary - Utility types

7 keyof and typeof

  1. keyof operator
  2. typeof operator
  3. Indexing with keyof
  4. Summary - keyof and typeof

8 Mapped types

  1. What is a mapped type?
  2. Readonly mapped type
  3. Optional mapped type
  4. Summary - Mapped types

9 Conditional types

  1. What is a conditional type?
  2. Distributive conditional
  3. Logic with conditional
  4. Summary - Conditional types

10 infer

  1. infer keyword
  2. Extracting from Promise
  3. Array element type
  4. Summary - infer

11 Decorators

  1. What is a decorator?
  2. Method decorator
  3. Property decorator
  4. Decorator factories
  5. Summary - Decorators

12 Modules

  1. export and import
  2. Default export
  3. Re-export
  4. Type-only import
  5. Summary - Modules

13 Namespaces

  1. What is a namespace?
  2. Nested namespace
  3. Namespace vs module
  4. Summary - Namespaces

14 Declaration files

  1. What is .d.ts?
  2. Ambient module
  3. Triple-slash directive
  4. Summary - Declaration files

15 Advanced classes

  1. abstract class
  2. implements
  3. Get and set
  4. Summary - Advanced classes

16 Type assertion

  1. as keyword
  2. angle bracket syntax
  3. Type assertion vs casting
  4. Casting from unknown
  5. Summary - Type assertion

17 Project: Library package

  1. Package design
  2. Exporting types
  3. Generic helpers
  4. Declaration file
  5. Integration

18 Project: Quiz app

  1. Question model
  2. Question pool
  3. Answer check
  4. Score calculation
  5. Integration

19 Project: Weather app

  1. Weather model
  2. API response type
  3. Data transform
  4. Display
  5. Integration

20 Intermediate final

  1. Challenge 1: Classes
  2. Challenge 2: Generics
  3. Challenge 3: Type systems
  4. Challenge 4: Mini app