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Summary - Type alias

Write a configuration type with aliases.

Summary - Type alias

Type aliases provide reuse by naming types, create new types with unions, and are more general than interfaces. Together they enable flexible and modular type definitions.

type Deger = number | string;
type Ayar = { ad: string; deger: Deger };

These features together create powerful configuration types.

Practice Write a configuration type

Define a type alias named Deger (number | string union). Define a type alias named Ayar ({ ad: string; deger: Deger }). Declare a variable named ayar of type Ayar with value { ad: "timeout", deger: 30 }. Print the value of ayar.ad with console.log.

Starter code

index.ts
// Deger adında bir type alias tanımla (number | string birleşimi)
// Ayar adında bir type alias tanımla ({ ad: string; deger: Deger })
// ayar değişkenini Ayar türünde tanımla, değer: { ad: "timeout", deger: 30 }
// ayar.ad değerini yazdır

Sample solution

index.ts
type Deger = number | string;
type Ayar = { ad: string; deger: Deger };
let ayar: Ayar = { ad: "timeout", deger: 30 };
console.log(ayar.ad);

Expected output

timeout

Questions and answers

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

1 What does using another alias in an alias definition enable?

  • Building complex types from simple ones Correct answer
  • Faster execution
  • Less memory usage
  • Types becoming immutable

Explanation: Aliases can reference other aliases; this enables building complex types modularly from simpler parts.

2 Complete the configuration value type

Question 
type Ayar = {
  ad: string;
  deger: ____;
};
Answer 
type Ayar = {
  ad: string;
  deger: number | string;
};

Explanation: Since a configuration value can be both a number and a string, the type must be <code>number | string</code>; only <code>number</code> or <code>string</code> excludes the other type, and <code>boolean</code> is the wrong type.

3 Type alias summary

Term Meaning
type ID = number Primitive type naming
type X = A | B Type union
type vs interface Type is more general
Type alias Naming a type

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app