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Enum members

Learn enum member access and reverse mapping.

Accessing enum members

You access enum members with dot notation: EnumName.Member. In a numeric enum, this access returns the member's number value. The value is the explicitly assigned number or, otherwise, the auto-incremented number.

enum Yon { Kuzey, Guney }
console.log(Yon.Kuzey);  // 0
console.log(Yon.Guney);  // 1

If a previous member was assigned a value, the next member automatically gets one more.

Reverse mapping

Numeric enums have reverse mapping: you can go from a number to the member name. A usage like Kod[10] returns the name of the member with that value as a string. This feature works only for numeric enums, not for string enums.

enum Kod { A = 10, B }
console.log(Kod[10]);  // "A"
console.log(Kod.B);   // 11

Reverse mapping lets you convert a number back to a name.

Practice Reverse mapping in a color constant

Define a const named Renk mapping Kirmizi to 0 and Mavi to 1. Add reverse mapping by using number values as keys (e.g. key 1 maps to "Mavi"). Print the name obtained via Renk[1] with console.log.

Starter code

index.ts
// Renk adında bir const tanımla
// Kirmizi 0 ve Mavi 1 ile eşle
// ters eşleme için sayıları da anahtar yap
// Renk[1] ile ismi yazdır

Sample solution

index.ts
const Renk = { Kirmizi: 0, Mavi: 1, 0: "Kirmizi", 1: "Mavi" };
console.log(Renk[1]);

Expected output

Mavi

Questions and answers

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

1 In <code>enum Kod { A = 10, B }</code>, what is <code>Kod.B</code>?

  • 1
  • 11 Correct answer
  • 10
  • 0

Explanation: Since <code>A = 10</code>, <code>B</code> automatically gets one more, that is <code>11</code>.

2 Find the name via reverse mapping

Question 
enum Kod { A = 10, B }
let isim = Kod[____];
Answer 
enum Kod { A = 10, B }
let isim = Kod[10];

Explanation: Since <code>A = 10</code>, reverse mapping <code>Kod[10]</code> returns <code>"A"</code>; <code>0</code>, <code>1</code> and <code>100</code> do not correspond to any member.

3 Access and reverse mapping

Term Meaning
Yon.Kuzey Returns the member's number value
Kod[10] Returns the name via reverse mapping
Reverse mapping Works only for numeric enums
String enum Does not support reverse mapping

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