Enum member access in Typescript

Introduction

Enums help group related values under one name. Accessing enum members lets you use these values clearly and safely in your code.

When you want to represent a fixed set of options like days of the week.

When you need readable names for numeric or string constants.

When you want to avoid using 'magic numbers' or strings scattered in your code.

When you want to check or compare values against a known set.

When you want to improve code clarity and reduce errors.

Syntax

Typescript

enum EnumName {
  Member1,
  Member2 = 5
}

// Access enum member
EnumName.Member1
EnumName[0]  // for numeric enums

Enum members can be numbers or strings.

Numeric enums have reverse mapping: you can get name from value.

Examples

Accessing numeric enum member by name.

Typescript

enum Direction {
  Up,
  Down,
  Left,
  Right
}

let move = Direction.Up;

Enum with custom numeric values.

Typescript

enum Status {
  Success = 1,
  Failure = 0
}

let result = Status.Success;

String enum members accessed by name.

Typescript

enum Color {
  Red = "RED",
  Green = "GREEN",
  Blue = "BLUE"
}

let favorite = Color.Green;

Accessing enum member name by numeric value (reverse mapping).

Typescript

enum Direction {
  Up,
  Down
}

let name = Direction[0];

Sample Program

This program uses enum member access to decide what action to take for each traffic light color.

Typescript

enum TrafficLight {
  Red = "RED",
  Yellow = "YELLOW",
  Green = "GREEN"
}

function getAction(light: TrafficLight): string {
  switch (light) {
    case TrafficLight.Red:
      return "Stop";
    case TrafficLight.Yellow:
      return "Get Ready";
    case TrafficLight.Green:
      return "Go";
  }
}

console.log(getAction(TrafficLight.Red));
console.log(getAction(TrafficLight.Yellow));
console.log(getAction(TrafficLight.Green));

Important Notes

Use enum member names to avoid mistakes with raw values.

String enums do not have reverse mapping like numeric enums.

Accessing enum members by name is safer and clearer than using raw values.

Summary

Enums group related values under one name.

Access enum members using dot notation: EnumName.Member.

Numeric enums support reverse mapping; string enums do not.

Practice

1.

Which of the following is the correct way to access a member named Red from an enum called Color?

easy

A. Color.Red

B. Red.Color

C. Color[Red]

D. Red.Color()

Solution

1. Step 1: Understand enum member access syntax

   In TypeScript, enum members are accessed using dot notation with the enum name first, then the member name.

2. Step 2: Apply correct syntax to the example

   For enum Color and member Red, the correct access is Color.Red.

3. Final Answer:

   Color.Red -> Option A

4. Quick Check:

   Enum member access = Color.Red [OK]

Hint: Use EnumName.Member to access enum values quickly [OK]

Common Mistakes:

• Reversing the order like Red.Color
• Using brackets without quotes for string enums
• Calling enum members as functions

2.

Which of the following is the correct syntax to define a numeric enum named Direction with members Up, Down, Left, and Right?

?

easy

A. enum Direction (Up, Down, Left, Right)

B. enum Direction = { Up, Down, Left, Right }

C. enum Direction { Up, Down, Left, Right }

D. enum Direction: { Up, Down, Left, Right }

Solution

1. Step 1: Recall enum declaration syntax

   In TypeScript, enums are declared using the keyword enum followed by the name and curly braces containing members.

2. Step 2: Match correct syntax

   enum Direction { Up, Down, Left, Right } uses enum Direction { Up, Down, Left, Right }, which is the correct syntax.

3. Final Answer:

   enum Direction { Up, Down, Left, Right } -> Option C

4. Quick Check:

   Enum declaration uses curly braces {} [OK]

Hint: Use curly braces {} after enum name to list members [OK]

Common Mistakes:

• Using equals sign (=) after enum name
• Using colon (:) instead of braces
• Using parentheses () instead of braces

3.

Given the enum:

enum Status { Active = 1, Inactive, Pending }

What is the output of console.log(Status[2])?

medium

A. "Pending"

B. "Inactive"

C. 2

D. undefined

Solution

1. Step 1: Understand numeric enum reverse mapping

   Numeric enums in TypeScript create a reverse mapping from value to name. Here, Active = 1, so Inactive is 2, Pending is 3.

2. Step 2: Access enum member by value

   Status[2] returns the name of the member with value 2, which is Inactive.

3. Final Answer:

   "Inactive" -> Option B

4. Quick Check:

   Numeric enum reverse lookup = "Inactive" [OK]

Hint: Numeric enums map values back to names automatically [OK]

Common Mistakes:

• Assuming string enums have reverse mapping
• Confusing member names with values
• Expecting numeric output instead of string

4.

Consider this enum and code snippet:

enum Fruit { Apple = "APPLE", Banana = "BANANA" }
console.log(Fruit["APPLE"]);

What will happen when this code runs?

medium

A. undefined

B. "APPLE"

C. "Apple"

D. SyntaxError

Solution

1. Step 1: Understand string enum member access

   String enums do not have reverse mapping from value to name like numeric enums.

2. Step 2: Analyze the code behavior

   Accessing Fruit["APPLE"] looks for a member named "APPLE", but enum keys are "Apple" and "Banana". So it returns undefined.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   String enums lack reverse mapping = undefined [OK]

Hint: String enums do not support reverse lookup by value [OK]

Common Mistakes:

• Expecting reverse mapping for string enums
• Confusing enum keys with values
• Assuming case-insensitive access

5.

You have this enum and function:

enum Status { Success = 200, NotFound = 404, ServerError = 500 }

function getStatusName(code: number): string {
  return Status[code] || "Unknown";
}

What will getStatusName(404) return, and why?

hard

A. "Unknown" because reverse mapping is not supported

B. "404" because it returns the number as string

C. Runtime error due to invalid access

D. "NotFound" because numeric enums support reverse mapping

Solution

1. Step 1: Recognize numeric enum reverse mapping

   Numeric enums in TypeScript create a reverse mapping from numeric values to member names.

2. Step 2: Evaluate function behavior with input 404

   Calling getStatusName(404) returns Status[404], which is "NotFound". If no match, it returns "Unknown".

3. Final Answer:

   "NotFound" because numeric enums support reverse mapping -> Option D

4. Quick Check:

   Numeric enum reverse lookup returns member name [OK]

Hint: Numeric enum values map back to names automatically [OK]

Common Mistakes:

• Thinking it returns the number as string
• Assuming reverse mapping doesn't exist
• Expecting runtime errors on valid enum access