Enum member access in Typescript - Mini Project: Build & Apply

Enum member access

📖 Scenario: You are creating a simple program to manage directions in a navigation app.

🎯 Goal: You will create an enum for directions and access its members to display a chosen direction.

📋 What You'll Learn

Create an enum called Direction with members Up, Down, Left, and Right assigned to 1, 2, 3, and 4 respectively.

Create a variable called chosenDirection and assign it the enum member Direction.Left.

Create a variable called directionName that gets the name of the enum member from chosenDirection.

Print the value of chosenDirection and directionName.

💡 Why This Matters

🌍 Real World

Enums help represent fixed sets of related values like directions, statuses, or categories in apps.

💼 Career

Understanding enums is important for writing clear and maintainable code in TypeScript, common in web development jobs.

1

Create the Direction enum

Create an enum called Direction with members Up = 1, Down = 2, Left = 3, and Right = 4.

Typescript

// Your code here to create the Direction enum

Hint

Use the enum keyword followed by the name Direction. Assign numbers to each member.

2

Assign an enum member to a variable

Create a variable called chosenDirection and assign it the enum member Direction.Left.

Typescript

enum Direction {
  Up = 1,
  Down = 2,
  Left = 3,
  Right = 4
}

// Your code here to assign Direction.Left to chosenDirection

Hint

Use let chosenDirection = Direction.Left; to assign the enum member.

3

Get the name of the enum member

Create a variable called directionName that gets the name of the enum member from chosenDirection using the enum reverse mapping.

Typescript

enum Direction {
  Up = 1,
  Down = 2,
  Left = 3,
  Right = 4
}

let chosenDirection = Direction.Left;

// Your code here to get the name of the enum member into directionName

Hint

Use the enum as an indexer: Direction[chosenDirection] to get the name.

4

Print the enum value and name

Print the value of chosenDirection and the value of directionName using console.log.

Typescript

enum Direction {
  Up = 1,
  Down = 2,
  Left = 3,
  Right = 4
}

let chosenDirection = Direction.Left;

let directionName = Direction[chosenDirection];

// Your code here to print chosenDirection and directionName

Hint

Use console.log(chosenDirection); and console.log(directionName); to print the values.

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