Enum member access in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an enum in TypeScript?

An enum is a way to define a set of named constants. It helps group related values under one name for easier use and better code clarity.

beginner

How do you access a numeric enum member by its name?

You use dot notation with the enum name and member name, like EnumName.MemberName.

intermediate

How can you get the name of an enum member from its numeric value?

TypeScript enums create a reverse mapping, so you can use EnumName[value] to get the member name as a string.

beginner

What happens if you try to access an enum member that does not exist?

You get undefined because the member name or value is not found in the enum.

intermediate

Can you access enum members using variables? How?

Yes, you can use bracket notation with a variable holding the member name or value, like EnumName[variable].

How do you access the enum member 'Red' in enum Color { Red, Green, Blue }?

A Red.Color

B Color.Red

C Color->Red

D Color['Red']()

You access enum members using dot notation: Color.Red.

What does Color[0] return if enum Color { Red, Green, Blue }?

A 'Green'

B 0

C 'Red'

D undefined

Numeric enums have reverse mapping, so Color[0] returns the member name 'Red'.

If you try Color['Yellow'] and 'Yellow' is not a member, what is the result?

A Throws error

B null

C Returns 'Yellow'

D undefined

Accessing a non-existent enum member returns undefined.

Can you use a variable to access an enum member dynamically?

A Yes, using bracket notation like Color[varName]

B No, only dot notation works

C Yes, but only with string enums

D No, enums are static

Bracket notation allows dynamic access using variables.

What type of enum member access allows getting the member name from its value?

A Reverse mapping in numeric enums

B Dot notation

C Bracket notation with string enums

D You cannot get member name from value

Numeric enums have reverse mapping to get member names from values.

Explain how to access enum members by name and by value in TypeScript.

Think about how you use the enum name and member name or value.

Describe what happens when you try to access an enum member that does not exist.

Consider what JavaScript returns when a property is missing.

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