String enums in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a string enum in TypeScript?

A string enum is a special type in TypeScript where each enum member is initialized with a string literal. It helps group related string constants under one name.

beginner

How do you declare a string enum in TypeScript?

Use the enum keyword followed by member names and assign string values to each member, like:

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

intermediate

Why use string enums instead of numeric enums?

String enums provide readable values which make debugging and logging easier. They also avoid accidental numeric value changes and improve code clarity.

intermediate

Can string enum members have computed values?

No, string enum members must be initialized with string literals or other string enum members. Computed or runtime values are not allowed.

beginner

How do you access a string enum member's value?

You access it by using the enum name and member name, like Colors.Red, which returns the string value assigned to that member.

Which of the following is a correct string enum declaration in TypeScript?

A enum Direction { Up = 1, Down = 2 }

B enum Direction { Up, Down }

C enum Direction { Up = true, Down = false }

D enum Direction { Up = "UP", Down = "DOWN" }

Only option C assigns string values to enum members, which is required for string enums.

What will Colors.Green return if enum Colors { Red = "RED", Green = "GREEN" }?

A "GREEN"

B 1

C "Green"

D undefined

The member Green is assigned the string "GREEN", so accessing Colors.Green returns "GREEN".

Can you assign a number to a string enum member?

A Yes, numbers are allowed in string enums.

B No, string enum members must be strings.

C Only if the number is converted to a string.

D Only for the first member.

String enum members must be initialized with string literals, not numbers.

Which is a benefit of using string enums?

A They make code less readable.

B They automatically increment numbers.

C They provide meaningful string values for debugging.

D They allow computed values.

String enums provide clear string values that help with debugging and readability.

Are computed values allowed in string enums?

A No, string enums require string literals.

B Yes, always.

C Only if the computed value is a number.

D Only if the enum is const.

String enums must have string literal values; computed values are not allowed.

Explain what a string enum is and why you might use it in TypeScript.

Think about grouping related strings under one name.

Describe how to declare and access members of a string enum with an example.

Show a simple enum with colors or directions.

Practice

1. What is a key benefit of using string enums in TypeScript?

easy

A. They group related string constants under named values for clarity.

B. They automatically assign numeric values to members.

C. They allow mixing numbers and strings without errors.

D. They eliminate the need to declare enum members explicitly.

Solution

1. Step 1: Understand string enums purpose

   String enums group related string values under named constants to improve code clarity and safety.

2. Step 2: Compare options

   Only They group related string constants under named values for clarity. correctly describes this benefit. Numeric auto-assignment and mixing types are not features of string enums.

3. Final Answer:

   They group related string constants under named values for clarity. -> Option A

4. Quick Check:

   String enums improve clarity = A [OK]

Hint: String enums give names to strings for clearer code [OK]

Common Mistakes:

• Confusing string enums with numeric enums
• Thinking enum members can be implicit strings
• Assuming enums mix string and number types

2. Which of the following is the correct syntax to declare a string enum in TypeScript?

easy

A. const enum Colors { Red = RED, Green = GREEN, Blue = BLUE }

B. enum Colors { Red, Green, Blue }

C. enum Colors { Red = 'RED', Green = 'GREEN', Blue = 'BLUE' }

D. enum Colors = { Red: 'RED', Green: 'GREEN', Blue: 'BLUE' }

Solution

1. Step 1: Review string enum syntax

   String enums require explicit string values assigned to each member using = 'value' inside an enum block.

2. Step 2: Check each option

   enum Colors { Red = 'RED', Green = 'GREEN', Blue = 'BLUE' } correctly uses enum Colors { Red = 'RED', ... }. enum Colors { Red, Green, Blue } lacks string assignments. enum Colors = { Red: 'RED', Green: 'GREEN', Blue: 'BLUE' } uses invalid syntax with = outside enum. const enum Colors { Red = RED, Green = GREEN, Blue = BLUE } misses quotes around string values.

3. Final Answer:

   enum Colors { Red = 'RED', Green = 'GREEN', Blue = 'BLUE' } -> Option C

4. Quick Check:

   String enum syntax = D [OK]

Hint: String enums need explicit string values with quotes [OK]

Common Mistakes:

• Omitting string values for enum members
• Using object literal syntax instead of enum
• Missing quotes around string values

3. What will be the output of this TypeScript code?

enum Direction {
  Up = "UP",
  Down = "DOWN",
  Left = "LEFT",
  Right = "RIGHT"
}

console.log(Direction.Left);

medium

A. 3

B. "LEFT"

C. Left

D. undefined

Solution

1. Step 1: Understand enum member values

   Each member in the string enum has an explicit string value, e.g., Left = "LEFT".

2. Step 2: Check console.log output

   Logging Direction.Left prints the string value assigned, which is "LEFT".

3. Final Answer:

   "LEFT" -> Option B

4. Quick Check:

   Enum member value = "LEFT" [OK]

Hint: Enum member prints assigned string value, not name or index [OK]

Common Mistakes:

• Expecting member name instead of value
• Confusing string enums with numeric enums
• Assuming enum members print numeric indexes

4. Identify the error in this string enum declaration:

enum Status {
  Active = ACTIVE,
  Inactive = "INACTIVE"
}

medium

A. Enum name Status is invalid

B. Enum members cannot have different string values

C. Enum must use numeric values only

D. Missing quotes around ACTIVE in Active member

Solution

1. Step 1: Check string enum member values

   String enum members must have explicit string values in quotes, e.g., "ACTIVE".

2. Step 2: Identify error in Active member

   The Active member uses ACTIVE without quotes, causing a reference error.

3. Final Answer:

   Missing quotes around ACTIVE in Active member -> Option D

4. Quick Check:

   String values need quotes = A [OK]

Hint: Always put string enum values inside quotes [OK]

Common Mistakes:

• Forgetting quotes around string values
• Mixing string and numeric enum members
• Assuming enum names have restrictions

5. Given this string enum:

enum Role {
  Admin = "ADMIN",
  User = "USER",
  Guest = "GUEST"
}

Which code snippet correctly checks if a variable r is a valid Role value?

hard

A. if (Object.values(Role).includes(r)) { /* valid */ }

B. if (r in Role) { /* valid */ }

C. if (r === Role) { /* valid */ }

D. if (Role[r]) { /* valid */ }

Solution

1. Step 1: Understand enum value checking

   To check if r is a valid enum value, compare it against the enum's values, not keys.

2. Step 2: Evaluate each option

   if (Object.values(Role).includes(r)) { /* valid */ } uses Object.values(Role).includes(r), correctly checking if r matches any enum string value. if (r in Role) { /* valid */ } checks keys, not values. if (r === Role) { /* valid */ } compares a string to the enum object itself. if (Role[r]) { /* valid */ } tries to index enum with string value, which fails.

3. Final Answer:

   if (Object.values(Role).includes(r)) { /* valid */ } -> Option A

4. Quick Check:

   Check enum values with Object.values() = B [OK]

Hint: Use Object.values(enum).includes(value) to check string enums [OK]

Common Mistakes:

• Checking keys instead of values
• Comparing value directly to enum object
• Using enum indexing with string values