Enum vs union literal type trade-offs in Typescript - Hands-On Comparison

Enum vs Union Literal Type Trade-offs in TypeScript

📖 Scenario: Imagine you are building a simple app that tracks the status of orders in a store. You want to represent the order status using TypeScript types. You will explore two ways to do this: using enum and using union literal types. This will help you understand the differences and trade-offs between these two approaches.

🎯 Goal: You will create two ways to represent order statuses: one with an enum and one with a union literal type. Then you will write a function that accepts only valid statuses and prints a message. Finally, you will see how the output looks for both approaches.

📋 What You'll Learn

Create an enum called OrderStatusEnum with values Pending, Shipped, and Delivered

Create a union literal type called OrderStatusUnion with the string literals 'Pending', 'Shipped', and 'Delivered'

Write a function printStatusEnum that takes a parameter status of type OrderStatusEnum and prints a message

Write a function printStatusUnion that takes a parameter status of type OrderStatusUnion and prints a message

Call both functions with the Shipped status and print the results

💡 Why This Matters

🌍 Real World

Enums and union literal types are common ways to represent fixed sets of values in TypeScript, such as status codes, user roles, or options in forms.

💼 Career

Understanding these types helps you write safer and clearer code in TypeScript, which is widely used in web development and many software projects.

1

Create the OrderStatusEnum enum

Create an enum called OrderStatusEnum with these exact members: Pending, Shipped, and Delivered.

Typescript

# Your code here

Hint

Use the enum keyword followed by the name OrderStatusEnum. List the members inside curly braces.

2

Create the OrderStatusUnion union literal type

Create a type alias called OrderStatusUnion that is a union of the string literals 'Pending', 'Shipped', and 'Delivered'.

Typescript

enum OrderStatusEnum {
    Pending,
    Shipped,
    Delivered
}

# Your code here

Hint

Use the type keyword followed by the name OrderStatusUnion and assign it to the union of string literals separated by |.

3

Write functions to print status messages

Write a function called printStatusEnum that takes a parameter status of type OrderStatusEnum and prints "Enum status: " followed by the status name. Also write a function called printStatusUnion that takes a parameter status of type OrderStatusUnion and prints "Union status: " followed by the status string.

Typescript

enum OrderStatusEnum {
    Pending,
    Shipped,
    Delivered
}

type OrderStatusUnion = 'Pending' | 'Shipped' | 'Delivered';

# Your code here

Hint

Use console.log with template strings to print the messages. For the enum, use OrderStatusEnum[status] to get the name.

4

Call the functions with the Shipped status and print output

Call printStatusEnum with OrderStatusEnum.Shipped and call printStatusUnion with the string 'Shipped'. These calls should print the status messages.

Typescript

enum OrderStatusEnum {
    Pending,
    Shipped,
    Delivered
}

type OrderStatusUnion = 'Pending' | 'Shipped' | 'Delivered';

function printStatusEnum(status: OrderStatusEnum): void {
    console.log(`Enum status: ${OrderStatusEnum[status]}`);
}

function printStatusUnion(status: OrderStatusUnion): void {
    console.log(`Union status: ${status}`);
}

# Your code here

Hint

Call the functions exactly as shown to print the status messages.

Practice

1. Which statement best describes a key difference between TypeScript enums and union literal types?

easy

A. Enums create runtime objects, while union literal types only exist at compile time.

B. Union literal types generate JavaScript code, enums do not.

C. Enums cannot restrict values, union literal types can.

D. Union literal types allow dynamic value assignment, enums do not.

Solution

1. Step 1: Understand enums and union literal types

   Enums create named objects that exist in the generated JavaScript code at runtime. Union literal types only exist during TypeScript's type checking and do not produce runtime code.

2. Step 2: Compare runtime presence

   Since union literal types are erased after compilation, they cannot be used as runtime objects, unlike enums.

3. Final Answer:

   Enums create runtime objects, while union literal types only exist at compile time. -> Option A

4. Quick Check:

   Runtime object presence = A [OK]

Hint: Enums exist at runtime; union types do not [OK]

Common Mistakes:

• Thinking union types generate runtime code
• Believing enums cannot restrict values
• Confusing dynamic assignment capabilities

2. Which of the following is the correct syntax to define a union literal type for colors 'red', 'green', and 'blue' in TypeScript?

easy

A. enum Colors { 'red', 'green', 'blue' }

B. type Colors = 'red' | 'green' | 'blue';

C. type Colors = ['red', 'green', 'blue'];

D. enum Colors = 'red' | 'green' | 'blue';

Solution

1. Step 1: Identify union literal type syntax

   Union literal types use the syntax: type Name = 'value1' | 'value2' | 'value3';

2. Step 2: Check options for correct syntax

   type Colors = 'red' | 'green' | 'blue'; correctly uses the union literal type syntax. Options B and D misuse enum syntax, and C defines a tuple type, not a union.

3. Final Answer:

   type Colors = 'red' | 'green' | 'blue'; -> Option B

4. Quick Check:

   Union literal type syntax = A [OK]

Hint: Union types use | between quoted literals [OK]

Common Mistakes:

• Using enum syntax for union types
• Using array syntax instead of union
• Assigning enum with = and union syntax mixed

3. Given the enum and union type below, what will be the output of the console.log statements?

enum StatusEnum { Active = 'active', Inactive = 'inactive' }
type StatusUnion = 'active' | 'inactive';

const enumValue = StatusEnum.Active;
const unionValue: StatusUnion = 'active';

console.log(enumValue === 'active');
console.log(unionValue === 'active');

medium

A. false false

B. true false

C. false true

D. true true

Solution

1. Step 1: Understand enum and union values

   StatusEnum.Active is assigned the string 'active'. The unionValue is also the string 'active'.

2. Step 2: Evaluate the console.log comparisons

   Both enumValue === 'active' and unionValue === 'active' compare string 'active' to 'active', so both are true.

3. Final Answer:

   true true -> Option D

4. Quick Check:

   String equality for enum and union = D [OK]

Hint: Enum string values equal union strings if assigned same literal [OK]

Common Mistakes:

• Assuming enum values are numbers by default
• Thinking enumValue is an object, not a string
• Confusing enum and union value types

4. Identify the error in the following TypeScript code using enums and union types:

enum Direction {
  Up = 'UP',
  Down = 'DOWN'
}

type DirectionUnion = 'UP' | 'DOWN';

function move(dir: DirectionUnion) {
  console.log(dir);
}

move(Direction.Up);
move('LEFT');

medium

A. Calling move with Direction.Up causes a type error.

B. Enum Direction is missing a 'LEFT' value causing an error.

C. Calling move with 'LEFT' causes a type error.

D. Function move should accept enum Direction, not union type.

Solution

1. Step 1: Check function parameter type

   Function move expects a parameter of type DirectionUnion, which allows only 'UP' or 'DOWN'.

2. Step 2: Analyze function calls

   Calling move(Direction.Up) passes 'UP', which is valid. Calling move('LEFT') passes a string not in the union, causing a type error.

3. Final Answer:

   Calling move with 'LEFT' causes a type error. -> Option C

4. Quick Check:

   Invalid union argument = B [OK]

Hint: Union types restrict allowed strings strictly [OK]

Common Mistakes:

• Assuming enum values are incompatible with union types
• Thinking missing enum values cause errors
• Believing function parameter types are flexible

5. You want to create a set of fixed string values representing user roles that can be checked at runtime and also provide type safety. Which approach is best and why?

// Option 1
enum UserRoleEnum { Admin = 'admin', User = 'user', Guest = 'guest' }

// Option 2
type UserRoleUnion = 'admin' | 'user' | 'guest';

hard

A. Use UserRoleEnum because it creates a runtime object for checks and type safety.

B. Use UserRoleUnion because it generates runtime code and is simpler.

C. Use UserRoleUnion because enums cannot restrict string values.

D. Use UserRoleEnum because union types cannot be used in type annotations.

Solution

1. Step 1: Understand runtime and type safety needs

   To check values at runtime, you need an actual JavaScript object. Enums generate this object. Union types only exist at compile time and do not generate code.

2. Step 2: Compare options for runtime checks and type safety

   UserRoleEnum provides both runtime presence and type safety. UserRoleUnion provides only type safety but no runtime object for checks.

3. Final Answer:

   Use UserRoleEnum because it creates a runtime object for checks and type safety. -> Option A

4. Quick Check:

   Runtime object + type safety = C [OK]

Hint: Enums create runtime objects; unions only type safety [OK]

Common Mistakes:

• Thinking union types generate runtime code
• Believing enums cannot restrict string values
• Assuming union types can be used for runtime checks