When enums add unnecessary runtime code in Typescript - Mini Project: Build & Apply

When enums add unnecessary runtime code

📖 Scenario: Imagine you are building a simple app that needs to represent a few fixed colors. You want to use TypeScript enums but learn how they add extra code when compiled. This project will help you see how to avoid that extra code by using a different approach.

🎯 Goal: You will create a TypeScript enum for colors, then replace it with a simpler constant object to avoid unnecessary runtime code. You will see the difference in output.

📋 What You'll Learn

Create a TypeScript enum called Colors with three colors: Red, Green, and Blue.

Create a constant object called ColorsConst with the same color keys and string values.

Write a function getColorName that takes a color value and returns its name using the enum.

Write a function getColorNameConst that takes a color value and returns its name using the constant object.

Print the results of calling both functions with the value 1.

💡 Why This Matters

🌍 Real World

Enums are useful for fixed sets of values but can add extra code that is not always needed. Using constant objects or union types can keep your code smaller and faster.

💼 Career

Understanding how enums compile helps you write efficient TypeScript code, which is important for frontend and backend development jobs.

1

Create a TypeScript enum for colors

Create a TypeScript enum called Colors with these entries: Red = 0, Green = 1, and Blue = 2.

Typescript

# Create the enum Colors with Red=0, Green=1, Blue=2

Hint

Use the enum keyword followed by the name Colors. Inside curly braces, list the colors with their numeric values.

2

Create a constant object for colors

Create a constant object called ColorsConst with keys Red, Green, and Blue and values "Red", "Green", and "Blue" respectively.

Typescript

enum Colors {
    Red = 0,
    Green = 1,
    Blue = 2
}

// Create the constant object ColorsConst with string values

Hint

Use const ColorsConst = { Red: "Red", Green: "Green", Blue: "Blue" } as const; to create a readonly object.

3

Write functions to get color names

Write a function called getColorName that takes a parameter color of type Colors and returns the color name using the enum. Also write a function called getColorNameConst that takes a parameter color of type keyof typeof ColorsConst and returns the color name from ColorsConst.

Typescript

enum Colors {
    Red = 0,
    Green = 1,
    Blue = 2
}

const ColorsConst = {
    Red: "Red",
    Green: "Green",
    Blue: "Blue"
} as const;

// Write function getColorName(color: Colors): string
// Write function getColorNameConst(color: keyof typeof ColorsConst): string

Hint

Use the enum as an index to get the name in getColorName. Use the key to get the value from ColorsConst in getColorNameConst.

4

Print the results of both functions

Write two console.log statements to print the results of calling getColorName(1) and getColorNameConst("Green").

Typescript

enum Colors {
    Red = 0,
    Green = 1,
    Blue = 2
}

const ColorsConst = {
    Red: "Red",
    Green: "Green",
    Blue: "Blue"
} as const;

function getColorName(color: Colors): string {
    return Colors[color];
}

function getColorNameConst(color: keyof typeof ColorsConst): string {
    return ColorsConst[color];
}

// Print getColorName(1) and getColorNameConst("Green")

Hint

Use console.log(getColorName(1)) and console.log(getColorNameConst("Green")) to print the color names.

Practice

1. What is a main reason to avoid using regular enums in TypeScript when you only need type checking?

easy

A. They add extra runtime code that may be unnecessary.

B. They cause syntax errors in modern TypeScript versions.

C. They cannot be used with functions.

D. They do not support string values.

Solution

1. Step 1: Understand what regular enums generate

   Regular enums create extra JavaScript code that runs when your program runs, even if you only want type checks.

2. Step 2: Identify the impact of extra runtime code

   This extra code can increase bundle size and slow down execution unnecessarily if you only need compile-time type safety.

3. Final Answer:

   They add extra runtime code that may be unnecessary. -> Option A

4. Quick Check:

   Extra runtime code = Avoid regular enums when only type checks needed [OK]

Hint: Remember: regular enums add runtime code, const enums don't [OK]

Common Mistakes:

• Thinking enums cause syntax errors
• Believing enums can't hold string values
• Assuming enums can't be used with functions

2. Which syntax correctly declares a const enum in TypeScript to avoid runtime code?

easy

A. enum Colors { Red, Green, Blue }

B. const Colors = enum { Red, Green, Blue }

C. const enum Colors { Red, Green, Blue }

D. enum const Colors { Red, Green, Blue }

Solution

1. Step 1: Recall the correct const enum syntax

   The correct way to declare a const enum is by placing the keyword const before enum like const enum Colors { ... }.

2. Step 2: Identify incorrect options

   enum Colors { Red, Green, Blue } is a regular enum, not const. const Colors = enum { Red, Green, Blue } and D have invalid syntax order or structure.

3. Final Answer:

   const enum Colors { Red, Green, Blue } -> Option C

4. Quick Check:

   const enum syntax = const enum Name { ... } [OK]

Hint: const enum always starts with 'const enum' keywords [OK]

Common Mistakes:

• Placing const after enum
• Using invalid syntax like 'const Colors = enum'
• Confusing regular enum with const enum

3. What will be the output of this TypeScript code when compiled and run in JavaScript?

const enum Status { Active, Inactive }
const current = Status.Active;
console.log(current);

medium

A. Status.Active

B. 0

C. undefined

D. Error at runtime

Solution

1. Step 1: Understand const enum compilation

   Const enums are inlined by the compiler, so Status.Active becomes the numeric value 0 directly in the output JavaScript.

2. Step 2: Predict the console output

   Since current is assigned Status.Active, which is 0, the console.log prints 0.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   const enum values inline as numbers [OK]

Hint: const enum values become numbers directly in output [OK]

Common Mistakes:

• Expecting enum object at runtime
• Thinking it prints the enum name
• Assuming runtime error due to const enum

4. Identify the problem in this TypeScript code snippet that uses a regular enum:

enum Direction { Up, Down }
function move(dir: Direction) {
  console.log(dir);
}
move(Direction.Left);

medium

A. The function move should return a value.

B. Enums cannot be used as function parameters.

C. The enum must be declared as const enum.

D. Direction.Left does not exist, causing a compile error.

Solution

1. Step 1: Check enum members

   The enum Direction only has Up and Down. Left is not defined.

2. Step 2: Understand the error cause

   Using Direction.Left causes a compile-time error because Left is not a valid member.

3. Final Answer:

   Direction.Left does not exist, causing a compile error. -> Option D

4. Quick Check:

   Using undefined enum member = compile error [OK]

Hint: Use only defined enum members to avoid errors [OK]

Common Mistakes:

• Assuming enums can't be function parameters
• Thinking const enum is required here
• Expecting runtime error instead of compile error

5. You want to define a set of string values for user roles without adding runtime code. Which TypeScript approach is best?

hard

A. Use a union of string literal types.

B. Use a regular enum with string values.

C. Use a const enum with string values.

D. Use a class with static string properties.

Solution

1. Step 1: Understand runtime code from enums

   Regular enums with string values generate runtime code. Const enums with string values are not allowed in TypeScript.

2. Step 2: Identify the best alternative

   Using a union of string literal types (e.g., 'admin' | 'user' | 'guest') provides type safety without any runtime code.

3. Final Answer:

   Use a union of string literal types. -> Option A

4. Quick Check:

   String unions avoid runtime code unlike enums [OK]

Hint: Use string unions for roles to skip runtime code [OK]

Common Mistakes:

• Trying to use const enum with strings (not allowed)
• Choosing regular enums causing extra code
• Using classes adds unnecessary complexity