TypeScript Enums

TypeScript Enums: Named Constant Sets & Const Enums

Enums (enumerations) allow developers to define a set of named constants. TypeScript supports numeric enums, string enums, and performance-optimized const enum declarations.

Enum Variations

1. Numeric Enums: Auto-incrementing numbers starting at 0 (or a custom start value).
2. String Enums: Each member is explicitly initialized with a string value.
3. Const Enums: Inlined directly at compile-time to eliminate runtime object overhead.

Practical Code Example

// Numeric Enum
enum Priority {
  Low = 1,
  Medium,   // 2
  High,     // 3
  Critical  // 4
}

// String Enum (Recommended for API transparency)
enum OrderStatus {
  Pending = "PENDING",
  Processing = "PROCESSING",
  Shipped = "SHIPPED",
  Delivered = "DELIVERED",
  Cancelled = "CANCELLED"
}

// Const Enum (Inlined into JS, no runtime object generated)
const enum LogLevel {
  Debug = "DEBUG",
  Info = "INFO",
  Error = "ERROR"
}

interface Order {
  id: string;
  status: OrderStatus;
  priority: Priority;
}

function processOrder(order: Order): void {
  console.log(`Order ${order.id} status: ${order.status} | Priority Level: ${order.priority}`);
}

processOrder({
  id: "ORD-9921",
  status: OrderStatus.Processing,
  priority: Priority.High
});

Best Practices & Gotchas

• Prefer String Enums Over Numeric Enums: String enums display readable text during debugging and API serialization instead of cryptic numbers.
• Use Union of Literals as an Alternative: Many TypeScript teams prefer type OrderStatus = 'PENDING' | 'SHIPPED' over enums for lightweight ergonomics.
• Avoid Reverse Mapping Pitfalls: Numeric enums generate reverse mappings in JavaScript (Priority[1] === "Low"), which increases bundle size.

Self-Check Challenge

Declare a string enum UserRole with values Admin = "ADMIN" and User = "USER". Write a function that accepts UserRole as a parameter.