TypeScript Enum Alternatives: Navigating Const Assertions and Union Types for Robust Code

TypeScript, a powerful superset of JavaScript, brings static typing to the dynamic world of web development. Among its many features, the enum keyword has long been a go-to for defining a set of named constants. Enums provide a clear way to represent a fixed collection of related values, enhancing readability and type safety.

However, as the TypeScript ecosystem matures and projects grow in complexity and scale, developers globally are increasingly questioning the traditional utility of enums. While straightforward for simple cases, enums introduce certain behaviors and runtime characteristics that can sometimes lead to unexpected issues, impact bundle size, or complicate tree-shaking optimizations. This has led to a widespread exploration of alternatives.

This comprehensive guide delves deep into two prominent and highly effective alternatives to TypeScript enums: Union Types with String/Numeric Literals and Const Assertions (as const). We will explore their mechanisms, practical applications, benefits, and trade-offs, providing you with the knowledge to make informed design decisions for your projects, regardless of their size or the global team working on them. Our goal is to empower you to write more robust, maintainable, and efficient TypeScript code.

The TypeScript Enum: A Quick Recap

Before we dive into alternatives, let's briefly revisit the traditional TypeScript enum. Enums allow developers to define a set of named constants, making code more readable and preventing "magic strings" or "magic numbers" from being scattered throughout an application. They come in two primary forms: numeric and string enums.

Numeric Enums

By default, TypeScript enums are numeric. The first member is initialized with 0, and each subsequent member is auto-incremented.

            enum Direction {
  Up,
  Down,
  Left,
  Right,
}

let currentDirection: Direction = Direction.Up;
console.log(currentDirection); // Outputs: 0
console.log(Direction.Left);    // Outputs: 2

            

              
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You can also manually initialize numeric enum members:

            enum StatusCode {
  Success = 200,
  NotFound = 404,
  ServerError = 500,
}

let status: StatusCode = StatusCode.NotFound;
console.log(status); // Outputs: 404

            

              
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A peculiar characteristic of numeric enums is reverse mapping. At runtime, a numeric enum compiles into a JavaScript object that maps both names to values and values back to names.

            enum UserRole {
  Admin = 1,
  Editor,
  Viewer,
}

console.log(UserRole[1]); // Outputs: "Admin"
console.log(UserRole.Editor); // Outputs: 2
console.log(UserRole[2]); // Outputs: "Editor"

/*
Compiles to JavaScript:
var UserRole;
(function (UserRole) {
    UserRole[UserRole["Admin"] = 1] = "Admin";
    UserRole[UserRole["Editor"] = 2] = "Editor";
    UserRole[UserRole["Viewer"] = 3] = "Viewer";
})(UserRole || (UserRole = {}));
*/

            

              
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String Enums

String enums are often preferred for their readability at runtime, as they don't rely on auto-incrementing numbers. Each member must be initialized with a string literal.

            enum UserPermission {
  Read = "READ_PERMISSION",
  Write = "WRITE_PERMISSION",
  Delete = "DELETE_PERMISSION",
}

let permission: UserPermission = UserPermission.Write;
console.log(permission); // Outputs: "WRITE_PERMISSION"

            

              
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String enums do not get a reverse mapping, which is generally a good thing for avoiding unexpected runtime behavior and reducing the generated JavaScript output.

Key Considerations and Potential Pitfalls of Enums

While enums offer convenience, they come with certain characteristics that warrant careful consideration:

• Runtime Objects: Both numeric and string enums generate JavaScript objects at runtime. This means they contribute to your application's bundle size, even if you only use them for type-checking. For small projects, this might be negligible, but in large-scale applications with many enums, it can add up.
• Lack of Tree-Shaking: Because enums are runtime objects, they are often not effectively tree-shaken by modern bundlers like Webpack or Rollup. If you define an enum but only use one or two of its members, the entire enum object might still be included in your final bundle. This can lead to larger file sizes than necessary.
• Reverse Mapping (Numeric Enums): The reverse mapping feature of numeric enums, while sometimes useful, can also be a source of confusion and unexpected behavior. It adds extra code to the JavaScript output and might not always be the desired functionality. For instance, serializing numeric enums can sometimes lead to just the number being stored, which might not be as descriptive as a string.
• Transpilation Overhead: The compilation of enums into JavaScript objects adds a slight overhead to the build process compared to simply defining constant variables.
• Limited Iteration: Directly iterating over enum values can be non-trivial, especially with numeric enums due to the reverse mapping. You often need helper functions or specific loops to get just the desired values.

These points highlight why many global development teams, especially those focused on performance and bundle size, are looking towards alternatives that provide similar type safety without the runtime footprint or other complexities.

Alternative 1: Union Types with Literals

One of the most straightforward and powerful alternatives to enums in TypeScript is the use of Union Types with String or Numeric Literals. This approach leverages TypeScript's robust type system to define a set of specific, allowed values at compile-time, without introducing any new constructs at runtime.

What are Union Types?

A union type describes a value that can be one of several types. For example, string | number means a variable can hold either a string or a number. When combined with literal types (e.g., "success", 404), you can define a type that can only hold a specific set of predefined values.

Practical Example: Defining Statuses with Union Types

Let's consider a common scenario: defining a set of possible statuses for a data processing job or a user's account. With union types, this looks clean and concise:

            type JobStatus = "PENDING" | "IN_PROGRESS" | "COMPLETED" | "FAILED";

function processJob(status: JobStatus): void {
  if (status === "COMPLETED") {
    console.log("Job finished successfully.");
  } else if (status === "FAILED") {
    console.log("Job encountered an error.");
  } else {
    console.log(`Job is currently ${status}.`);
  }
}

let currentJobStatus: JobStatus = "IN_PROGRESS";
processJob(currentJobStatus);

// This would result in a compile-time error:
// let invalidStatus: JobStatus = "CANCELLED"; // Error: Type '"CANCELLED"' is not assignable to type 'JobStatus'.

            

              
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For numeric values, the pattern is identical:

            type HttpCode = 200 | 400 | 404 | 500;

function handleResponse(code: HttpCode): void {
  if (code === 200) {
    console.log("Operation successful.");
  } else if (code === 404) {
    console.log("Resource not found.");
  }
}

let responseStatus: HttpCode = 200;
handleResponse(responseStatus);

            

              
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Notice how we're defining a type alias here. This is purely a compile-time construct. When compiled to JavaScript, JobStatus simply vanishes, and the literal strings/numbers are used directly.

Benefits of Union Types with Literals

This approach offers several compelling advantages:

• Purely Compile-Time: Union types are entirely erased during compilation. They do not generate any JavaScript code at runtime, leading to smaller bundle sizes and faster application startup times. This is a significant advantage for performance-critical applications and those deployed globally where every kilobyte counts.
• Excellent Type Safety: TypeScript rigorously checks assignments against the defined literal types, providing strong guarantees that only valid values are used. This prevents common bugs associated with typos or incorrect values.
• Optimal Tree-Shaking: Since there's no runtime object, union types inherently support tree-shaking. Your bundler only includes the actual string or numeric literals you use, not an entire object.
• Readability: For a fixed set of simple, distinct values, the type definition is often very clear and easy to understand.
• Simplicity: No new language constructs or complex compilation artifacts are introduced. It's just leveraging fundamental TypeScript type features.
• Direct Value Access: You directly work with the string or number values, which simplifies serialization and deserialization, especially when interacting with APIs or databases that expect specific string identifiers.

Drawbacks of Union Types with Literals

While powerful, union types also have some limitations:

• Repetition for Associated Data: If you need to associate additional data or metadata with each "enum" member (e.g., a display label, an icon, a color), you cannot do this directly within the union type definition. You would typically need a separate mapping object.
• No Direct Iteration of All Values: There's no built-in way to get an array of all possible values from a union type at runtime. For example, you can't easily get ["PENDING", "IN_PROGRESS", "COMPLETED", "FAILED"] directly from JobStatus. This often necessitates maintaining a separate array of values if you need to display them in a UI (e.g., a dropdown menu).
• Less Centralized: If the set of values is needed both as a type and as an array of runtime values, you might find yourself defining the list twice (once as a type, once as a runtime array), which can introduce potential for desynchronization.

Despite these drawbacks, for many scenarios, union types provide a clean, performant, and type-safe solution that aligns well with modern JavaScript development practices.

Alternative 2: Const Assertions (as const)

The as const assertion, introduced in TypeScript 3.4, is another incredibly powerful tool that offers an excellent alternative to enums, especially when you need a runtime object and robust type inference. It allows TypeScript to infer the narrowest possible type for literal expressions.

What are Const Assertions?

When you apply as const to a variable, an array, or an object literal, TypeScript treats all properties within that literal as readonly and infers their literal types instead of broader types (e.g., "foo" instead of string, 123 instead of number). This makes it possible to derive highly specific union types from runtime data structures.

Practical Example: Creating a "Pseudo-Enum" Object with as const

Let's revisit our job status example. With as const, we can define a single source of truth for our statuses, which acts as both a runtime object and a basis for type definitions.

            const JobStatuses = {
  PENDING: "PENDING",
  IN_PROGRESS: "IN_PROGRESS",
  COMPLETED: "COMPLETED",
  FAILED: "FAILED",
} as const;

// JobStatuses.PENDING is now inferred as type "PENDING" (not just string)
// JobStatuses is inferred as type {
//   readonly PENDING: "PENDING";
//   readonly IN_PROGRESS: "IN_PROGRESS";
//   readonly COMPLETED: "COMPLETED";
//   readonly FAILED: "FAILED";
// }

            

              
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At this point, JobStatuses is a JavaScript object at runtime, just like a regular enum. However, its type inference is far more precise.

Combining with typeof and keyof for Union Types

The real power emerges when we combine as const with TypeScript's typeof and keyof operators to derive a union type from the object's values or keys.

            const JobStatuses = {
  PENDING: "PENDING",
  IN_PROGRESS: "IN_PROGRESS",
  COMPLETED: "COMPLETED",
  FAILED: "FAILED",
} as const;

// Type representing the keys (e.g., "PENDING" | "IN_PROGRESS" | ...)
type JobStatusKeys = keyof typeof JobStatuses;

// Type representing the values (e.g., "PENDING" | "IN_PROGRESS" | ...)
type JobStatusValues = typeof JobStatuses[keyof typeof JobStatuses];

function processJobWithConstAssertion(status: JobStatusValues): void {
  if (status === JobStatuses.COMPLETED) {
    console.log("Job finished successfully.");
  } else if (status === JobStatuses.FAILED) {
    console.log("Job encountered an error.");
  } else {
    console.log(`Job is currently ${status}.`);
  }
}

let currentJobStatusFromObject: JobStatusValues = JobStatuses.IN_PROGRESS;
processJobWithConstAssertion(currentJobStatusFromObject);

// This would result in a compile-time error:
// let invalidStatusFromObject: JobStatusValues = "CANCELLED"; // Error!

            

              
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This pattern provides the best of both worlds: a runtime object for iteration or direct property access, and a compile-time union type for strict type checking.

Benefits of Const Assertions with Derived Union Types

• Single Source of Truth: You define your constants once in a plain JavaScript object, and derive both runtime access and compile-time types from it. This significantly reduces duplication and improves maintainability across diverse development teams.
• Type Safety: Similar to pure union types, you get excellent type safety, ensuring only predefined values are used.
• Iterability at Runtime: Since JobStatuses is a plain JavaScript object, you can easily iterate over its keys or values using standard JavaScript methods like Object.keys(), Object.values(), or Object.entries(). This is invaluable for dynamic UIs (e.g., populating dropdowns) or logging.
• Associated Data: This pattern naturally supports associating additional data with each "enum" member.
• Better Tree-Shaking Potential (Compared to Enums): While as const creates a runtime object, it's a standard JavaScript object. Modern bundlers are generally more effective at tree-shaking unused properties or even entire objects if they are not referenced, compared to TypeScript's enum compilation output. However, if the object is large and only a few properties are used, the entire object might still be included if it's imported in a way that prevents granular tree-shaking.
• Flexibility: You can define values that are not just strings or numbers but more complex objects if needed, making this a highly flexible pattern.

            const FileOperations = {
  UPLOAD: {
    label: "Upload File",
    icon: "upload-icon.svg",
    permission: "can_upload"
  },
  DOWNLOAD: {
    label: "Download File",
    icon: "download-icon.svg",
    permission: "can_download"
  },
  DELETE: {
    label: "Delete File",
    icon: "delete-icon.svg",
    permission: "can_delete"
  },
} as const;

type FileOperationType = keyof typeof FileOperations; // "UPLOAD" | "DOWNLOAD" | "DELETE"
type FileOperationDetail = typeof FileOperations[keyof typeof FileOperations]; // { label: string; icon: string; permission: string; }

function performOperation(opType: FileOperationType) {
  const details = FileOperations[opType];
  console.log(`Performing: ${details.label} (Permission: ${details.permission})`);
}

performOperation("UPLOAD");

            

              
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Drawbacks of Const Assertions

• Runtime Object Presence: Unlike pure union types, this approach still creates a JavaScript object at runtime. While it's a standard object and often better for tree-shaking than enums, it's not entirely erased.
• Slightly More Verbose Type Definition: Deriving the union type (keyof typeof ... or typeof ...[keyof typeof ...]) requires a bit more syntax than simply listing literals for a union type.
• Potential for Misuse: If not used carefully, a very large as const object could still contribute significantly to bundle size if its contents are not effectively tree-shaken across module boundaries.

For scenarios where you need both robust compile-time type checking and a runtime collection of values that can be iterated over or provide associated data, as const is often the preferred choice among TypeScript developers worldwide.

Comparing the Alternatives: When to Use What?

Choosing between union types and const assertions largely depends on your specific requirements concerning runtime presence, iterability, and whether you need to associate additional data with your constants. Let's break down the decision-making factors.

Simplicity vs. Robustness

• Union Types: Offer ultimate simplicity when you only need a type-safe set of distinct string or numeric values at compile-time. They are the most lightweight option.
• Const Assertions: Provide a more robust pattern when you need both compile-time type safety and a runtime object that can be queried, iterated, or extended with additional metadata. The initial setup is slightly more verbose, but it pays off in features.

Runtime vs. Compile-time Presence

• Union Types: Are purely compile-time constructs. They generate absolutely no JavaScript code. This is ideal for applications where minimizing bundle size is paramount, and the values themselves are sufficient without needing to access them as an object at runtime.
• Const Assertions: Generate a plain JavaScript object at runtime. This object is accessible and usable in your JavaScript code. While it adds to the bundle size, it's generally more efficient than TypeScript enums and better candidates for tree-shaking.

Iterability Requirements

• Union Types: Do not offer a direct way to iterate over all possible values at runtime. If you need to populate a dropdown menu or display all options, you'll need to define a separate array of these values, potentially leading to duplication.
• Const Assertions: Excel here. Since you're working with a standard JavaScript object, you can easily use Object.keys(), Object.values(), or Object.entries() to get an array of keys, values, or key-value pairs, respectively. This makes them perfect for dynamic UIs or any scenario requiring runtime enumeration.

            const PaymentMethods = {
  CREDIT_CARD: "Credit Card",
  PAYPAL: "PayPal",
  BANK_TRANSFER: "Bank Transfer",
} as const;

type PaymentMethodType = keyof typeof PaymentMethods;

// Get all keys (e.g., for internal logic)
const methodKeys = Object.keys(PaymentMethods) as PaymentMethodType[];
console.log(methodKeys); // ["CREDIT_CARD", "PAYPAL", "BANK_TRANSFER"]

// Get all values (e.g., for display in a dropdown)
const methodLabels = Object.values(PaymentMethods);
console.log(methodLabels); // ["Credit Card", "PayPal", "Bank Transfer"]

// Get key-value pairs (e.g., for mapping)
const methodEntries = Object.entries(PaymentMethods);
console.log(methodEntries); // [["CREDIT_CARD", "Credit Card"], ...]

            

              
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Tree-Shaking Implications

• Union Types: Are inherently tree-shakeable as they are compile-time only.
• Const Assertions: While they create a runtime object, modern bundlers can often tree-shake unused properties of this object more effectively than with TypeScript's generated enum objects. However, if the entire object is imported and referenced, it will likely be included. Careful module design can help.

Best Practices and Hybrid Approaches

It's not always an "either/or" situation. Often, the best solution involves a hybrid approach, especially in large, internationalized applications:

• For simple, purely internal flags or identifiers that never need to be iterated or have associated data, Union Types are generally the most performant and cleanest choice.
• For sets of constants that need to be iterated over, displayed in UIs, or have rich associated metadata (like labels, icons, or permissions), the Const Assertions pattern is superior.
• Combining for Readability and Localization: Many teams use as const for the internal identifiers and then derive localized display labels from a separate internationalization (i18n) system.

            // src/constants/order-status.ts
const OrderStatuses = {
  PENDING: "PENDING",
  PROCESSING: "PROCESSING",
  SHIPPED: "SHIPPED",
  DELIVERED: "DELIVERED",
  CANCELLED: "CANCELLED",
} as const;

type OrderStatus = typeof OrderStatuses[keyof typeof OrderStatuses];

export { OrderStatuses, type OrderStatus };

// src/i18n/en.json
{
  "orderStatus": {
    "PENDING": "Pending Confirmation",
    "PROCESSING": "Processing Order",
    "SHIPPED": "Shipped",
    "DELIVERED": "Delivered",
    "CANCELLED": "Cancelled"
  }
}

// src/components/OrderStatusDisplay.tsx
import { OrderStatuses, type OrderStatus } from "../constants/order-status";
import { useTranslation } from "react-i18next"; // Example i18n library

interface OrderStatusDisplayProps {
  status: OrderStatus;
}

function OrderStatusDisplay({ status }: OrderStatusDisplayProps) {
  const { t } = useTranslation();
  const displayLabel = t(`orderStatus.${status}`);
  return <span>Status: {displayLabel}</span>;
}

// Usage:
// <OrderStatusDisplay status={OrderStatuses.DELIVERED} />

            

              
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This hybrid approach leverages the type safety and runtime iterability of as const while keeping localized display strings separate and manageable, a critical consideration for global applications.

Advanced Patterns and Considerations

Beyond the basic usage, both union types and const assertions can be integrated into more sophisticated patterns to further enhance code quality and maintainability.

Using Type Guards with Union Types

When working with union types, especially when the union includes diverse types (not just literals), type guards become essential for narrowing down types. With literal union types, discriminated unions offer immense power.

            type SuccessEvent = { type: "SUCCESS"; data: any; };
type ErrorEvent = { type: "ERROR"; message: string; code: number; };

type SystemEvent = SuccessEvent | ErrorEvent;

function handleSystemEvent(event: SystemEvent) {
  if (event.type === "SUCCESS") {
    console.log("Data received:", event.data);
    // event is now narrowed to SuccessEvent
  } else {
    console.log("Error occurred:", event.message, "Code:", event.code);
    // event is now narrowed to ErrorEvent
  }
}

handleSystemEvent({ type: "SUCCESS", data: { user: "Alice" } });
handleSystemEvent({ type: "ERROR", message: "Network failure", code: 503 });

            

              
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This pattern, often called "discriminated unions," is incredibly robust and type-safe, providing compile-time guarantees about the structure of your data based on a common literal property (the discriminator).

Object.values() with as const and Type Assertions

When using the as const pattern, Object.values() can be very useful. However, TypeScript's default inference for Object.values() might be broader than desired (e.g., string[] instead of a specific union of literals). You might need a type assertion for strictness.

            const Statuses = {
  ACTIVE: "Active",
  INACTIVE: "Inactive",
  PENDING: "Pending",
} as const;

type StatusValue = typeof Statuses[keyof typeof Statuses]; // "Active" | "Inactive" | "Pending"

// Object.values(Statuses) is inferred as (string | "Active" | "Inactive" | "Pending")[]
// We can assert it more narrowly if needed:
const allStatusValues: StatusValue[] = Object.values(Statuses);

console.log(allStatusValues); // ["Active", "Inactive", "Pending"]

// For a dropdown, you might pair values with labels if they differ
const statusOptions = Object.entries(Statuses).map(([key, value]) => ({
  value: key, // Use the key as the actual identifier
  label: value // Use the value as the display label
}));
console.log(statusOptions);
/*
[
  { value: "ACTIVE", label: "Active" },
  { value: "INACTIVE", label: "Inactive" },
  { value: "PENDING", label: "Pending" }
]
*/

            

              
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This demonstrates how to get a strongly typed array of values suitable for UI elements while maintaining the literal types.

Internationalization (i18n) and Localized Labels

For global applications, managing localized strings is paramount. While TypeScript enums and their alternatives provide internal identifiers, display labels often need to be separated for i18n. The as const pattern beautifully complements i18n systems.

You define your internal, immutable identifiers using as const. These identifiers are consistent across all locales and serve as keys for your translation files. The actual display strings are then fetched from an i18n library (e.g., react-i18next, vue-i18n, FormatJS) based on the user's selected language.

            // app/features/product/constants.ts
export const ProductCategories = {
  ELECTRONICS: "ELECTRONICS",
  APPAREL: "APPAREL",
  HOME_GOODS: "HOME_GOODS",
  BOOKS: "BOOKS",
} as const;

export type ProductCategory = typeof ProductCategories[keyof typeof ProductCategories];

// app/i18n/locales/en.json
{
  "productCategories": {
    "ELECTRONICS": "Electronics",
    "APPAREL": "Apparel & Accessories",
    "HOME_GOODS": "Home Goods",
    "BOOKS": "Books"
  }
}

// app/i18n/locales/es.json
{
  "productCategories": {
    "ELECTRONICS": "Electrónica",
    "APPAREL": "Ropa y Accesorios",
    "HOME_GOODS": "Artículos para el hogar",
    "BOOKS": "Libros"
  }
}

// app/components/ProductCategorySelector.tsx
import { ProductCategories, type ProductCategory } from "../features/product/constants";
import { useTranslation } from "react-i18next";

function ProductCategorySelector() {
  const { t } = useTranslation();

  return (
    <select>
      {Object.values(ProductCategories).map(categoryKey => (
        <option key={categoryKey} value={categoryKey}>
          {t(`productCategories.${categoryKey}`)}
        </option>
      ))}
    </select>
  );
}

            

              
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This separation of concerns is crucial for scalable, global applications. The TypeScript types ensure you're always using valid keys, and the i18n system handles the presentation layer based on the user's locale. This avoids having language-dependent strings directly embedded within your core application logic, a common anti-pattern for international teams.

Conclusion: Empowering Your TypeScript Design Choices

As TypeScript continues to evolve and empower developers across the globe to build more robust and scalable applications, understanding its nuanced features and alternatives becomes increasingly important. While TypeScript's enum keyword offers a convenient way to define named constants, its runtime footprint, tree-shaking limitations, and reverse mapping complexities often make modern alternatives more appealing for performance-sensitive or large-scale projects.

Union Types with String/Numeric Literals stand out as the leanest and most compile-time-centric solution. They provide uncompromising type safety without generating any JavaScript at runtime, making them ideal for scenarios where minimal bundle size and maximal tree-shaking are priorities, and runtime enumeration isn't a concern.

On the other hand, Const Assertions (as const) combined with typeof and keyof offer a highly flexible and powerful pattern. They provide a single source of truth for your constants, strong compile-time type safety, and the critical ability to iterate over values at runtime. This approach is particularly well-suited for situations where you need to associate additional data with your constants, populate dynamic UIs, or integrate seamlessly with internationalization systems.

By carefully considering the trade-offs – runtime footprint, iterability needs, and complexity of associated data – you can make informed decisions that lead to cleaner, more efficient, and more maintainable TypeScript code. Embracing these alternatives is not just about writing "modern" TypeScript; it's about making deliberate architectural choices that enhance your application's performance, developer experience, and long-term sustainability for a global audience.

Empower your TypeScript development by choosing the right tool for the right job, moving beyond the default enum when better alternatives exist.