TypeScript Enums vs Types Explained with Examples

While building a TypeScript project, you may need to define a set of constant values, like user roles, status codes, or payment methods. At this point, you might wonder: Should I use an Enum or a Type alias?

This is a common question for both beginners and experienced developers. TypeScript provides several ways to define a set of named constants, including enums and union types.

In this tutorial, we will explore the differences between TypeScript Enums vs Types Explained with Examples. By the end, you’ll have a clear understanding of when to use each and how they can enhance the readability and maintainability of your code.

Table of Contents

What are TypeScript Enums?

Enums, short for “enumerations,” allow developers to define a set of named constants. They provide a way to create a set of distinct cases, making it easier to document intent and create more readable code. Enums are real objects at runtime, which can be desirable in some cases.

Here’s an example of a numeric enum:

enum Direction {
  Up = 1,
  Down,
  Left,
  Right,
}

In this example, Up is initialized with 1, and the subsequent members are incremented by 1. You can also specify string values for enum members:

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

One unique feature of enums is the ability to perform reverse mapping. You can access an enum value by its key, but you can also get the key by its value:

enum Direction {
  Up,
  Down,
  Left,
  Right
}
console.log(Direction.Up);    // 0
console.log(Direction[0]);    // "Up"

Here is the output of the above TypeScript code.

TypeScript Enums vs Types Explained with Examples

What are TypeScript Types?

TypeScript types, specifically union types, offer an alternative approach to defining a set of named constants. Union types allow you to specify a type that can be one of several types. They are not real objects at runtime, but rather a way to define a type that can be one of several possible values.

Here’s an example of a union type:

type Direction = "Up" | "Down" | "Left" | "Right";

In this example, the Direction type can be one of the four string literals: “Up“, “Down“, “Left“, or “Right“.

Enums vs. Types: Key Differences

Runtime Existence: Enums are real objects at runtime, while union types are not. This means that enums can be used as values, while union types are purely a type-level construct.
Type Safety: Both enums and union types provide type safety, ensuring that variables can only be assigned valid values. However, union types offer more flexibility and can be used in conjunction with other types, such as null or undefined.
Readability: Enums can enhance code readability by providing named constants, thereby improving readability. Union types can also enhance readability, especially when the possible values are self-explanatory.
Extensibility: Union types are more extensible than enums. You can easily add new values to a union type without modifying existing code. With enums, adding new values requires modifying the enum definition.
Performance: Union types have a slight performance advantage over enums, as they don’t create an object at runtime. However, the difference is usually negligible.

When to Use Enums vs. Types

Use Enums When:

You have a fixed set of named constants that rarely change.
You need to use the constants as values at runtime.
You want to improve code readability by providing named constants.

Use Union Types When:

You have a set of possible values that may change over time.
You don’t need to use the constants as values at runtime.
You want more flexibility in defining the possible values.
You need to use the constants with other types, like null or undefined.

Here’s a summary table comparing enums and union types:

Feature	Enums	Union Types
Runtime Existence	Yes	No
Type Safety	Yes	Yes
Readability	Improves readability with named constants	Can improve readability, especially with self-explanatory values
Extensibility	Requires modifying the enum definition	Easily extensible without modifying existing code
Performance	Slightly slower due to runtime object creation	Slightly faster, as they don’t create an object at runtime

Conclusion

In this tutorial, we’ve explored the differences between TypeScript enums and types, specifically union types. We’ve seen how enums provide a way to define named constants that exist at runtime, while union types offer a more flexible way to define a set of possible values without creating an object at runtime.

When deciding between enums and union types, consider factors such as runtime existence, type safety, readability, extensibility, and performance. If you have a fixed set of named constants that won’t change often and need to use them as values at runtime, enums are a good choice. If you need more flexibility, extensibility, or better performance, union types may be a better option.

Ultimately, the choice between enums and union types depends on your specific use case and project requirements. By understanding the strengths and weaknesses of each, you can make an informed decision that will lead to more readable, maintainable, and efficient code.