2024-08-15
1785
#typescript
Kealan Parr
121310
Aug 15, 2024 ⋅ 6 min read

How to extend enums in TypeScript

Kealan Parr Software engineer, technical writer and member of the Unicode Consortium.

Recent posts:

Building A Telegram Bot With GrammY

Building a Telegram bot with grammY

Build a Telegram bot with Node.js and grammY to automate text, audio, and image responses using the Telegram API and Google Gemini.

Amazing Enyichi Agu
Feb 20, 2025 ⋅ 7 min read
Styling with the CSS box-shadow Property

Styling with the CSS box-shadow property

Use the `box-shadow` CSS property to create layered, neon, and neumorphic shadows in this detailed tutorial.

Oscar Jite-Orimiono
Feb 19, 2025 ⋅ 9 min read
Using Typescript Enums In React Native Web

A complete guide to TypeScript enums in React Native

Learn everything you need to know about how to use React Context — a great feature that enables you to manage and share state across the React application.

Kayode Adeniyi
Feb 19, 2025 ⋅ 8 min read
what are the event loop and call stack in JavaScript

What are the event loop and call stack in JavaScript?

Learn how the call stack, event loop, and various queues help JavaScript handle asynchronous operations while maintaining its single-threaded nature.

Ikeh Akinyemi
Feb 18, 2025 ⋅ 6 min read
View all posts

2 Replies to "How to extend enums in TypeScript"

  1. >In the above code block, we used an intersection type. The intersection acts like an “or,” which simply means that the DoorState type will either be of type Door or of type DoorFrame.

    Isn’t that a union type? Although, the resultant type will let code only compile if the type is used such that only properties in the *intersection* of the types being unioned is accessed.

  2. > Can you extend enums?
    > The short answer is no, you can’t extend enums because TypeScript offers no language feature to > extend them.

    Depends on what you mean by “extend”… but either way, this sentence is pretty falsy. You might not be able to use an `extends` heritage clause as you would with interfaces or classes, but enums are subject to declaration merging in the same manner as namespaces (both `namespace` and the legacy `module` keyword), and interfaces/classes (at the type-level).

    You can also extend the functionality of an enum with static methods, in the same way you would by defining a namespace with the same name as an existing class or function to add types or static methods/properties.

    Here’s a brief real world example:
    “`ts
    // svg path segment commands
    enum Command {
    MoveToAbs = “M”,
    MoveToRel = “m”,
    LineToAbs = “L”,
    LineToRel = “l”,
    // …
    }

    namespace Command {
    // adding types, available as `Command.Absolute` or `Command.Relative`:
    export type Absolute = Command.MoveToAbs | Command.LineToAbs;
    export type Relative = Command.MoveToRel | Command.LineToRel;

    // add runtime functionality with type guards
    export function isAbsolute(it: unknown): it is Absolute {
    return it === Command.MoveToAbs || it === Command.LineToAbs;
    }

    export function isRelative(it: unknown): it is Relative {
    return it === Command.MoveToRel || it === Command.LineToRel;
    }
    }
    “`

Leave a Reply