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:

​​How HTML’s Selectedcontent Element Improves Dropdowns

​​How HTML’s <selectedcontent> element improves dropdowns

is an experimental HTML element that gives developers control over how a selected option is displayed, using just HTML and CSS.

Temitope Oyedele
Jun 27, 2025 ⋅ 6 min read
advanced caching in Node.js with Valkey

How to get faster data access in Node.js with Valkey

Learn how to implement an advanced caching layer in a Node.js app using Valkey, a high-performance, Redis-compatible in-memory datastore.

Muhammed Ali
Jun 27, 2025 ⋅ 7 min read
how to properly handle rejected promises in TypeScript

How to properly handle rejected promises in TypeScript

Learn how to properly handle rejected promises in TypeScript using Angular, with tips for retry logic, typed results, and avoiding unhandled exceptions.

Lewis Cianci
Jun 26, 2025 ⋅ 4 min read
Your AI Has Agency — Here’s How To Architect Its Frontend

Your AI has agency — here’s how to architect its frontend

AI’s not just following orders anymore. If you’re building the frontend, here’s how to design interfaces that actually understand your agent’s smarts.

Rosario De Chiara
Jun 25, 2025 ⋅ 5 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