Methods for a TypeScript type alias

Question

I have this Dir type alias that is a set of 8 cardinal directions, used as an enum:

type Dir = "N" | "NE" | "E" | "SE" | "S" | "SW" | "W" | "NW";

and I have a class Dirs that contains static members for only these values, as well as some utility methods on Dir:

class Dirs {
    public static readonly N: Dir = "N";
    public static readonly NE: Dir = "NE";
    public static readonly E: Dir = "E";
    public static readonly SE: Dir = "SE";
    public static readonly S: Dir = "S";
    public static readonly SW: Dir = "SW";
    public static readonly W: Dir = "W";
    public static readonly NW: Dir = "NW";

    public static unitOffset(dir: Dir): MetroPosition {
        // ...
    }

    public static opposite(dir: Dir): Dir {
        // ...
    }
}

Then, I can use them like this:

Dirs.unitOffset(Dirs.opposite(Dirs.N))

However, I want to change it to chained methods like this, for ergonomics:

Dirs.N.opposite().unitOffset()

In Rust, this is simple (disregarding the different actual values):

enum Dir {N,NE,E,SE,S,SW,W,NW}

impl Dir {
    unitOffset(self) -> MetroPosition {
        // ...
    }
    opposite(self) -> Self {
        // ...
    }
}

// usage
Dir::N.opposite().unitOffset()

Is there a way to give methods to a type alias?

do you need to use Dirs.N as string like typeof Dirs.N === 'string'? You cannot attach methods to a specific pure JS string — Alexander Nenashev
– Alexander Nenashev, Commented Jun 4 at 9:31
@AlexanderNenashev no, but I'm serializing this to JSON eventually and having it as a string helps simplify marshalling/unmarshalling. — bigyihsuan
– bigyihsuan, Commented Jun 4 at 13:11

Iaroslav Sobolev · Accepted Answer · 2025-06-04 04:36:46Z

2

Unfortunately, Typescript aliases are not objects or classes; They are just type-safe aliases to string literals. It means you can not directly attach behaviour to it (like their own methods) You could try to create a wrapper class with the desired methods, something like this:


type Dir = "N" | "NE" | "E" | "SE" | "S" | "SW" | "W" | "NW";

class DirWrapper {
  constructor(public dir: Dir) {}

  opposite(): DirWrapper {
    const opposites: Record<Dir, Dir> = {
      N: "S", E: "W", S: "N", W: "E", 
     // etc...
    };
    return new DirWrapper(opposites[this.dir]);
  }

  unitOffset() {
  //...
  }
  ... 
}

class Dirs {
  static readonly N = new DirWrapper("N");
  static readonly NE = new DirWrapper("NE");
  static readonly E = new DirWrapper("E");
  static readonly SE = new DirWrapper("SE");
  static readonly S = new DirWrapper("S");
  static readonly SW = new DirWrapper("SW");
  static readonly W = new DirWrapper("W");
  static readonly NW = new DirWrapper("NW");
}

And then you can use it like this:

const offset = Dirs.N.opposite().unitOffset();

answered Jun 4 at 4:36

Iaroslav Sobolev

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1 Comment

bigyihsuan Jun 6 at 5:41

Throwing you an upvote because I've decided to use this approach, but I want to see if there's any other methods of doing this.

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