How can I export an overloaded function in typescript

Question

I have a function that I want to overload, so that it knows the type of arg2 based on the value of arg1 (which has a list of know values).

A very rough example to demonstrate what I'm trying to do:

interface CatArgs {legs : number}

interface FishArgs {fins: number}

type CarOrFishArgs = CatArgs | FishArgs;
type Animal = Cat | Fish;

type Type = 'Cat' | 'Fish';

class Cat{
    constructor(args:CatArgs){}
}

class Fish {
    constructor(args:FishArgs){}
}

export declare function getAnimal(type:'Cat', args:CatArgs): Cat;
export declare function getAnimal(type:'Fish', args:FishArgs): Fish;

export function getAnimal(type:Type, args:CarOrFishArgs): Animal {
    switch (type) {
        case 'Cat':
            return new Cat(args as CatArgs);
        case 'Fish':
            return new Fish(args as FishArgs);
    }
}

But I'm getting "Overload signatures must all be ambient or non-ambient".

Is this type of feature supported by TypeScript? What am I doing wrong?

Answer 1

To specify overload signatures you don't need declare:

export function getAnimal(type:'Cat', args:CatArgs): Cat;
export function getAnimal(type:'Fish', args:FishArgs): Fish;
export function getAnimal(type:Type, args:CarOrFishArgs): Animal {
    switch (type) {
        case 'Cat':
            return new Cat(args as CatArgs);
        case 'Fish':
            return new Fish(args as FishArgs);
    }
}