CODEWITHSUNDEEP
typescript
sky
sky

Reputation: 133

What's the type of a mixin class

I cannot figure out how to get the type of a typescript mixin class without resorting so some hack (exemplified below)

type Constructor<T = {}> = new(...args: any[]) => T;

function MyMixin<T extends Constructor>(BaseClass: T) {
  return class extends BaseClass {doY() {}}
}

// Option A: Ugly/wasteful
const MixedA = MyMixin(class {doX() {}});
const dummy = new MixedA();
type MixedA = typeof dummy;

class OtherA {
  field: MixedA = new MixedA();
  a() {this.field.doX(); this.field.doY();}
}

// Option B: Verbose
class Cls {doX() {}}
interface MixinInterface {doY(): void}

const MixedB = MyMixin(Cls);
type MixedB = Cls & MixinInterface;

class OtherB {
  field: MixedB = new MixedB();
  a() {this.field.doX(); this.field.doY();}
}

It's really sad to me that typescript doesn't support honest mixins/traits, but is there some other way to declare the type of field without resorting to typeof an instance or having to re-declare the signatures in an interface (I tried typeof(new MixedBaseA()) but typeof doesn't accept arbitrary expressions)?

Upvotes: 2

Views: 747

Answers (1)

Caramiriel
Caramiriel

Reputation: 7257

Probably not what you want, but here's a less wasteful alternative. Given the definition:

type Constructor<T = {}> = new(...args: any[]) => T;

function MyMixin<T extends Constructor>(BaseClass: T) {
    return class extends BaseClass {
        doY() { }
    }
}

const MixedA = MyMixin(class { doX() {} });

You can get the type using:

function getReturnType<R>(fn: (new(...args: any[]) => R)): R {
  return {} as R;
}

const dummy = getReturnType(MixedA);
type MixedAType = typeof dummy;

const mixedA : MixedAType = new MixedA();
mixedA.doX();
mixedA.doY();

Playground

Proposal for getting the type of any expression is still open for discussion: https://github.com/Microsoft/TypeScript/issues/6606. This will get rid of the dummy variable and the function.

Or, to get a clean type MixedAType = MyMixinY & X, you can choose to return the correct constructor type in the mixin:

type Constructor<T = {}> = new(...args: any[]) => T;

interface MyMixinY {
    doY()
} 

function MixinY<T extends Constructor>(BaseClass: T)
    : Constructor<MyMixinY> & T {

    return <any> class Y extends BaseClass implements MyMixinY {
        doY() { 
            console.log("in Y");
        }
    }
}

const MixedA = MixinY(class X {
    doX() {
        console.log("in X");
    }
});

function getReturnType<R>(fn: (new(...args: any[]) => R)): R {
  return {} as R;
}

const dummy = getReturnType(MixedA);
type MixedAType = typeof dummy; // now is `type MixedAType = MyMixinY & X`

const mixedA: MixedAType = new MixedA();
mixedA.doX();
mixedA.doY();

Upvotes: 4

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