CODEWITHSUNDEEP
typescripttypescript1.8typescript-generics
wendellmva
wendellmva

Reputation: 1944

How to make a generic type constraint instanciable

I have these two code fragments how can I change them so that MemberModel can be passed in as generic type and remains instanciatable so that I can extract the follow method into its own exported function?

export interface IFollowable {
    follow(user: IUserModel): IMemberModel;
}

export class CardModel implements ICardModel, IFollowable {
    public follow(user: IUserModel): IMemberModel {
        let member = new MemberModel({userId: user.id});
        member.role = UserRoles.follower;
        this.members.push(member);
        return member;
    }
}

How do I make follow and IFollowable generic so that the whatever type I pass in remains newable like the MemberModel is right now?

Upvotes: 1

Views: 182

Answers (1)

basarat
basarat

Reputation: 276343

so that the whatever type I pass in remains newable like the MemberModel is right now

Because typescript generics are erased at compile time you need to pass them in explicitly e.g. using a factory as shown below: 

type User = {name:string};
type Member = {name: string};
type MemberCreator = {new (user:User): Member};

interface Followable {
    follow(user: User, memberCreator: MemberCreator): Member;
}

export class CardModel implements Followable {
    public follow(user: User, memberCreator: MemberCreator): Member {
       return new memberCreator(user);
    }
}

Upvotes: 2

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