Typescript serializing generic

Question

I have the following base class:

export default class BaseModel{
    Id:string;
    CreatedDate:Date;
}

With following child class:

export default class ProfileModel extends BaseModel{
    UserName:string;
    Email:string;
    Password:string;
    FirstName:string;
    LastName:string;
    Vendor:Boolean;
    Authenticated:Boolean;
    constructor(aUserName, anEmail, aPassword, aFirstName, aLastName, 
aVendor, isAuthenticated){
    super();
    this.UserName = aUserName;
    this.Email = anEmail;
    this.Password = aPassword;
    this.FirstName = aFirstName;
    this.LastName = aLastName;
    this.Vendor = aVendor;
    this.Authenticated = isAuthenticated;
    }
    public static NewNonVendor(aUserName, anEmail, aPassword, aFirstName, 
aLastName):ProfileModel{
        return new ProfileModel(aUserName, anEmail, aPassword, aFirstName, 
    aLastName, null, false);
    }
}

I would like to move the fromJSON method to base class to handle serialization for all child classes. I want to basically do the following but having trouble coming up with the correct syntax:

export default class BaseModel<T>{
    Id:string;
    CreatedDate:Date;
    static fromJSON(d: Object): T {
        return Object.assign(new T(), d);
    }
}

CertainPerformance · Accepted Answer · 2020-11-29 19:04:05Z

1

If you type the T generic as T extends BaseModel, you can type the this for the static method as this: new () => T:

class BaseModel {
  Id: string;
  CreatedDate: Date;
  static fromJSON<T extends BaseModel>(this: new () => T, d: Object): T {
    return Object.assign(new this(), d);
  }
  constructor(Id: string, CreatedDate: Date) {
    this.Id = Id;
    this.CreatedDate = CreatedDate;
  }
}

answered Nov 29, 2020 at 19:04

CertainPerformance

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9 Comments

Maxqueue Over a year ago

@CeertainPerformance Yes this appears to work. Was there a reason you added the constructor?

CertainPerformance Over a year ago

Only so that the TypeScript would compile properly on the playground without errors (use your original constructor implementation, of course - it isn't in the question, so it was throwing a TS error)

Maxqueue Over a year ago

so now when trying to call fromJSON using following: let test = ProfileModel.fromJSON(json as Object); I get following error: The 'this' context of type 'typeof ProfileModel' is not assignable to method's 'this' of type 'new () => ProfileModel'.

CertainPerformance Over a year ago

It looks to compile without errors here: typescriptlang.org/play?#code/…

CertainPerformance Over a year ago

Also note that JSON is a way of representing an object in a string. If what you have is an object, it's not a JSON - maybe call it fromPlainObject instead of fromJSON? Or pass in the JSON and use JSON.parse on it inside the method?

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