TypeScript: Multiple constructor implementations are not allowed

Question

I have an Object where I use it in multiple services, and each one should take some parameters, so I create two constructors, but TypeScript did not allow me to do this. My example is:

class User {
    id: number;
    username: string;
    password: string;
    email: string;
    firstName: string;
    lastName: string;
    roles: string[];

    constructor(username: string, password: string){
        this.username = username;
        this.password = password;
    }

    constructor(id: number, username: string, firstname: string, lastname: string, roles: string[]){
        this.id = id;
        this.username= username;
        this.firstname= firstname;
        this.lastname= lastname;
        this.roles = roles;
    }
    //.. and maybe another constructor also
}

Is there a trick to solve this issue, please?

---

When I use the optional ? in constructors for example:

constructor(
    public id?: number,
    public username?: string,
    public email?: string,
    public password?: string,
    public firstName?: string,
    public lastName?: string,
    public roles?: string[]) {
}

and when I get my data from backend:

this.service.usersList().subscribe(users => {
  console.log(users);
  this.dataSource.data = users;
});

The roles is set in the password and not in the roles failed:

{
  "id": 1,
  "username": "user1",
  "email": "user1@email.com",
  "password": [
    "USER",
    "MODR"
  ]
}

For that I'm not sure about this trick.

---

Maybe I was not precise, but I use this method to parse my data:

static fromJson(item: Object): any {
    return new User(
        item['id'],
        item['username'],
        item['email'],
        item['roles']
    );
}

For that, when I create a constructor with optional, it will set the attributes in order of my call.

Answer 1

You can't use multiple constructors, but you can add a few optional parameters and verify if it exists, like the following:

class User {
    id: number;
    username: string;
    password: string;
    email: string;
    firstname: string;
    lastname: string;
    roles: string[];
    // The "?" says that its optional parameter
    constructor(id?: number, username?: string, firstname?: string,
        lastname?: string, roles?: string[], password?: string) {
        if (id) { // if id exists , you can implement the first constructor
            this.id = id;
            this.username = username;
            this.firstname = firstname;
            this.lastname = lastname;
            this.roles = roles;
        }
        if (password) { // if password exists : you can implement the second one
            this.username = username;
            this.password = password;
        }
    }
}

Your response should be like this before this works fine:

static fromJson(item: Object): any {
    return new User({
        id : item['id'],
        username : item['username'],
        email : item['email'],
        roles : item['roles']
    });
}

So your constructor should be like this:

constructor(user: any){
    if (user.id) { // if id exists , you can implement the first constructor
        this.id = user.id;
        this.username = user.username;
        this.firstname = user.firstname;
        this.lastname = user.lastname;
        this.roles = user.roles;
    }
    if (user.password) { // if password exists : you can implement the second one
        this.username = user.username;
        this.password = user.password;
    }
}

Or if you don't want to do that, you can set the response regarding the order, like this:

static fromJson(item: Object): any {
    return new User(
        item['id'],
        item['username'],
        undefined,
        item['email'],
        undefined,
        undefined,
        item['roles']
    );
}

Answer 2

I found the solution:

What happened?

When you create a constructor with optional parameters and try to call this constructor, it will set the attributes with the order of call. For that, when I call:

new User(
    item['id'],
    item['username'],
    item['email'],
    item['roles']
);

The roles in set in the firstName or password.

Solution

To solve this, it's required to change the order or parameters in the constructor:

constructor(
    public id?: number,
    public username?: string,
    public email?: string,
    public roles?: string[],

    public password?: string,
    public firstName?: string,
    public lastName?: string) {
}

Or if you won't change the order, just use undefined for example:

new User(
    item['id'],
    item['username'],
    undefined,
    item['email'],
    undefined,
    undefined,
    item['roles']
);

Until you arrive to the position of your attribute.

Answer 3

Use:

export class A {
   constructor() {
      // Something here
   }
   secondConstructor() {
      // Something here
      return this;
   }
}

And then you use just like this:

const a = new A();
const a2 = new A().secondConstructor();