Optional parameter of class in typescript

Question

given following code:

class ConnectionOptions
{
    host: string = "localhost";
    port: number = 5672;
    username: string = "guest";
    password: string = "guest";
}

class ConnectorClass
{
    options: ConnectionOptions = new ConnectionOptions();

    SetOptions(options: ConnectionOptions)
    {
        console.log(options);
        this.options = options;
    }
}

// Adopt singleton pattern
var Connector = (function () {
    var instance : ConnectorClass;

    function createInstance() {
        return new ConnectorClass();
    }
 
    return {
        I: function () : ConnectorClass {
            if (!instance) {
                instance = createInstance();
            }
            return instance;
        }
    };
})();

// TestApp
Connector.I().SetOptions({ 'host': "192.168.17.5" });

In the last line there is following typescript error:

index.ts:50:26 - error TS2345: Argument of type '{ host: string; }' is not assignable to parameter of type 'ConnectionOptions'.
  Type '{ host: string; }' is missing the following properties from type 'ConnectionOptions': port, username, password

50 Connector.I().SetOptions({ 'host': "192.168.17.5" });

I understand why the error is thrown: TS is expecting 4 properties but I only want to set 1 of them, isnt this possible in Typescript?

Answer 1

For your code, an interface seems more suitable than a class. You have several classes implementing the interface if required. Then, if you want some of arguments to be optional, you may use the optional modifier ( ? ):

interface ConnectionOptions {
    host?: string;
    port?: number;
    username?: string;
    password?: string;
}

const DefaultConnectionOptions: ConnectionOptions = {
  host: 'localhost',
  port: 5672,
  username: 'guest',
  password: 'guest',
}

Now your SetOptions method can look something like this:

    options: ConnectionOptions = DefaultConnectionOptions;

    SetOptions(options: ConnectionOptions) {
        console.log(options);
        this.options = { ...DefaultConnectionOptions, ...options };
    }

Your call Connector.I().SetOptions({ 'host': "192.168.17.5" }); should now work because no property is mandatory.

---

It may be that you want all connection options to be mandatory, but when calling SetOptions they are not mandatory. In that case do not use the optional modifier on the properties. Change your SetOptions signature to use Partial<T> instead:

    SetOptions(options: Partial<ConnectionOptions>) {
        console.log(options);
        this.options = { ...DefaultConnectionOptions, ...options };
    }

The above will still force this.options to have every single property, but the argument options does not have to provide all properties.