What is the equivalent of protected in TypeScript?

Question

What is the equivalent of protected in TypeScript?

I need to add some member variables in the base class to be used on

Answer 1

How about the following approach:

interface MyType {
    doit(): number;
}

class A implements MyType {
    public num: number;

    doit() {
        return this.num; 
    }
}

class B extends A {
    constructor(private times: number) {
        super();
    }

    doit() {
        return super.num * this.times; 
    }
}

Since the num variable is defined as public, this will work:

var b = new B(4);
b.num;

But since it's not defined in the interface, this:

var b: MyType = new B(4);
b.num;

will result in The property 'num' does not exist on value of type 'MyType'.
You can try it in this playground.

You can also wrap it in a module while exporting only the interface, then from other exported methods you can return the instances (factory), that way the public scope of the variables will be "contained" in the module.

module MyModule {
    export interface MyType {
        doit(): number;
    }

    class A implements MyType {
        public num: number;

        doit() {
            return this.num; 
        }
    }

    class B extends A {
        constructor(private times: number) {
            super();
        }

        doit() {
            return super.num * this.times; 
        }
    }

    export function factory(value?: number): MyType {
        return value != null ? new B(value) : new A();
    }
}

var b: MyModule.MyType = MyModule.factory(4);
b.num; /// The property 'num' does not exist on value of type 'MyType'

Modified version in this playground.

I know it's not exactly what you asked for, but it's pretty close.