Get properties of a class

This TypeScript code

class A {
    private a1;
    public a2;
}

compiles to this JavaScript code

class A {
}

That's because properties in JavaScript start extisting only after they have some value. You have to assign the properties some value.

class A {
    private a1 = "";
    public a2 = "";
}

it compiles to

class A {
    constructor() {
        this.a1 = "";
        this.a2 = "";
    }
}

Still, you cannot get the properties from mere class (you can get only methods from prototype). You must create an instance. Then you get the properties by calling Object.getOwnPropertyNames().

let a = new A();
let array = return Object.getOwnPropertyNames(a);

array[0] === "a1";
array[1] === "a2";

Applied to your example

class Describer {
    static describe(instance): Array<string> {
        return Object.getOwnPropertyNames(instance);
    }
}

let a = new A();
let x = Describer.describe(a);

Some answers are partially wrong, and some facts in them are partially wrong as well.

Answer your question: Yes! You can.

In Typescript

class A {
    private a1;
    private a2;


}

Generates the following code in Javascript:

var A = /** @class */ (function () {
    function A() {
    }
    return A;
}());

as @Erik_Cupal said, you could just do:

let a = new A();
let array = return Object.getOwnPropertyNames(a);

But this is incomplete. What happens if your class has a custom constructor? You need to do a trick with Typescript because it will not compile. You need to assign as any:

let className:any = A;
let a = new className();// the members will have value undefined

A general solution will be:

class A {
    private a1;
    private a2;
    constructor(a1:number, a2:string){
        this.a1 = a1;
        this.a2 = a2;
    }
}

class Describer{

   describeClass( typeOfClass:any){
       let a = new typeOfClass();
       let array = Object.getOwnPropertyNames(a);
       return array;//you can apply any filter here
   }
}

For better understanding this will reference depending on the context.

Another solution, You can just iterate over the object keys like so, Note: you must use an instantiated object with existing properties:

printTypeNames<T>(obj: T) {
    const objectKeys = Object.keys(obj) as Array<keyof T>;
    for (let key of objectKeys)
    {
       console.log('key:' + key);
    }
}