How to map properties from an angular reactive form to an object only with class member properties?

I have tried mapping reactive form properties to an object class using (Object.Assign) but it adds extra properties that doesn't belong to the class or interface, ex:

export interface MyInterface {
    a: string |null,
    b: number|null
}

and when I submit the form I do the following:

onSubmit() {
    const mynewobject:MyInterface = Object.assign({},this.myForm.getRawValue());

    console.warn(mynewobject);
  }

but the result I receive is :

{a: 'text', b: 1, c: 'additional property from form'}

I only want to map properties a and b.

Is there a way to map only properties that belong to the class or interface?

The most closer is iterate over the keys of the object. filter this keys by some method and use reduce to asign the variables to a new object.

e.g.

const obj:any={a:'fool',b:10,d:'other'}
const obj2=Object.keys(obj).
    filter((key) => ['a','b'].includes(key)).
    reduce((cur, key) => { return Object.assign(cur, { [key]: obj[key] })}, {});
console.log(obj2) //{a:fool,b:10}

In this case you filter if the key is in the array ['a',`b']. If all the properties you need starts with some letter or you can delete some especific "keys" it's easy create the filter.

NOTE: Remember that formgroup.value only give you the formControls not disabled, so another approach can be disable this controls.