Typescript: type error emerged on creating a "copy" constructor for class

I would like to add a constructor to MyClass which takes 2 parameters, first one is instance of MyClass and second one is partial of MyClass non-function properties. Example is I have below MyClass

class MyClass {
    public variable1: string | undefined;
    public variable2: number | undefined;

    public function1(): void {
        console.log("log from function1.");
    }

    constructor(variable1: string, variable2: number) {
        this.variable1 = p1;
        this.variable2 = p2;
    }
}

and

const instance1 = new MyClass("a", 1);
const instance2 = new MyClass(instance1, { variable1: "b" });  <= now instance2 is instance with variable1 as "b" and variable2 remains 1

I changed MyClass as below

type PickByValue<T, ValueType> = Pick<T, {
    [Key in keyof T]-?: T[Key] extends ValueType ? never : Key;
}[keyof T]>;

type NonFunction = PickByValue<MyClass, Function>;
type UUKeys = keyof NonFunction;

class MyClass {
    public variable1: string | undefined;
    public variable2: number | undefined;

    public function1(): void {
        console.log(this.variable1 ?? "variable missing " + this.variable2?.toString() ?? "variable2 missing");
    }

    // First constructor
    constructor(variable1: string, variable2: number);
    // Second constructor
    constructor(p1: MyClass, p2: Partial<NonFunction>);
    // Implementation
    constructor(p1: MyClass | string, p2?: Partial<NonFunction> | number) {
        if (typeof p1 === 'string' && typeof p2 === 'number') {
            this.variable1 = p1;
            this.variable2 = p2;
        } else if (p1 instanceof MyClass && p2 && typeof p2 !== "number" && p2 satisfies Partial<NonFunction>) {
            for (const key of Object.keys(p1)) {
                if (typeof p1[key as keyof MyClass] !== "function") {
                    const k = key as UUKeys;
                    this[k] = p2[k] ?? p1[k];
                    ^^^^^^  <= ERROR: Type 'string | number | undefined' is not assignable to type 'undefined'. Type 'string' is not assignable to type 'undefined'.
                }
            }
        } else {
            this.variable1 = '';
            this.variable2 = 0;
        }
    }
}

Error emerged confused my which I could not understand where undefined coming from as I already exclude undefined by converting key to UUKeys.

> Type 'string | number | undefined' is not assignable to type
> 'undefined'. Type 'string' is not assignable to type 'undefined'

BTW, I am using Typescript v5.1.6.

It makes more sense if we look at the type of the key k and the value v.

if (typeof p1[key as keyof MyClass] !== 'function') {
  const k = key as UUKeys;  // 'variable1' | 'variable2'
  const v = p2[k] ?? p1[k]; // number | string | undefined
  this[k] = v;
}

• The key type is an enum of the properties of the class (that's not a function).
• The value type is a union of the types of all (non-function) properties of the class. In this case, the value can either be a number, string, or undefined.

If the key is variable1 and the value is a number, it would be invalid. If the key is variable2 and the value is a string, it would also be invalid. The only valid value for all properties is undefined.

In this case, you're already sure that the value has the correct type for the key, because p2 is a partial of MyClass.

This is a typical case of "I know what I'm doing". Ease down on the type-checking.

(this as any)[key] = p2[key] ?? p1[key];