class Holder {

    constructor(init) {

        Holder.internalAssign(this, init);

    }

    static has(instance, property) {

        return Object.prototype.hasOwnProperty.call(instance, property);

    }

    static internalAssign(target, source) {

        if (Holder.has(source, "a"))

            target.a = source.a;

        if (Holder.has(source, "b"))

            target.b = source.b;

        if (Holder.has(source, "c"))

            target.c = source.c;

    }

    clone() {

        return new Holder(this);

    }

​

    cloneWith(props) {

        const cloned = this.clone();

        Holder.internalAssign(cloned, props);

        return cloned;

    }

    cloneWith2(props) {

        const cloned = this.clone();

        Object.assign(this, props);

        return cloned;

    }

​

    mutate(mutation) {

        const cloned = this.clone();

        mutation(cloned);

        return cloned;

    }

}

​

var initHolder = new Holder({

    a: "value of a",

    b: "value of b",

    c: 1

});