Way of specifying non-enumerable properties

Object assign is defined like below in TS:

interface ObjectConstructor {
    /**
      * Copy the values of all of the enumerable own properties from one or more source objects to a
      * target object. Returns the target object.
      * @param target The target object to copy to.
      * @param source The source object from which to copy properties.
      */
    assign<T, U>(target: T, source: U): T & U;
}

Though from MDN it is only copying members that are enumerable:

The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

So if U above has non-enumerable members, TS will copy them anyway. This is slightly incorrect and unsafe.

One issue I recently ran into was

const selection = Object.assign({}, window.getSelection());

I was assuming I was copying all members of the Selection object to {}:

interface Selection {
    readonly anchorNode: Node;
    readonly anchorOffset: number;
    // etc ...
}

declare var Selection: {
    prototype: Selection;
    new(): Selection;
}

Though it didn't copy any members at all, because the Selection object only contains non-enumerable members.

Proposal

Mark properties as non-enumerable

interface Selection {
    readonly nonenum anchorNode: Node;
    readonly nonenum anchorOffset: number;
    // etc ...
}

And have an operator to get the only "enum side" of a type:

interface ObjectConstructor {
    /**
      * Copy the values of all of the enumerable own properties from one or more source objects to a
      * target object. Returns the target object.
      * @param target The target object to copy to.
      * @param source The source object from which to copy properties.
      */
    assign<T, U>(target: T, source: U): T & enumsof U;
}

Object.assign の ObjectConstructor 宣言と Selection の例から始め、提案されている nonenum マーカーおよび enumsof 演算子を、JavaScript の列挙可能な自身のプロパティに関する挙動と比較します。型システムが列挙可能なプロパティを表現するための合意された方法を持ち、Object.assign がコピーされるメンバーを過大に見積もらなくなり、示されたケースをカバーできれば完了です。