Is there any way to use a numeric type as an object key?

No, this is not possible. The key will always be converted to a string. See Property Accessor docs

Property names must be strings. This means that non-string objects cannot be used as keys in the object. Any non-string object, including a number, is typecasted into a string via the toString method.

> var foo = {}
undefined

> foo[23213] = 'swag'
'swag'

> foo
{ '23213': 'swag' }

> typeof(Object.keys(foo)[0])
'string'

In an object, no, but I have found Map extremely useful for this application. Here is where I have used it for numeric keys, a key-based event.

onKeydown(e) {
  const { toggleSidebar, next, previous } = this.props;

  const keyMapping = new Map([
    [ 83, toggleSidebar ],  // user presses the s button
    [ 37, next          ],  // user presses the right arrow
    [ 39, previous      ]   // user presses the left arrow
  ]);

  if (keyMapping.has(e.which)) {
    e.preventDefault();
    keyMapping.get(e.which)();
  }
}

Appears to be by design in ECMA-262-5:

The Property Identifier type is used to associate a property name with a Property Descriptor. Values of the Property Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a Property Descriptor value.

However, I don't see a definite specification for it in ECMA-262-3. Regardless, I wouldn't attempt to use non-strings as property names.