what do the parentheses of object attributes mean in es6?

in the change event of the input box of the react form

handleChange = (e) => {
    console.log(e);
    this.setState({
      value: e.target.value
    });
}

the event object here is a proxy object. Print it to the console and output it as follows

[[Handler]] : Object
  set :  (target, prop, value)
  __proto__ : Object
[[Target]] : SyntheticEvent
  bubbles : (...)
  cancelable : (...)
  currentTarget : (...)
  defaultPrevented : (...)
  dispatchConfig : null
  eventPhase : (...)
  isDefaultPrevented : null
  isPropagationStopped : null
  isTrusted : (...)
  nativeEvent : (...)
  target : (...)
  timeStamp : (...)
  type : (...)
  _dispatchInstances : null
  _dispatchListeners : null
  _targetInst : null
  preventDefault : (...)
  stopPropagation : (...)
  get bubbles :  ()
  set bubbles :  (val)
  get cancelable :  ()
  set cancelable :  (val)
  get currentTarget :  ()
  set currentTarget :  (val)
  get defaultPrevented :  ()
  set defaultPrevented :  (val)
  get eventPhase :  ()
  set eventPhase :  (val)
  get isTrusted :  ()
  set isTrusted :  (val)
  get nativeEvent :  ()
  set nativeEvent :  (val)
  get target :  ()
  set target :  (val)
  get timeStamp :  ()
  set timeStamp :  (val)
  get type :  ()
  set type :  (val)
  get preventDefault :  ()
  set preventDefault :  (val)
  get stopPropagation :  ()
  set stopPropagation :  (val)
  __proto__ : Object
[[IsRevoked]] : false

if it is a function object, it will have such a structure

arguments : (...)
caller : (...)
length : 0
name : ""
prototype : {constructor: }
__proto__ :  ()
[[FunctionLocation]] : emptyFunction.js:13
[[Scopes]] : Scopes[2]

what"s the point of naming the [[]] (both parentheses) attribute in this object?