Hash Tables & JavaScript Objects

What object enhancement pattern is demonstrated here?

const obj = {};
Object.defineProperties(obj, {
  size: {
    get() { return Object.keys(this).length },
    enumerable: false
  },
  add: {
    value: function(key, val) { this[key] = val },
    enumerable: false
  }
});

Property shadowing
Method delegation
Dynamic properties
This demonstrates dynamic properties because: 1) Defines getter for computed size property, 2) Adds non-enumerable utility methods, 3) Creates hash table-like interface, 4) Properties update automatically with object changes, 5) Methods don't appear in property enumeration, 6) Maintains clean object iteration, 7) Common pattern for implementing data structures, 8) Combines properties and behavior effectively.
Computed properties

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