Property with a custom getter to get merged Set objects as a single Set

main.es6

let obj = {
  
  a: new Set([1,2,3]),
  b: new Set([4,2,6,7]),
  c: new Set([6,2,1,0])
};

Object.defineProperties(obj, {
  
  all: {
    
    get() { return new Set([...Object.keys(this).reduce((a, b) => [...(this[a] || a), ...this[b]])]); }
  }
});