https://eloquentjavascript.net/07_robot.html#i_s+ntyh5xrm

Eloquent JavaScript 0703 Persistent Group.js

class PGroup {
  // Your code here
  constructor(values) {
    this.values = values;
  }
  
  add(elem) {
    if (this.has(elem)) {
      return this;
    } else {
      return new PGroup([...this.values, elem]);
    }
  }
  
  delete(elem) {
    if (this.has(elem)) {
      return new PGroup(this.values.filter(e => e !== elem));
    } else {
      return this;
    }
  }
  
  has(elem) {
    return this.values.includes(elem);
  }
}

// To add a property (empty) to a constructor that is not a method,
// you have to add it to the constructor after the class definition, as a regular property.
PGroup.empty = new PGroup([]);

let a = PGroup.empty.add("a");
let ab = a.add("b");
let b = ab.delete("a");

console.log(b.has("b"));
// → true
console.log(a.has("b"));
// → false
console.log(b.has("a"));
// → false