7.3 Persistent Group (Eloquent JavaScript Solutions)

7-3 Eloquent Javascript Solutions.js

function logSetElements(value1, value2, set) {
  console.log('set[' + value1 + '] = ' + value2);
}

class PGroup {
  constructor(pg = new Set()) {
    this.group = pg;
  }
  add(item) {
    let g = PGroup.from(this.group);
    g.group.add(item);
    return g;
  }
  delete(item) {
    let g = PGroup.from(this.group);
    g.group.delete(item);
    return g;
  }
  has(item) {
    return this.group.has(item);
  }
  static empty() {
    return new PGroup();
  }
  static from(a) {
    let g = new PGroup();
    for (let item of a) {
      g.group.add(item);
    }
    return g;
  }
}

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

a.group.forEach(logSetElements);
ab.group.forEach(logSetElements);
b.group.forEach(logSetElements);

Hints

Note: I didn't use the following hints so my solution uses sets instead of arrays.

The most convenient way to represent the set of member values remains is still an array, since those are easy to copy.

When a value is added to the group, you can create a new group with a copy of the original array that has the value added (for example using concat). When a value is deleted, you filter it from the array.

The class’ constructor can take such an array as argument, and store it as the instance’s (only) property. This array is never updated.

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.

You only need one empty instance because all empty groups are the same and instances of the class don’t change. You can create many different groups from that single empty group without affecting it.