usage.js

a = new VersionedMap(); // v0
console.assert(a.version === 0, "a.version === 0");
console.assert(a.isSame(a), "a.isSame(a)");

// mutate a producing b v1
b = a.set(1, 1);


console.assert(a.version === 0, "a.version === 0");
console.assert(b.version === 1, "b.version === 1");
console.assert(!a.isSame(a), "!a.isSame(a)");
console.assert(!a.isSame(b), "!a.isSame(b)");
console.assert(b.isSame(b), "!a.isSame(b)");

// no change so stay on v1
c = b.set(1, 1);
console.assert(b.version === 1, "b.version === 1");
console.assert(b.version === 1, "b.version === 1");
console.assert(b.isSame(b), "b.isSame(b)");
console.assert(b.isSame(c), "b.isSame(c)");
console.assert(c.isSame(c), "c.isSame(c)");

VersionedMap.js

class VersionedMap {
  constructor(iterableOrMap, version = 0) {
    this.map =
      iterableOrMap instanceof Map ? iterableOrMap : new Map(iterableOrMap);
    this.version = version;
    this.mutated = false;
  }

  set(key, value) {
    if (this.map.get(key) !== value) {
      this.map.set(key, value);
      this.mutated = true;
      return new VersionedMap(this.map, this.version + 1);
    }
    return this;
  }

  delete(key) {
    if (this.map.has(key)) {
      this.map.delete(key);
      this.mutated = true;
      return new VersionedMap(this.map, this.version + 1);
    }
    return this;
  }

  isSame(versionedMap) {
    return (
      !this.mutated &&
      versionedMap instanceof VersionedMap &&
      !versionedMap.mutated &&
      versionedMap.map instanceof Map &&
      versionedMap.map === this.map &&
      versionedMap.version === this.version
    );
  }
}