PairWeakMap.ts

class PairWeakMap<A extends object, B extends object, C> {
  map: WeakMap<A, WeakMap<B, C>>

  constructor() {
    this.map = new WeakMap()
  }

  set(a: A, b: B, c: C): this {
    const { map } = this
    let submap = map.get(a)
    if (!submap) {
      submap = new WeakMap()
      map.set(a, submap)
    }
    submap.set(b, c)
    return this
  }

  get(a: A, b: B): C | undefined {
    const submap = this.map.get(a)
    if (!submap) {
      return
    }
    return submap.get(b)
  }
}

const a = {}
const b = {}
console.log(new PairWeakMap().set(a, b, 123).get(a, b))