code: function heuristic() {
      if (this.type === typeEnum.unit) {
        // Remove the excess.
        return Math.max(0, this.cache.size - this.capacity)
      } else if (this.type === typeEnum.heap) {
        if (getHeapSize() >= this.capacity) {
          console.log('LRU HEURISTIC heap:', getHeapSize())
          // Remove half of them.
          return this.cache.size >> 1
        } else {
          return 0
        }
      } else {
        console.error(`Unknown heuristic '${this.type}' for LRU cache.`)
        return 1
      }
    }
docs: Returns the number of elements to remove if we're past the limit.