set(key, value) {
    const bucketIndex = this.getIndex(key);
    if (this.buckets[bucketIndex]) {
      this.buckets[bucketIndex].push({ key, value });
      if (this.buckets[bucketIndex].length > 1) { this.collisions++; }
    } else {
      this.buckets[bucketIndex] = [{ key, value }];
    }
    return this;
  }
insert a key/value pair into the hash map
@param {any} key
@param {any} value