Find X-Sum of All K-Long Subarrays I

export function findXSum(nums: number[], k: number, x: number): number[] {
  const n = nums.length;
  const counter = new Counter(nums.slice(0, k));
  const answer = [counter.xSum(x)];
  for (let i = k; i < n; i++) {
    counter.sub(nums[i - k]);
    counter.add(nums[i]);
    answer.push(counter.xSum(x));
  }
  return answer;
}
 
class Counter {
  private readonly map: Map<number, number>;
  constructor(nums: number[]) {
    this.map = new Map();
    nums.forEach((num) => {
      this.add(num);
    });
  }
 
  get(key: number): number {
    return this.map.get(key) ?? 0;
  }
 
  add(key: number) {
    return this.map.set(key, this.get(key) + 1);
  }
 
  sub(key: number) {
    return this.map.set(key, this.get(key) - 1);
  }
 
  xSum(x: number) {
    return [...this.map.entries()]
      .sort((a, b) => (a[1] !== b[1] ? b[1] - a[1] : b[0] - a[0]))
      .slice(0, x)
      .reduce((prev, curr) => prev + curr[0] * curr[1], 0);
  }
}