```ts
import { assert } from 'assert';

export class LruCache<K, V> {
  private size = 0;
  private capacity: number;
  private map = new Map<K, [V, number]>();
  private orderedKeys = [];

  constructor(capacity: number) {
    if (capacity <= 0) {
      throw new Error('Capacity must be a positive integer');
    }
    this.capacity = capacity;
  }

  get(key: K): V | undefined {
    const entry = this.map.get(key);
    if (entry) {
      this.orderedKeys.splice(this.orderedKeys.indexOf(key), 1);
      this.orderedKeys.push(key);
      return entry[0];
    }
    return undefined;
  }

  set(key: K, value: V): void {
    const entry = this.map.get(key);
    if (entry) {
      entry[0] = value;
      entry[1]++;
      this.orderedKeys.splice(this.orderedKeys.indexOf(key), 1);
      this.orderedKeys.push(key);
    } else {
      this.size++;
      this.map.set(key, [value, 1]);
      if (this.size > this.capacity) {
        const oldestKey = this.orderedKeys.shift()!;
        this.map.delete(oldestKey);
      }
    }
  }

  get size(): number {
    return this.size;
  }
}
```