medium_stack_queue.js

class Queue {
  pos = -1;
  queue = [];

  empty() {
    while (this.pos > -1) {
      this.pop();
    }
  }

  pop() {
    if (this.pos === -1) { 
      return;
    }
    
    const res = this.queue[this.pos];
    this.queue.length = this.pos--;
    return res;
  }

  push(v) {
    this.queue[++this.pos] = v;
  }

  top() {
    return this.queue[this.pos];
  }
}