implement-stack

class Stack {
  constructor() {
    this.body = {};
    this.minimum;
  }
  
  push(elem) {
    this.body = {
      val: elem,
      below: Object.assign({}, this.body)
    };
    
    if (this.minimum===undefined || elem<this.minimum) this.minimum = elem;
    return this.count();
  }
  
  pop() {
    const top = this.body.val;
    this.body = this.body.below || {};
    return top;
  }
  
  count() {
    const recurs = function recurs(obj) {
      if (!obj.val) return 0;
      return 1 + recurs(obj.below);
    }
      
    return recurs(this.body);
  }
  
  min() {
    return this.minimum;
  }
}

const myStack = new Stack();

myStack.push(12);
myStack.push(15);
myStack.push(17);

myStack.min();   // 12