Min Stack - Design a stack that supports push, pop, top, and retrieving the minimum element in constant time

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

/**
 * initialize your data structure here.
 */
var MinStack = function () {
  this.stack = [];
  this.length = 0;
  this.min = undefined;
};

/**
 * @param {number} x
 * @return {void}
 */
MinStack.prototype.push = function (x) {
  this.stack.push(x);
  if (this.length === 0) {
    this.min = {
      previousMin: this.min,
      min: x,
    };
  } else {
    this.min =
      this.min.min < x
        ? this.min
        : {
            previousMin: this.min,
            min: x,
          };
  }
  this.length++;
};

/**
 * @return {void}
 */
MinStack.prototype.pop = function () {
  let popped = this.stack.pop();
  if (this.min && this.min.min === popped) {
    this.min = this.min.previousMin;
  }
  this.length--;
};

/**
 * @return {number}
 */
MinStack.prototype.top = function () {
  return this.stack[this.length - 1];
};

/**
 * @return {number}
 */
MinStack.prototype.getMin = function () {
  return this.min ? this.min.min : undefined;
};

/**
 * Your MinStack object will be instantiated and called as such:
 * var obj = new MinStack()
 * obj.push(x)
 * obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.getMin()
 */