minstack.js

var MinStack = function() {
  this.items = [];
  this.auxItems = [];
};

MinStack.prototype.push = function(x) {
  const firstAuxItem = this.auxItems[this.auxItems.length - 1];
  this.items.push(x);
  if (!this.auxItems.length) {
    this.auxItems.push(x);
  } else {
    if (x < firstAuxItem) {
      this.auxItems.push(x);
    } else {
      this.auxItems.push(firstAuxItem);
    }
  }
};

MinStack.prototype.pop = function() {
  this.items.pop();  
  this.auxItems.pop();
};

MinStack.prototype.getMin = function() {
  return this.auxItems[this.auxItems.length - 1];
};

var newStack = new MinStack();
newStack.push(4);
newStack.push(10);
newStack.push(1);
newStack.push(2);
newStack.push(1524);
newStack.pop();
newStack.pop();
newStack.pop();
newStack.getMin();