minstack.js

const MinStack = function() {
  this.items = [];
  this.minValueTracker = [];
};

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

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

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

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