How to implement a stack with an O(1) min method

JavaScript, data structures, stacks

Here is one way to implement a stack with a min method which returns the smallest item in the stack (assuming the stack is composed only of numbers). push, pop, and min should be O(1) time.

StackMinTry in REPL
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function StackMin() {
  this.top = null;
  this.size = 0;
  this.minStack = [];
}
function Node(data) {
  this.data = data;
  this.next = null;
}
StackMin.prototype.push = function(data) {
    if (!this.minStack.length) {
      this.minStack.push(data);
    } else if (data < this.minStack[this.minStack.length-1]) {
      this.minStack.push(data);
    }
    const node = new Node(data);
    node.next = this.top;
    this.top = node;
    this.size++;
};
StackMin.prototype.pop = function() {
    if (this.top === null) return;
    const data = this.top.data;
    if (this.minStack[this.minStack.length-1] === this.top.data) this.minStack.pop();
    this.top = this.top.next;
    this.size = Math.max(0, this.size - 1);
    return data;
};
StackMin.prototype.peek = function() {
  if (!this.top) return;
  return this.top.data;
};
StackMin.prototype.isEmpty = function() {
  return this.size === 0 && !this.top;
};
StackMin.prototype.min = function() {
  return this.minStack[this.minStack.length-1];
};

Tests can be found at REPL.it