JavaScript Data Structure - Stack

stack.js

var Stack = function() {
  this.data = [];
};

Stack.prototype.size = function() {
  return this.data.length;
};

Stack.prototype.empty = function() {
  return this.size() === 0;
};

Stack.prototype.push = function(val) {
  this.data.push(val);
};

Stack.prototype.pop = function() {
  return !this.empty() ? this.data.pop() : -1;
};

Stack.prototype.top = function() {
  return !this.empty() ? this.data[this.data.length-1] : -1;
};