countSet.js

// A countSet: a Javascript data structure
// I tend to build from scratch often.

function countSet() {
  this.set = {};
}

countSet.prototype.incr = function(key) {
  if (!this.set[key]) this.set[key] = 0;
  this.set[key]++;
  return this;
};

countSet.prototype.decr = function(key) {
  if (!this.set[key]) this.set[key] = 0;
  this.set[key]--;
  return this;
};

countSet.prototype.value = function() {
  return this.set;
};

var set = new countSet();

set.incr('keys').decr('foo').value();