multimethods.js

window.onload = function() {
  window.multi = function(dispatchFn) {
    var methods = [];
    
    var fn = function() {
      var val = dispatchFn.apply(this, arguments);
      return (typeof methods[val] !== "undefined" ? methods[val] : methods['default'])();
    }
    
    fn.method = function(dispatchVal, tailFn) {
      methods[dispatchVal] = tailFn;
    }
    
    return fn;
  }
  
  var foo = multi(function(x) { return [x.rank, x.lvl]; });
  
  foo.method(['silver', 'one'], function() { return "silver one"; });
  foo.method(['silver', 'two'], function() { return "silver two"; });
  foo.method(['gold', 'one'], function() { return "gold one"; });
  foo.method(['gold', 'two'], function() { return "gold two"; })
  foo.method('default', function() { return "default"; });
  
  alert(foo({ rank: 'silver', lvl: 'one' }));
  alert(foo({ rank: 'silver', lvl: 'two' }));
  alert(foo({ rank: 'gold', lvl: 'one' }));
  alert(foo({ rank: 'gold', lvl: 'two' }));
  alert(foo({ rank: 'platinum', lvl: 'one' }));
}