pf.js is a simple Javascript library that attempts to implement partial functions in a format akin to those found in Scala. Specifically this means that each partial function is a function that only returns something for a discrete set of values.

Further reading:
http://www.scala-lang.org/node/134
http://suereth.blogspot.com/2008/11/using-partial-functions-and-pattern.html
http://en.wikipedia.org/wiki/Partial_function

An example:
var myCondition = Condition(1, function() { return "one" });
var myPF = PF(myCondition)
myPF.isDefinedAt(1) // -> true
myPF.isDefinedAt("dog") // -> false
myPF(1) // -> "one"
myPF("dog") // -> error thrown

var anotherCondition = Condition("dog", function() { return "man's  best friend" });
var anotherPF = PF(anotherCondition)
var thirdPF = myPF.orElse(anotherPF)
thirdPF.isDefinedAt(1) // -> true
thirdPF.isDefinedAt("dog") // -> true
thirdPF(1) // -> "one"
thirdPF("dog") // -> "man's best friend"