Show HN: multimethod.js - a Clojure-inspired multimethod library for JavaScript (opens in new tab)

Good question.

Performance will depend on a number of factors, such as how complex your dispatch function is, how complex your dispatched values and match values are (deep equality becomes more expensive as the compared objects grow in complexity), and how many methods are registered. In general, it is much more expensive than native switch statements or prototype chain lookups.

If performance is of utmost importance and you can substitute a switch statement or prototype hierarchy for a multimethod, you should. Multimethods are a useful pattern for achieving things that either aren't possible in switch (add/remove cases dynamically, deep equality case matching) or are not naturally/painlessly expressed with a prototype hierarchy.

While fib is a good example of concisely illustrating the control flow of a multimethod, it is a bad example of where it makes sense to use a multimethod. What is interesting about a multimethod is you can change your dispatch logic and bind new cases well after your multimethod function is constructed. In situations like fib where you know every case you need to handle up front you should certainly use 'switch'.

To illustrate this flexibility in the context of fib, multimethod.js, and CoffeeScript:

   > fib = multimethod()
           .when(0, 0)
           .when(1, 1)
           .default (n) -> fib(n)
   > fib 20
   6765

   > fib.dispatch (n) -> n.numberVal or n
   > fib 20
   6765

   > fib { numberVal: 20 }
   6765

   > fib.when 2, 1
   > fib.when 3, 2
   > fib 20
   6765 (now "optimized" with fewer recursive calls than before)