Dead-simple Clojure-style multimethods for JavaScript. Public Domain.

multimethod.js

/**
 * Dead-simple multimethods for JavaScript.
 *
 * Usage:
 *
 *   var count = multimethod([
 *     String, function(self) {
 *       return self.length;
 *     },
 *     Number, function(self) {
 *       return self;
 *     },
 *     Error, function(self) {
 *       return self.stack.length;
 *     }
 *   ]);
 */
(function(global) {
  var multimethodId = 0;
  function multimethod(name, implArrayMap) {
    if (implArrayMap === undefined) {
      implArrayMap = name;
      name = 'multimethod' + (++multimethodId);
    }
    console.log(name);
    var map = new Map();
    for (var i = 0; i < implArrayMap.length;) {
      var constructor_ =  implArrayMap[i];
      if (typeof constructor_ === "string") {
        constructor_ = global[constructor_];
      }
      map.set(implArrayMap[i++], implArrayMap[i++]);
    }
    name = "$multi_" + name;
    var typeMap = new WeakMap();
    return function(self) {
      var impl = self[name];
      if (!impl) {
        impl = map.get(self.constructor) || map.get("default");
        if (!impl) {
          throw new Error("No method defined for type: " + self.constructor);
        }
        self.constructor.prototype[name] = impl;
      }
      return impl.apply(null, arguments);
    }
  }

  global.multimethod = multimethod;
})(typeof window !== "undefined" || this);