A relatively small, pretty crappy inheritance/class thingy

defineClass.js

// Here's the defineClass function
    (function() {
      function defineClass() {
        var kDefineClassMagicValue = 'defineClass_do_not_call_user_provided_constructor';
        var args = Array.prototype.slice.apply(arguments);
        var className, superClassName, constructor, properties;

        className = args.shift();
        if (typeof args[0] == 'string') {
          superClassName = args.shift();
        }
        if (typeof args[0] == 'function') {
          constructor = args.shift();
        } else if(superClassName) {
          constructor = window[superClassName].prototype.constructor;
        } else {
          constructor = function() {};
        }
        if (typeof args[0] == 'object') {
          properties = args.shift();
        } else {
          properties = {};
        }

        (function(className, superClassName, constructor, properties) {
          function theNewClass() {
            this.className = className;
            this.constructor = constructor;

            if (arguments[0] != kDefineClassMagicValue) {
              this.constructor.apply(this, arguments);
            }
          }

          if (superClassName) {

            theNewClass.prototype = new window[superClassName](kDefineClassMagicValue);
          }
          theNewClass.constructor = constructor;

          theNewClass.className = className;
          Object.keys(properties).forEach(function(key) {
            theNewClass.prototype[key] = properties[key];
          }.bind(this));

          window[className] = theNewClass;
        })(className, superClassName, constructor, properties);
      }
      window.defineClass = defineClass;
    })();

// This is a class declaration
      defineClass('SMAgent', function(engine) {
        this.engine = engine;
      }, {
        tick: function() {
          console.log('agent tick');
        }
      });

// and here's a subclass
      defineClass('SMPlayer', 'SMAgent', {
        isPlayer: true,
        tick: function() {
          console.log('player tick');
        }
      });