A lightweight plugin / dependency system for libraries. Keep code modularized & extensible. MicroPlugin is a lightweight drop-in plugin architecture for your JavaScript library. Plugins can declare dependencies to other plugins and can be initialized with options (in a variety of formats). It AMD-compatible and it works identically in Node.js and in a browser.

/**
 * microplugin.js
 * https://github.com/brianreavis/microplugin.js
 * Copyright (c) 2013 Brian Reavis & contributors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this
 * file except in compliance with the License. You may obtain a copy of the License at:
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under
 * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF
 * ANY KIND, either express or implied. See the License for the specific language
 * governing permissions and limitations under the License.
 *
 * @author Brian Reavis <brian@thirdroute.com>
 */

(function(root, factory) {
    if (typeof define === "function" && define.amd) {
        define(factory);
    } else if (typeof exports === "object") {
        module.exports = factory();
    } else {
        root.MicroPlugin = factory();
    }
})(this, function() {
    var MicroPlugin = {};

    MicroPlugin.mixin = function(Interface) {
        Interface.plugins = {};

        /**
         * Initializes the listed plugins (with options).
         * Acceptable formats:
         *
         * List (without options):
         *   ['a', 'b', 'c']
         *
         * List (with options):
         *   [{'name': 'a', options: {}}, {'name': 'b', options: {}}]
         *
         * Hash (with options):
         *   {'a': { ... }, 'b': { ... }, 'c': { ... }}
         *
         * @param {mixed} plugins
         */
        Interface.prototype.initializePlugins = function(plugins) {
            var i, n, key;
            var self = this;
            var queue = [];

            self.plugins = {
                names: [],
                settings: {},
                requested: {},
                loaded: {}
            };

            if (utils.isArray(plugins)) {
                for (i = 0, n = plugins.length; i < n; i++) {
                    if (typeof plugins[i] === "string") {
                        queue.push(plugins[i]);
                    } else {
                        self.plugins.settings[plugins[i].name] =
                            plugins[i].options;
                        queue.push(plugins[i].name);
                    }
                }
            } else if (plugins) {
                for (key in plugins) {
                    if (plugins.hasOwnProperty(key)) {
                        self.plugins.settings[key] = plugins[key];
                        queue.push(key);
                    }
                }
            }

            while (queue.length) {
                self.require(queue.shift());
            }
        };

        Interface.prototype.loadPlugin = function(name) {
            var self = this;
            var plugins = self.plugins;
            var plugin = Interface.plugins[name];

            if (!Interface.plugins.hasOwnProperty(name)) {
                throw new Error('Unable to find "' + name + '" plugin');
            }

            plugins.requested[name] = true;
            plugins.loaded[name] = plugin.fn.apply(self, [
                self.plugins.settings[name] || {}
            ]);
            plugins.names.push(name);
        };

        /**
         * Initializes a plugin.
         *
         * @param {string} name
         */
        Interface.prototype.require = function(name) {
            var self = this;
            var plugins = self.plugins;

            if (!self.plugins.loaded.hasOwnProperty(name)) {
                if (plugins.requested[name]) {
                    throw new Error(
                        'Plugin has circular dependency ("' + name + '")'
                    );
                }
                self.loadPlugin(name);
            }

            return plugins.loaded[name];
        };

        /**
         * Registers a plugin.
         *
         * @param {string} name
         * @param {function} fn
         */
        Interface.define = function(name, fn) {
            Interface.plugins[name] = {
                name: name,
                fn: fn
            };
        };
    };

    var utils = {
        isArray:
            Array.isArray ||
            function(vArg) {
                return (
                    Object.prototype.toString.call(vArg) === "[object Array]"
                );
            }
    };

    return MicroPlugin;
});