Powerful Javascript Middleware Pattern Implementation, apply middleweares to any object.

Middleware.js

'use strict';
/* eslint-disable consistent-this */

let middlewareManagerHash = [];

/**
 * Composes single-argument functions from right to left. The rightmost
 * function can take multiple arguments as it provides the signature for
 * the resulting composite function.
 *
 * @param {...Function} funcs The functions to compose.
 * @returns {Function} A function obtained by composing the argument functions
 * from right to left. For example, compose(f, g, h) is identical to doing
 * (...args) => f(g(h(...args))).
 */
export function compose(...funcs) {
  if (funcs.length === 0) {
    return arg => arg;
  }

  funcs = funcs.filter(func => typeof func === 'function');

  if (funcs.length === 1) {
    return funcs[0];
  }

  const last = funcs[funcs.length - 1];
  const rest = funcs.slice(0, -1);
  return (...args) => rest.reduceRight((composed, f) => f(composed), last(...args));
}

/**
 * Manage and apply middlewares for an object.
 * Middleware functions are functions that have access to the target function and it's arguments,
 * and the target object and the next middleware function in the target function cycle.
 * The next middleware function is commonly denoted by a variable named next.
 *
 * Middleware functions can perform the following tasks:
 *  - Execute any code.
 *  - Make changes to the function's arguments.
 *  - End the target function.
 *  - Call the next middleware in the stack.
 *
 * If the current middleware function does not end the target function cycle,
 * it must call next() to pass control to the next middleware function. Otherwise,
 * the target function will be left hanging.
 *
 * e.g.
 *  ```
 *  const walk = target => next => (...args) => {
 *     this.log(`walk function start.`);
 *     const result = next(...args);
 *     this.log(`walk function end.`);
 *     return result;
 *   }
 *  ```
 *
 * Middleware object is an object that contains function's name as same as the target object's function name.
 *
 * e.g.
 *  ```
 *  const Logger = {
 *      walk: target => next => (...args) => {
 *        console.log(`walk function start.`);
 *        const result = next(...args);
 *        console.log(`walk function end.`);
 *        return result;
 *      }
 *   }
 *  ```
 *
 * Function's name start or end with "_" will not be able to apply middleware.
 *
 * @example
 *
 * ## Basic
 *
 * We define a Person class.
 * // the target object
 * class Person {
 *   // the target function
 *   walk(step) {
 *     this.step = step;
 *   }
 *
 *   speak(word) {
 *     this.word = word;
 *   }
 * }
 *
 * Then we define a middleware function to print log.
 *
 * // middleware for walk function
 * const logger = target => next => (...args) => {
 *   console.log(`walk start, steps: ${args[0]}.`);
 *   const result = next(...args);
 *   console.log(`walk end.`);
 *   return result;
 * }
 *
 * Now we apply the log function as a middleware to a Person instance.
 *
 * // apply middleware to target object
 * const p = new Person();
 * const middlewareManager = new MiddlewareManager(p);
 * middlewareManager.use('walk', walk);
 * p.walk(3);
 *
 * Whenever a Person instance call it's walk method, we'll see logs from the looger middleware.
 *
 * ## Middleware object
 * We can also apply a middleware object to a target object.
 * Middleware object is an object that contains function's name as same as the target object's function name.
 *
 * const PersonMiddleware {
 *   walk: target => next => step => {
 *     console.log(`walk start, steps: step.`);
 *     const result = next(step);
 *     console.log(`walk end.`);
 *     return result;
 *   },
 *   speak: target => next => word => {
 *     word = 'this is a middleware trying to say: ' + word;
 *     return next(word);
 *   }
 * }
 *
 * // apply middleware to target object
 * const p = new Person();
 * const middlewareManager = new MiddlewareManager(p);
 * middlewareManager.use(PersonMiddleware);
 * p.walk(3);
 * p.speak('hi');
 *
 * ## middlewareMethods
 * Or we can use `middlewareMethods` to define function names for middleware target within a class.
 *
 * class CuePointMiddleware {
 *   constructor() {
 *     //Define function names for middleware target.
 *     this.middlewareMethods = ['walk', 'speak'];
 *   }
 *   log(text) {
 *     console.log('Middleware log: ' + text);
 *   }
 *   walk(target) {
 *     return next => step => {
 *       this.log(`walk start, steps: step.`);
 *       const result = next(step);
 *       this.log(`walk end.`);
 *       return result;
 *     }
 *   }
 *   speak(target) {
 *     return next => word => {
 *       this.log('this is a middleware tring to say: ' + word);
 *       return next(word);
 *     }
 *   }
 * }
 *
 * // apply middleware to target object
 * const p = new Person();
 * const middlewareManager = new MiddlewareManager(p);
 * middlewareManager.use(new PersonMiddleware())
 * p.walk(3);
 * p.speak('hi');
 *
 */
export class MiddlewareManager {
  /**
   * @param {object} target The target object.
   * @param {...object} middlewareObjects Middleware objects.
   * @return {object} this
   */
  constructor(target, ...middlewareObjects) {
    let instance = middlewareManagerHash.find(function (key) {
      return key._target === target;
    });
    // a target can only has one MiddlewareManager instance
    if (instance === undefined) {
      this._target = target;
      this._methods = {};
      this._methodMiddlewares = {};
      middlewareManagerHash.push(this);
      instance = this;
    }
    instance.use(...middlewareObjects);

    return instance;
  }

  _applyToMethod(methodName, ...middlewares) {
    if (typeof methodName === 'string' && !/^_+|_+$/g.test(methodName)) {
      let method = this._methods[methodName] || this._target[methodName];
      if (typeof method === 'function') {
        this._methods[methodName] = method;
        if (this._methodMiddlewares[methodName] === undefined) {
          this._methodMiddlewares[methodName] = [];
        }
        middlewares.forEach(middleware =>
          typeof middleware === 'function' && this._methodMiddlewares[methodName].push(middleware(this._target))
        );
        this._target[methodName] = compose(...this._methodMiddlewares[methodName])(method.bind(this._target));
      }
    }
  }

  /**
   * Apply (register) middleware functions to the target function or apply (register) middleware objects.
   * If the first argument is a middleware object, the rest arguments must be middleware objects.
   *
   * @param {string|object} methodName String for target function name, object for a middleware object.
   * @param {...function|...object} middlewares The middleware chain to be applied.
   * @return {object} this
   */
  use(methodName, ...middlewares) {
    if (typeof methodName === 'object') {
      Array.prototype.slice.call(arguments).forEach(arg => {
        // A middleware object can specify target functions within middlewareMethods (Array).
        // e.g. obj.middlewareMethods = ['method1', 'method2'];
        // only method1 and method2 will be the target function.
        typeof arg === 'object' && (arg.middlewareMethods || Object.keys(arg)).forEach(key => {
          typeof arg[key] === 'function' && this._applyToMethod(key, arg[key].bind(arg));
        });
      });
    } else {
      this._applyToMethod(methodName, ...middlewares);
    }

    return this;
  }
}