koa-router源码学习小结

/**
 * Returns separate middleware for responding to `OPTIONS` requests with
 * an `Allow` header containing the allowed methods, as well as responding
 * with `405 Method Not Allowed` and `501 Not Implemented` as appropriate.
 *
 * @example
 *
 * ```javascript
 * var Koa = require('koa');
 * var Router = require('koa-router');
 *
 * var app = new Koa();
 * var router = new Router();
 *
 * app.use(router.routes());
 * app.use(router.allowedMethods());
 * ```
 *
 * **Example with [Boom](https://github.com/hapijs/boom)**
 *
 * ```javascript
 * var Koa = require('koa');
 * var Router = require('koa-router');
 * var Boom = require('boom');
 *
 * var app = new Koa();
 * var router = new Router();
 *
 * app.use(router.routes());
 * app.use(router.allowedMethods({
 *  throw: true,
 *  notImplemented: () => new Boom.notImplemented(),
 *  methodNotAllowed: () => new Boom.methodNotAllowed()
 * }));
 * ```
 *
 * @param {Object=} options
 * @param {Boolean=} options.throw throw error instead of setting status and header
 * @param {Function=} options.notImplemented throw the returned value in place of the default NotImplemented error
 * @param {Function=} options.methodNotAllowed throw the returned value in place of the default MethodNotAllowed error
 * @returns {Function}
 */
Router.prototype.allowedMethods = function (options) {
 options = options || {};
 var implemented = this.methods;
 return function allowedMethods(ctx, next) {
   // 所有中间件执行完之后执行allowedMethod方法
  return next().then(function() {
   var allowed = {};
      // 没有响应状态码或者响应了404
   if (!ctx.status || ctx.status === 404) {
     // 在match方法中，匹配的路由的layer实例对象组成的数组
    ctx.matched.forEach(function (route) {
     route.methods.forEach(function (method) {
       // 把匹配的路由的http方法保存起来，认为是允许的http请求方法
      allowed[method] = method;
     });
    });
    var allowedArr = Object.keys(allowed);
        // 如果该方法在router实例的methods中不存在
    if (!~implemented.indexOf(ctx.method)) {
      // 如果在初始化router时配置了throw属性为true
     if (options.throw) {
      var notImplementedThrowable;
      if (typeof options.notImplemented === 'function') {
        // 指定了报错函数
       notImplementedThrowable = options.notImplemented(); // set whatever the user returns from their function
      } else {
        // 没有指定则抛出http异常
       notImplementedThrowable = new HttpError.NotImplemented();
      }
      throw notImplementedThrowable;
     } else {
       // 没有配置throw则响应501								 
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