koa-Context

 * Context prototype.
 */

const proto = module.exports = {
  ...
};

throw(...args) {
  throw createError(...args);
},
onerror(err) {
  // don't do anything if there is no error.
  // this allows you to pass `this.onerror`
  // to node-style callbacks.s allows you to pass `this.onerror`
  // to node-style callbacks.
  if (null == err) return;

  if (!(err instanceof Error)) err = new Error(util.format('non-error thrown: %j', err));

  let headerSent = false;
  if (this.headerSent || !this.writable) {
    headerSent = err.headerSent = true;
  }

  // delegate
  this.app.emit('error', err, this);

  // nothing we can do here other
  // than delegate to the app-level
  // handler and log.
  if (headerSent) {
    return;
  }

  const { res } = this;

  // first unset all headers
  if (typeof res.getHeaderNames === 'function') {
    res.getHeaderNames().forEach(name => res.removeHeader(name));
  } else {
    res._headers = {}; // Node < 7.7
  }

  // then set those specified
  this.set(err.headers);

  // force text/plain
  this.type = 'text';

  // ENOENT support
  if ('ENOENT' == err.code) err.status = 404;

  // default to 500
  if ('number' != typeof err.status || !statuses[err.status]) err.status = 500;

  // respond
  const code = statuses[err.status];
  const msg = err.expose ? err.message : code;
  this.status = err.status;
  this.length = Buffer.byteLength(msg);
  this.res.end(msg);
}

const delegate = require('delegates');

 * Response delegation.
 */

delegate(proto, 'response')
  .method('attachment')
  .method('redirect')
  .method('remove')
  .method('vary')
  .method('set')
  .method('append')
  .method('flushHeaders')
  .access('status')
  .access('message')
  .access('body')
  .access('length')
  .access('type')
  .access('lastModified')
  .access('etag')
  .getter('headerSent')
  .getter('writable');


 * Request delegation.
 */

delegate(proto, 'request')
  .method('acceptsLanguages')
  .method('acceptsEncodings')
  .method('acceptsCharsets')
  .method('accepts')
  .method('get')
  .method('is')
  .access('querystring')
  .access('idempotent')
  .access('socket')
  .access('search')
  .access('method')
  .access('query')
  .access('path')
  .access('url')
  .getter('origin')
  .getter('href')
  .getter('subdomains')
  .getter('protocol')
  .getter('host')
  .getter('hostname')
  .getter('URL')
  .getter('header')
  .getter('headers')
  .getter('secure')
  .getter('stale')
  .getter('fresh')
  .getter('ips')
  .getter('ip');

module.exports = Delegator;
function (proto, target) {
  if (!(this instanceof Delegator)) return new Delegator(proto, target);
  this.proto = proto;
  this.target = target;
  this.methods = [];
  this.getters = [];
  this.setters = [];
  this.fluents = [];
}

Delegator.prototype.method = function(name){
  var proto = this.proto;
  var target = this.target;
  this.methods.push(name);

  proto[name] = function(){
    return this[target][name].apply(this[target], arguments);
  };

  return this;
};

Delegator.prototype.access = function(name){
  return this.getter(name).setter(name);
};

Delegator.prototype.getter = function(name){
  var proto = this.proto;
  var target = this.target;
  this.getters.push(name);

  proto.__defineGetter__(name, function(){
    return this[target][name];
  });

  return this;
};

Delegator.prototype.setter = function(name){
  var proto = this.proto;
  var target = this.target;
  this.setters.push(name);

  proto.__defineSetter__(name, function(val){
    return this[target][name] = val;
  });

  return this;
};

Delegator.prototype.fluent = function (name) {
  var proto = this.proto;
  var target = this.target;
  this.fluents.push(name);

  proto[name] = function(val){
    if ('undefined' != typeof val) {
      this[target][name] = val;
      return this;
    } else {
      return this[target][name];
    }
  };

  return this;
};