fix: let MiddlewareFunc can extend by custom Context interface

example/extend/middleware.ts

@@ -0,0 +1,16 @@
+import { MiddlewareFunc, Context, type ContextDelegation } from '../../src/index.js';
+
+class CustomContext extends Context {
+  // Add your custom properties and methods here
+  get hello() {
+    return 'world';
+  }
+}
+
+type ICustomContext = CustomContext & ContextDelegation;
+
+export const middleware: MiddlewareFunc<ICustomContext> = async (ctx, next) => {
+  console.log('middleware start, %s', ctx.hello, ctx.writable);
+  await next();
+  console.log('middleware end');
+};

src/application.ts

@@ -11,18 +11,17 @@ import onFinished from 'on-finished';
 import statuses from 'statuses';
 import compose from 'koa-compose';
 import { HttpError } from 'http-errors';
-import { Context } from './context.js';
+import { Context, type ContextDelegation } from './context.js';
 import { Request } from './request.js';
 import { Response } from './response.js';
-import type { ContextDelegation } from './context.js';
 import type { CustomError, AnyProto } from './types.js';
 
 const debug = debuglog('@eggjs/koa/application');
 
 export type ProtoImplClass<T = object> = new(...args: any[]) => T;
 export type Next = () => Promise<void>;
-type _MiddlewareFunc = (ctx: ContextDelegation, next: Next) => Promise<void> | void;
-export type MiddlewareFunc = _MiddlewareFunc & { _name?: string };
+type _MiddlewareFunc<T> = (ctx: T, next: Next) => Promise<void> | void;
+export type MiddlewareFunc<T = ContextDelegation> = _MiddlewareFunc<T> & { _name?: string };
 
 /**
  * Expose `Application` class.
@@ -222,7 +221,7 @@ export class Application extends Emitter {
    */
   protected createContext(req: IncomingMessage, res: ServerResponse) {
     const context = new this.ContextClass(this, req, res);
-    return context as ContextDelegation;
+    return context;
   }
 
   /**

src/context.ts

@@ -11,7 +11,7 @@ import type { Response } from './response.js';
 import type { CustomError, AnyProto } from './types.js';
 
 export class Context {
-  [key: symbol]: unknown;
+  [key: symbol | string]: unknown;
   app: Application;
   req: IncomingMessage;
   res: ServerResponse;
@@ -158,7 +158,9 @@ export class Context {
     // We can probably remove it once jest fixes https://github.com/facebook/jest/issues/2549.
     const isNativeError = err instanceof Error ||
       Object.prototype.toString.call(err) === '[object Error]';
-    if (!isNativeError) err = new Error(util.format('non-error thrown: %j', err));
+    if (!isNativeError) {
+      err = new Error(util.format('non-error thrown: %j', err));
+    }
 
     let headerSent = false;
     if (this.response.headerSent || !this.response.writable) {
@@ -282,5 +284,4 @@ export type ContextDelegation = Context & Pick<Request, 'acceptsLanguages' | 'ac
 | 'path' | 'url' | 'accept' | 'origin' | 'href' | 'subdomains' | 'protocol' | 'host' | 'hostname'
 | 'URL' | 'header' | 'headers' | 'secure' | 'stale' | 'fresh' | 'ips' | 'ip'>
 & Pick<Response, 'attachment' | 'redirect' | 'remove' | 'vary' | 'has' | 'set' | 'append' | 'flushHeaders'
-| 'status' | 'message' | 'body' | 'length' | 'type' | 'lastModified' | 'etag' | 'headerSent' | 'writable'>
-& AnyProto;
+| 'status' | 'message' | 'body' | 'length' | 'type' | 'lastModified' | 'etag' | 'headerSent' | 'writable'>;

src/types.ts

@@ -7,5 +7,5 @@ export type CustomError = Error & {
 };
 
 export type AnyProto = {
-  [key: string]: any;
+  [key: string | symbol]: any;
 };