Merge pull request #32671 from astashov/typesafe-compose

[@types/koa-compose]: Provide way for typesafe compose

types/koa-compose/index.d.ts

@@ -2,6 +2,13 @@
 // Project: https://github.com/koajs/compose
 // Definitions by: jKey Lu <https://github.com/jkeylu>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.3
+
+import * as Koa from "koa";
+
+declare function compose<T, U, V, W>(
+    middleware: [Koa.Middleware<T, U>, Koa.Middleware<V, W>]
+): Koa.Middleware<T & V, U & W>;
 
 declare function compose<T>(middleware: Array<compose.Middleware<T>>): compose.ComposedMiddleware<T>;
 

types/koa-compose/koa-compose-tests.ts

@@ -1,4 +1,5 @@
 import compose = require('koa-compose');
+import * as Koa from "koa";
 
 const fn1: compose.Middleware<any> = (context: any, next: () => Promise<void>): Promise<any> =>
     Promise
@@ -11,3 +12,40 @@ const fn2: compose.Middleware<any> = (context: any, next: () => Promise<void>):
         .then(next);
 
 const fn = compose([fn1, fn2]);
+
+interface FooCtx {
+    foo: string;
+}
+
+const fooMiddleware: Koa.Middleware<FooCtx> = async (ctx, next) => {
+    ctx.state.foo = "foo";
+    await next();
+};
+
+interface BarCtx {
+    bar: string;
+}
+
+const barMiddleware: Koa.Middleware<BarCtx> = async (ctx, next) => {
+    ctx.state.bar = "bar";
+    await next();
+};
+
+interface WooCtx {
+    woo: string;
+}
+
+const wooMiddleware: Koa.Middleware<WooCtx> = async (ctx, next) => {
+    ctx.state.woo = "woo";
+    await next();
+};
+
+new Koa<{}, {}>()
+    .use(compose([compose([fooMiddleware, barMiddleware]), wooMiddleware]))
+    .use(async (ctx, next) => {
+        ctx.state.foo;
+        ctx.state.bar;
+        ctx.state.woo;
+        ctx.body = "Something";
+        await next();
+    });