— name: adapter-express description: > Mount tRPC as Express middleware with createExpressMiddleware() from @trpc/server/adapters/express. Access Express req/res in createContext via CreateExpressContextOptions. Mount at a path prefix like app.use('/trpc', …). Avoid global express.json() conflicting with tRPC body parsing for FormData. type: core library: trpc library_version: '11.16.0' requires: – server-setup sources: – www/docs/server/adapters/express.md – examples/express-server/src/server.ts — # tRPC — Adapter: Express ## Setup “`ts // server.ts import { initTRPC } from '@trpc/server'; import * as trpcExpress from '@trpc/server/adapters/express'; import express from 'express'; import { z } from 'zod'; const createContext = ({ req, res, }: trpcExpress.CreateExpressContextOptions) => { return { req, res }; }; type Context = Awaited<ReturnType<typeof createContext>>; const t = initTRPC.context<Context>().create(); const appRouter = t.router({ greet: t.procedure .input(z.object({ name: z.string() })) .query(({ input }) => ({ greeting: `Hello, ${input.name}!` })), }); export type AppRouter = typeof appRouter; const app = express(); app.use( '/trpc', trpcExpress.createExpressMiddleware({ router: appRouter, createContext, }), ); app.listen(4000, () => { console.log('Listening on http://localhost:4000'); }); “` ## Core Patterns ### Accessing Express req/res in context “`ts import * as trpcExpress from '@trpc/server/adapters/express'; const createContext = ({ req, res, }: trpcExpress.CreateExpressContextOptions) => { const token = req.headers.authorization?.split(' ')[1]; return { token, res }; }; type Context = Awaited<ReturnType<typeof createContext>>; “` `CreateExpressContextOptions` provides typed access to the Express `req` (IncomingMessage) and `res` (ServerResponse). ### Adding tRPC alongside existing Express routes “`ts import * as trpcExpress from '@trpc/server/adapters/express'; import cors from 'cors'; import express from 'express'; import { createContext } from './context'; import { appRouter } from './router'; const app = express(); app.use(cors()); app.get('/health', (_req, res) => { res.json({ status: 'ok' }); }); app.use( '/trpc', trpcExpress.createExpressMiddleware({ router: appRouter, createContext, }), ); app.listen(4000); “` ### Limiting batch size with maxBatchSize “`ts import * as trpcExpress from '@trpc/server/adapters/express'; import express from 'express'; import { createContext } from './context'; import { appRouter } from './router'; const app = express(); app.use( '/trpc', trpcExpress.createExpressMiddleware({ router: appRouter, createContext, maxBatchSize: 10, }), ); app.listen(4000); “` Requests batching more than `maxBatchSize` operations are rejected with a `400 Bad Request` error. Set `maxItems` on your client's `httpBatchLink` to the same value to avoid exceeding the limit. ## Common Mistakes ### HIGH Global express.json() consuming tRPC request body Wrong: “`ts const app = express(); app.use(express.json()); // global body parser app.use( '/trpc', trpcExpress.createExpressMiddleware({ router: appRouter, createContext, }), ); “` Correct: “`ts const app = express(); // Only apply body parser to non-tRPC routes app.use('/api', express.json()); app.use( '/trpc', trpcExpress.createExpressMiddleware({ router: appRouter, createContext, }), ); “` If `express.json()` is applied globally before the tRPC middleware, it consumes and parses the request body. tRPC then receives an already-parsed body, which breaks FormData and binary content type handling. Source: www/docs/server/non-json-content-types.md ## See Also – **server-setup** — `initTRPC.create()`, router/procedure definition, context – **adapter-standalone** — simpler adapter when Express middleware ecosystem is not needed – **auth** — extracting JWT from `req.headers.authorization` in context – Express docs: https://expressjs.com/en/guide/using-middleware.html