adapter-express
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.
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SKILL.md
tRPC — Adapter: Express
Setup
// 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
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
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
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:
const app = express();
app.use(express.json()); // global body parser
app.use(
'/trpc',
trpcExpress.createExpressMiddleware({
router: appRouter,
createContext,
}),
);
Correct:
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.authorizationin context - Express docs: https://expressjs.com/en/guide/using-middleware.html
Files
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