Example middleware for tanstack start + better auth

middleware.ts

export const userMiddleware = createMiddleware({
  type: 'function',
}).server(async ({ next }) => {
  const { headers } = getWebRequest()

  const session = await auth.api.getSession({
    headers,
    query: {
      // ensure session is fresh
      // https://www.better-auth.com/docs/concepts/session-management#session-caching
      disableCookieCache: true,
    },
  })

  return next({
    context: {
      user: session?.user,
    },
  })
})

export const userRequiredMiddleware = createMiddleware({
  type: 'function',
})
  .middleware([userMiddleware])
  .server(async ({ next, context }) => {
    if (!context.user) {
      throw json(
        { error: 'You must be logged in to access this resource' },
        { status: 401 },
      )
    }

    return next({
      context: {
        user: context.user, // make user non-nullable
      },
    })
  })

export const adminRequiredMiddleware = createMiddleware({
  type: 'function',
})
  .middleware([userRequiredMiddleware])
  .server(async ({ next, context }) => {
    const adminQuery = await db
      .select()
      .from(adminUsers)
      .where(eq(adminUsers.userId, context.user.id))

    if (adminQuery.length === 0) {
      throw json(
        { error: 'You must be an admin to access this resource' },
        { status: 403 },
      )
    }

    return next()
  })