Preflight Handlers (app.preflight)

In addition to standard middleware, Volten provides app.preflight().

How Preflight Differs from Middleware

• Standard middleware (app.use) runs after route matching has located the matching endpoint.
• Preflight handlers (app.preflight) run before route matching begins for every single incoming request.

Typical Preflight Use Cases

• Global CORS preflight response generation (OPTIONS method short-circuiting)
• Early request filtering / IP blocklists before consuming route tree resources
• Global tracing headers / Correlation IDs

app.preflight(async (ctx) => {
  // Inject early correlation ID
  ctx.setHeader("X-Request-Id", crypto.randomUUID());

  // Handle CORS preflight before routing
  if (ctx.method === "OPTIONS") {
    ctx.setHeader("Access-Control-Allow-Origin", "*");
    ctx.setHeader("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, PATCH, OPTIONS");
    ctx.setHeader("Access-Control-Allow-Headers", "Content-Type, Authorization");
    return ctx.status(204).send("");
  }
});

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