Fixing Missing Access-Control-Allow-Origin Header On 422 Error In Elysia.js

Jul 30, 2025 by ADMIN 76 views

Missing Access-Control-Allow-Origin Header on HTTP 422 Error in Elysia.js

Hey everyone! Let's dive into a tricky issue encountered while using Elysia.js, specifically version 1.3.6, with the elysia-cors plugin. We're going to break down the problem, why it's happening, and what the expected behavior should be. So, if you're wrestling with CORS issues in your Elysia.js application, you're in the right place!

The Problem: Missing CORS Header on 422 Errors

The core issue we're tackling today is the missing Access-Control-Allow-Origin header when an HTTP 422 Unprocessable Entity error is returned by an Elysia.js server. For those new to web development, this header is crucial for Cross-Origin Resource Sharing (CORS). CORS is a browser security feature that restricts web pages from making requests to a different domain than the one which served the web page. The Access-Control-Allow-Origin header is the server's way of saying, "Hey, it's okay for requests from this origin (or all origins) to access my resources."

To replicate this bug, a simple setup involving a POST request with body validation is used. When the request body fails validation, the server correctly returns a 422 status code, but it incorrectly omits the Access-Control-Allow-Origin header. This omission causes headaches, especially in browser environments, because the browser will block the response due to CORS policy, even though the pre-flight OPTIONS request might have succeeded.

Let's take a closer look at the provided code snippet to understand how this scenario unfolds. The provided code uses Elysia.js and the @elysiajs/cors plugin to set up a basic API. The API has a POST endpoint /recipes that expects a JSON body with a recipe property, which itself must be an object conforming to the RecipeDTO schema. This schema defines that the slug property must be a string with a minimum length of 1, a maximum length of 64, and a specific pattern that allows only lowercase letters, numbers, and hyphens.

import { Elysia, t } from "elysia";
import { cors } from '@elysiajs/cors';

const RecipeDTO = t.Object({
  slug: t.String({ minLength: 1, maxLength: 64, pattern: '^[a-z0-9\\-]+{{content}}#39; }),
});


const app = new Elysia()
  .use(cors({ origin: true }))
  .get("/", () => "Hello Elysia")
  .post("/recipes", ({
    body: { recipe }
  }) => {
    return recipe;
  }, {
    body: t.Object({ recipe: RecipeDTO })
  })
  .listen(3000);

console.log(
  `🦊 Elysia is running at ${app.server?.hostname}:${app.server?.port}`
);

The .use(cors({ origin: true })) part is where the CORS middleware is applied. Setting origin: true tells the server to reflect the request's origin in the Access-Control-Allow-Origin header, which is a common and flexible way to handle CORS in development. The problem arises when a request to /recipes is made without a valid body, or with a body that doesn't match the RecipeDTO schema. In these cases, Elysia.js correctly returns a 422 status code to indicate a validation failure, but it fails to include the Access-Control-Allow-Origin header in the response. This is the crux of the issue.

Expected Behavior vs. What We See

The expected behavior is that the Access-Control-Allow-Origin header should always be present in the response, regardless of whether the request is successful or results in an error. This is because the browser needs this header to determine if it's allowed to share the response with the requesting web page. Without it, the browser will block the response, leading to CORS errors and a frustrating developer experience.

However, what we actually see is that the Access-Control-Allow-Origin header is missing from the 422 response. This is evident from the curl command example provided:

< HTTP/1.1 422 Unprocessable Entity
< Content-Type: application/json
< Access-Control-Allow-Credentials: true
< Date: Tue, 29 Jul 2025 20:24:17 GMT
< Content-Length: 1115

Notice the absence of Access-Control-Allow-Origin in the headers. The presence of Access-Control-Allow-Credentials: true is also noteworthy, as it indicates that the server intends to support credentials in cross-origin requests, but the missing Access-Control-Allow-Origin header prevents this from working correctly.

Impact on Browsers and Development Tools

The absence of the Access-Control-Allow-Origin header has significant implications for both browser-based applications and developers using tools like Postman or Restfox.

In browsers, the pre-flight OPTIONS request typically succeeds, as it includes the necessary CORS headers. However, the actual POST request fails due to the missing Access-Control-Allow-Origin header in the 422 response. This results in a Network Error in the browser, which is a generic error that doesn't clearly indicate the underlying CORS issue. This can be confusing for developers who might not immediately realize that CORS is the culprit.

When using developer tools like Postman or Restfox, the situation is slightly different. These tools often report a network error or a same-origin policy violation. While this provides a clearer indication of the problem, it still doesn't pinpoint the exact cause – the missing header on the 422 response.

Diving Deeper: Why is this happening?

To really understand why this issue occurs, we need to think about how CORS middleware typically works and how Elysia.js handles errors. CORS middleware is usually implemented as a piece of code that intercepts HTTP requests and adds the necessary CORS headers to the response. This typically happens after the request has been processed by the application's route handlers.

In the case of Elysia.js, the @elysiajs/cors plugin likely works in a similar fashion. It adds the Access-Control-Allow-Origin header to successful responses and responses generated by the route handlers themselves. However, the issue arises when an error occurs before the route handler has a chance to execute fully. In this specific scenario, the 422 error is generated by Elysia's built-in validation mechanism before the route handler for /recipes is invoked.

Because the error occurs early in the request processing pipeline, the CORS middleware doesn't get a chance to add the necessary headers to the response. This is a common pitfall in web frameworks – error handling and middleware execution order can sometimes lead to unexpected behavior.

Potential Solutions and Workarounds

Now that we understand the problem and its root cause, let's explore some potential solutions and workarounds. These solutions generally fall into two categories: addressing the issue within the Elysia.js application or implementing a workaround on the client-side.

Server-Side Solutions (Elysia.js)

Custom Error Handling: One approach is to implement custom error handling within the Elysia.js application. This involves creating a global error handler that intercepts 422 errors and manually adds the Access-Control-Allow-Origin header to the response before it's sent to the client. This ensures that the header is present regardless of where the error originates.

To implement this, you can use Elysia's .onError hook. This hook allows you to define a function that will be executed whenever an error occurs during request processing. Inside this function, you can check if the error is a 422 error and, if so, add the Access-Control-Allow-Origin header to the response.
Middleware Ordering: Another potential solution involves adjusting the order in which middleware is applied. In some cases, ensuring that the CORS middleware is applied before any validation middleware might resolve the issue. However, this might not be feasible in all scenarios, as validation often needs to occur before other middleware can be executed.

In Elysia.js, the order in which you call .use() to apply middleware determines the order in which it's executed. So, if you have custom validation middleware, you would need to ensure that the cors() middleware is called before it.
Extending the CORS Plugin: A more robust solution might involve extending the @elysiajs/cors plugin itself. This could involve modifying the plugin to include a global error handler that automatically adds the Access-Control-Allow-Origin header to all error responses. This would ensure consistent CORS behavior across the application.

This approach would require a deeper understanding of the plugin's internals and might involve contributing changes back to the open-source project.

Client-Side Workarounds

While server-side solutions are generally preferred, there are some client-side workarounds that can be used in certain situations:

JSONP (Limited Applicability): JSON with Padding (JSONP) is a technique that allows cross-domain requests by leveraging the <script> tag. However, JSONP only supports GET requests and can be less secure than CORS. Therefore, it's not a suitable solution for POST requests or sensitive data.
Proxy Server: A proxy server can be used to forward requests to the Elysia.js server and add the Access-Control-Allow-Origin header to the responses. This can be a viable workaround in some cases, but it adds complexity to the application architecture.

The client-side application would make requests to the proxy server, which would then forward them to the Elysia.js server. The proxy server would add the necessary CORS headers to the response before sending it back to the client.

Conclusion: Ensuring Proper CORS Handling in Elysia.js

The missing Access-Control-Allow-Origin header on HTTP 422 errors in Elysia.js is a common issue that can lead to frustrating CORS problems. By understanding the root cause of the issue – the early generation of the error before CORS middleware can be applied – we can develop effective solutions.

Server-side solutions, such as custom error handling or extending the CORS plugin, are generally the most robust approach. Client-side workarounds can be used in certain situations, but they often come with limitations.

By implementing proper CORS handling, we can ensure that our Elysia.js applications are secure and accessible to clients from different origins. This is crucial for building modern web applications that interact with various APIs and services.

If you've encountered this issue, I hope this article has provided you with a clear understanding of the problem and potential solutions. Remember to always prioritize server-side solutions for CORS issues, as they provide the most control and security. Happy coding, guys!