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Problem Encountered with CORS in Deno Server

Published at

2/4/2024

Problem Encountered with CORS in Deno Server

Recently, I encountered an issue while developing a server using Deno. My objective was to enable CORS (Cross-Origin Resource Sharing) requests utilizing the oakCors middleware. Despite setting it up as per the documentation, CORS handling did not function as expected.

The Scenario

I employed Deno's Oak framework to create the server and handle incoming requests from the frontend. Below is a snippet of the code used (some parts are modified for brevity and clarity):

import { Application, Router } from "https://deno.land/x/oak/mod.ts";
import { oakCors } from "https://deno.land/x/cors/mod.ts";

const router = new Router();
router.post("/", async (context) => {
    const body = await context.request.body();
    console.log(body.text());
});

const app = new Application();
app.use(router.routes());
app.use(router.allowedMethods());

// Enabling CORS for port 5173 on localhost using oakCors
app.use(
  oakCors({
    origin: "http://localhost:5173",
    optionsSuccessStatus: 200,
    methods: "POST, OPTIONS",
  }),
);

await app.listen({ port: 8000 });

Despite configuring the server to accept CORS requests from http://localhost:5173, requests to the server were being rejected due to CORS errors.

Analysis

It appears that the 'oakCors' middleware is not working correctly, and the CORS headers are not being added, as indicated by the error message I received:

Access to fetch at 'http://localhost:8000/api/generate' from origin 'http://localhost:5173' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

Solution

After some trial and error, I discovered that moving the configuration of oakCors before defining the routes resolved the issue.

import { Application, Router } from "https://deno.land/x/oak/mod.ts";
import { oakCors } from "https://deno.land/x/cors/mod.ts";

const app = new Application();

// Enabling CORS for port 5173 on localhost using oakCors
// make sure to initialize oakCors before the routers
app.use(
  oakCors({
    origin: "http://localhost:5173",
    optionsSuccessStatus: 200,
    methods: "POST, OPTIONS",
  }),
);

const router = new Router();
router.post("/", async (context) => {
    const body = await context.request.body();
    console.log(body.text());
});


app.use(router.routes());
app.use(router.allowedMethods());

await app.listen({ port: 8000 });

I learned that I should configure middleware before the routes' are defined.

Remaining Challenge

According to the chapter "Enable CORS for a Single Route" of the official docs, these codes would work but didn't.

const corsOptions: oakCors.Options = {
  origin: "http://localhost:5173",
  optionsSuccessStatus: 200,
};

router.post("/", oakCors(corsOptions), (context) => {
    const body = await context.request.body;
    console.log(body.text());
});

Although this issue remains unresolved at present, it does not impede the functionality of the server as I have implemented a workaround that meets my current requirements.

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