missing "Acces-Control-Allow-Origin" header after nodejs API deployment

Question

SOLVED The problem was coming from my start script which was linked to an old version of the build, and i didn't remember changing it that's why i didn't check that... So nothing is wrong with the expressjs/cors package / TS compatibility ;)

I've created an API using NodeJS, Express & TypeScript, and i'm having troubles resolving a CORS-related issue. My API is accessible from the following domain patterns: https://api.domain.com, and the front from https://www.front.domain.com.

When I try login from the front-end, i'm receiving this error :

Access to XMLHttpRequest at 'https://api.domain.com/sessions/login' from origin 'https://www.front.domain.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

When i'm running the API locally, the 'Access-Control-Allow-Origin' header is present (I used Postman to check it), and the error isn't there. But when i deploy my API, the same header is absent from every request, and i get the above error message.

I've tried deploying it on OVH web cloud and Clever cloud, and since i'm having the issue on both services I think there is something wrong with my code but i can't find it.

My server code is the following (createServer.ts):

const createServer = () => {

    const app = express()

    app.use(cors({ credentials: true, origin: 'https://www.front.domain.com', methods: ["OPTIONS", "HEAD", "GET", "POST", "PUT", "DELETE"] }))
    app.use(express.json())
    app.use(cookieParser())
    app.use(deserializeToken)

    routes(app)
    return app

}

(app.ts)

const app = createServer()

app.listen(
    port,
    async () => {
        console.info(`API is running on: ${rootUrl}, port: ${port}`)
        await dbConnect()
    }
)

EDIT : The issue seems to come from the build. The header disapears after the build. here is my tsconfig.json file :

{
    "compilerOptions": {
        "outDir": "build",
        "target": "es6",
        "module": "commonjs",
        "esModuleInterop": true, 
        "forceConsistentCasingInFileNames": true, 
        "strict": true,
        "skipLibCheck": true
    }
}

Before build :

After build

Answer 1

Did you check the pre-flight request in your inspector ? It should be an OPTIONS request just before your original request with the same URL.