CORS errors in Node.js APIs usually happen due to restrictive headers. Adjusting the server’s CORS configuration or using middleware can resolve the issue.

CORS (Cross-Origin Resource Sharing) errors in a Node.js API occur when the browser blocks a request from one domain to another due to security policy. This happens because the server is either missing the required CORS headers or they are incorrectly configured, causing the browser to prevent the client from receiving responses. To address this, you can start by installing the cors middleware in Express, which simplifies the process of adding the necessary headers. Once set up, configure it to allow specific domains by passing an array of allowed origins to the cors function, or set it to accept requests from any origin during development by using *, though be cautious as this can introduce security risks in production. If your API supports custom headers or is handling specific HTTP methods like PUT, DELETE, or PATCH, ensure these are also included in the Access-Control-Allow-Methods header, which dictates which HTTP methods are allowed for cross-origin requests. For complex requests that require an additional preflight request, make sure the server responds to OPTIONS requests by setting Access-Control-Allow-Methods, Access-Control-Allow-Headers, and Access-Control-Allow-Origin headers appropriately. If using multiple subdomains, configure the server to dynamically set the allowed origin based on the Origin header in incoming requests. Alternatively, if you’re using a proxy setup (for instance, with a frontend app and API on the same server but different ports), ensure the proxy server passes the correct headers. Testing the API endpoint from different environments, such as Postman, can help confirm if CORS settings are correctly configured. Resolving CORS errors requires careful configuration of headers, particularly in production, where stricter policies should be applied for enhanced security.