404 errors for existing routes often occur if routes are misconfigured or if middleware is improperly ordered. Checking route paths and middleware sequence can resolve it.

In Express.js, 404 errors for routes that should exist typically indicate either a misconfigured route, middleware interference, or incorrect routing order. First, ensure that the route definitions in your Express app match the paths being requested; even minor typos or case sensitivity can cause route mismatches. It’s also important to note the order of middleware and routes in Express because the framework processes them sequentially. If a route is defined after a catch-all or app.use middleware, the catch-all will trigger before the route, leading to unexpected 404 errors. Another issue could be that middleware, such as body parsers or authentication checks, may inadvertently block routes from resolving. Placing general middleware before specific route handlers and testing each route individually can clarify which middleware, if any, may be interfering. In API-based applications, also check for prefix issues, such as routes being nested under /api/v1 or similar, which may cause 404 errors if the prefix is omitted in requests. Lastly, ensure that the correct HTTP methods are used in both route definition and requests, as a route defined with app.post won’t match a GET request. These checks will usually help fix route recognition issues in Express applications.