Access both static content and API with Express - node.js

Question

I have a Express REST API at /api and a frontend that I want to show at /ui. I have this:
api.js
const STATIC_FILES_PATH = path.resolve(__dirname, '..', 'frontend', 'build');
app.use(express.static(STATIC_FILES_PATH));
app.get('/ui', (_, res) => {
res.sendFile(path.resolve(STATIC_FILES_PATH, 'index.html'));
});
app.get('/api/stuff', (req, res) => {
//do stuff
});
When I navigate to /api/stuff, I still see the frontend instead of the API response. No matter the URI, it always show the frontend webpage.
What am I doing wrong?
EDIT
This is my project structure:
/api
api.js
/frontend
/build
index.html
When I navigate to /api/stuff, in my Chrome console, I see a HTTP 304! It's redirecting me to the frontend (cached) instead of showing me the JSON result (API response).

Answer 1

After adding this middleware:
app.get('/*', (req, res, next) => {
res.setHeader('Last-Modified', (new Date()).toUTCString());
next();
});
it's working fine. It seems Express is caching stuff. Thanks to all!

Answer 2

change your code with this snippet, it tested and will work correctly.
api.js
const express = require('express');
const app = express();
const path = require('path');
const STATIC_FILES_PATH = path.resolve(__dirname, 'frontend', 'build');
app.use(express.static(STATIC_FILES_PATH));
app.get('/ui', (req, res, next) => {
res.sendFile(path.resolve(STATIC_FILES_PATH, 'index.html'));
});
app.get('/api/stuff', (req, res, next) => {
res.json({title: 'staff'});
});
app.listen(3000);