.map is not a function when getting React files from build folder - node.js

Question

I have a node/express application that's using React for the frontend. Everything works fine in development mode where there's a proxy between the backend running on localhost:5000 and the frontend on localhost:3000. However, I'm running into an issue for my production build where I'm getting the static React files from the build folder by including the following in my main server file:
app.use(express.static(path.join(__dirname, "client", "build")));
app.get("*", function (req, res) {
res.sendFile("index.html", {
root: path.join(__dirname, "client", "build"),
});
});
With this setup, I'm able to see the React app on localhost:5000, but on certain routes/pages I receive TypeError: a.map is not a function. The errors all seem to be triggered by arrays that are created in a context provider that sets state from APIs using axios. Here's an example of one of the API calls:
const getNodesApi = {
url: "model/get-all-nodes",
method: "GET",
headers: {
token: localStorage.token,
},
};
async function getNodeData() {
await axiosPm(getNodesApi)
.then((response) => {
const resData = response.data;
setNodeData(resData);
setLoading(false);
})
.catch((error) => {
console.log(error.response);
});
}
Is there something I have to do to set the base URL for the APIs or could something else be causing the TypeError: a.map is not a function?
Edit 1: Example where .map is failing
const [options, setOptions] = useState(
processes.map((process, index) => ({
key: index,
value: process.process_id,
label: process.process_name,
}))
);
In this example processes is coming from the context provider. It's initially defined by:
const [processes, setProcesses] = useState([]);

Answer 1

Answering my own question for visibility. The issue was being caused because I was mounting my backend/API routes after doing app.get for the React index.html. To resolve, mount the backend routes first.
app.use(express.static(path.join(__dirname, "client", "build")));
//Routes - mounted from backend for API
mountRoutes(app);
//Routes - use from React app for frontend
app.get("*", function (req, res) {
res.sendFile("index.html", {
root: path.join(__dirname, "client", "build"),
});
});