server.js

// [...]
// Stream Chat server SDK
const StreamChat = require('stream-chat').StreamChat;

const serverClient = new StreamChat(
    '<STREAM_API_KEY>', 
    '<STREAM_API_SECRET>'
);

app.get('/token', (req, res) => {
    const { username } = req.query

    if (username) {
        const token = serverClient.createToken(username)
        res.status(200).json({ token, status: "sucess" })
    } else {
        res.status(401).json({ message: "invalid request", status: "error" })
    }
});

// [...]