Send Map in express res.send

June 11, 2022

I’m working on a game with a friend and we need to send a Map with some stuff in it, but express only sends the user {} instead of the actual Map. The problem is at sending it and not the code itself, console.log‘ging it does return the Map.
Code:

router.get("/list", async (req, res) => {
    try {
        const users = await userCollection.find();
        accessedListEmbed(req);
        let userData = new Map();
        users.forEach((user) => userData.set(user.userName, user.status));
        res.send(userData);
        console.log(userData);
    } catch (error) {
        res.send("unknown");
    }
});

>Solution :

Generally, you can only send serializable values over the network. Maps aren’t serializable:

const map = new Map();
map.set('key', 'value');
console.log(JSON.stringify(map));

Either send an array of arrays that can be converted into a Map on the client side, or use another data structure, like a plain object. For example:

router.get("/list", async (req, res) => {
    try {
        const users = await userCollection.find();
        accessedListEmbed(req);
        const userDataArr = [];
        users.forEach((user) => {
            userDataArr.push([user.userName, user.status]);
        });
        res.json(userDataArr); // make sure to use .json
    } catch (error) {
        // send JSON in the case of an error too so it can be predictably parsed
        res.json({ error: error.message });
    }
});

Then on the client-side:

fetch(..)
    .then(res => res.json())
    .then((result) => {
        if ('error' in result) {
            // do something with result.error and return
        }
        const userDataMap = new Map(result);
        // ...

Or something along those lines.