Trying to return a string value from a function and save it to mongoDB doesn't work

It's not a bug. It's a feature. JavaScript IO is asynchronous. Your best bet is to make url return a promise, so you can take advantage of async/await. MinIOClient methods return a promise f no callback is passed, so use that.

// If a custom image was selected by the client set the picture URL to Firebase's  CDNconst url = async () => {  // If the user has selected a file  if (req.file) {    // Upload user image to the database    await MinIOClient.fPutObject("local", `${req.body.email}.png`, req.file.path, {      "Content-Type": req.file.mimetype    });    // Getting the link for the object    const presignedUrl = await MinIOClient.presignedGetObject("local", `${req.body.email}.png`, 24 * 60 * 60)     return presignedUrl;  }  // If the user didn't select an image return a random image link(string) that will be used to serve default avatars from the server  else {    return avatarLinks[Math.floor(Math.random() * avatarLinks.length)];  }};

Then modify the controller

router.post("/", upload.single("avatar"), async (req, res) => {    const pictureUrl = await url();    let Account = new AccountSchema({        // ... other values        pictureUrl    });});