Something like this:
wixData.query("Members")
.eq('_id', userId)
.find()
.then( (results) => {
let items = results.items; // items is an array of results
let item = items[0]; // get the first and only item
// item contains the user's details
} )
.catch( (error) => {
let errorMsg = error.message;
let code = error.code;
}
);