Build a unique array of db fields

This is stripped down from a project I'm working on:

let userFields = {
    id: {
        dbFields: 'id',
    },
    email: {
        dbFields: 'email',
    },
    first_name: {
        dbFields: 'first_name',
    },
    last_name: {
        dbFields: 'last_name',
    },
    name: {
        dbFields: ['first_name', 'last_name'],
        resolve: user => [user.first_name, user.last_name].filter(n => n.length).join(' '),
    }
};

let selectFields = ['name', 'first_name', 'email'];

let dbFields = new Set();
selectFields.forEach(f => {
    if(userFields[f].dbFields) {
        if(userFields[f].dbFields instanceof Array) {
            userFields[f].dbFields.forEach(x => dbFields.add(x));
        } else {
            dbFields.add(userFields[f].dbFields);
        }
    }
});

console.log(Array.from(dbFields)); // [ 'first_name', 'last_name', 'email' ]

I don't like that big selectFields.forEach. Or the fact that Set doesn't appear to have an addMany method.

Is there any way I can compact this a bit more?

You can run this with babel-node if you need to. (npm install -g babel)