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'];

function ensureArray(x) {
    return x instanceof Array ? x : [x];
}

function concat(arrays) {
    return [].concat.apply([], arrays);
}

function uniquify(array) {
    let s = new Set(array);
    return Array.from(s);
}

var dbFieldsPerSelect =
    selectFields
    .map(name => userFields[name].dbFields)
    .map(ensureArray);

var dbFields = uniquify(concat(dbFieldsPerSelect));

console.log(dbFields);