Lodash map and return unique

You can also use uniqBy function which also accepts iteratee invoked for each element in the array. It accepts two arguments as below, where id is the iteratee parameter.

_.uniqBy(array, 'id')

Many ways, but uniq() isn't a method on array, it's a lodash method.

_.uniq(_.map(data, 'usernames'))

Or:

_.chain(data).map('usernames').uniq().value()

(The second is untested and possibly wrong, but it's close.)

As mentioned in the comment, depending on what your data actually is, you could do this all in one shot without first pulling out whatever usernames is.