The lodash flat map method works more or less the same way as the lodash map method in the sense that the first argument is a collection and the second argument is an iteratee method for each element in the collection. The difference is that an array of results for each element is what is expected to be returned by the iteratee and that array is to be flattened into an array of just one dimension.
To some extent the lodash flatten method is redundant as the same result can be achieved with the lodash map and lodash flatten method by way of chaining.
Maybe one of the best options to go with these days will still involve chaining a few native method that include array map, along with array reduce and array concat.
This solution might still break on older browsers, but often so will lodash code as well in some cases it it is a late version of lodash.