Returns 0 or more values for every value in the given iterable. Technically, it's just calling map(), followed by flatten(), but it's a very useful operation if you want to map over a structure, but not have a 1:1 input-output mapping. Instead, if you want to potentially return 0 or more values per input element, use flatmap():

For example, to return all numbers n in the input iterable n times:

>>> const repeatN = n => repeat(n, n);
>>> [...flatmap([0, 1, 2, 3, 4], repeatN)]
[1, 2, 2, 3, 3, 3, 4, 4, 4, 4]  // note: no 0