function flatMapDepth(collection, iteratee, depth) {
      depth = depth === undefined ? 1 : toInteger(depth);
      return baseFlatten(map(collection, iteratee), depth);
    }
This method is like `_.flatMap` except that it recursively flattens the
mapped results up to `depth` times.

@static
@memberOf _
@since 4.7.0
@category Collection
@param {Array|Object} collection The collection to iterate over.
@param {Function} [iteratee=_.identity] The function invoked per iteration.
@param {number} [depth=1] The maximum recursion depth.
@returns {Array} Returns the new flattened array.
@example

function duplicate(n) {
  return [[[n, n]]];
}

_.flatMapDepth([1, 2], duplicate, 2);
// => [[1, 1], [2, 2]]