function baseFlatten(array, depth, predicate, isStrict, result) {
  var index = -1,
      length = array.length;
  predicate || (predicate = isFlattenable);
  result || (result = []);

  while (++index < length) {
    var value = array[index];

    if (depth > 0 && predicate(value)) {
      if (depth > 1) {
        // Recursively flatten arrays (susceptible to call stack limits).
        baseFlatten(value, depth - 1, predicate, isStrict, result);
      } else {
        arrayPush(result, value);
      }
    } else if (!isStrict) {
      result[result.length] = value;
    }
  }

  return result;
}
The base implementation of `_.flatten` with support for restricting flattening.

@private
@param {Array} array The array to flatten.
@param {number} depth The maximum recursion depth.
@param {boolean} [predicate=isFlattenable] The function invoked per iteration.
@param {boolean} [isStrict] Restrict to values that pass `predicate` checks.
@param {Array} [result=[]] The initial result value.
@returns {Array} Returns the new flattened array.