Function _flatten

src/utils/array.js:489–498

(array)

  return flat

  function _flatten (array) {
    for (let i = 0; i < array.length; i++) {
      const item = array[i]
      if (Array.isArray(item)) {
        _flatten(item)
      } else {
        flat.push(item)
      }
    }
  }

  function _flattenRectangular (array) {
    if (Array.isArray(array[0])) {