function calculateBackwardIndex (viewSize) {
    const { index } = Glide

    if (Glide.isType('carousel')) {
      return index - viewSize
    }

    // ensure our back navigation results in the same index as a forward navigation
    // to experience a homogeneous paging
    const view = Math.ceil(index / viewSize)

    return (view - 1) * viewSize
  }
Calculates index value to move backward/to the left

@param viewSize
@returns {Number}