code: function getSelected (content, selection) {
    if (typeof content !== 'string') {
      throw new TypeError('The content must be a string.')
    }
  
    if (typeof selection !== 'object') {
      throw new TypeError('The selection must be an object.')
    }
  
    if (typeof selection.start !== 'number') {
      throw new TypeError('The selection start value must be a number.')
    }
  
    if (typeof selection.end !== 'number') {
      throw new TypeError('The selection end value must be a number.')
    }
  
    return content.slice(selection.start, selection.end)
  }
docs: Get the piece of content selected from a full content stringa and
  the selections positios
  @param  {string}                 content   The full content string
  @param  {SelectionType} selection The selections positions
  @return {string}                           The sliced string