_createDomExpandButton () {
    // create expand button
    const expand = document.createElement('button')
    expand.type = 'button'
    if (this._hasChilds()) {
      expand.className = this.expanded
        ? 'jsoneditor-button jsoneditor-expanded'
        : 'jsoneditor-button jsoneditor-collapsed'
      expand.title = translate('expandTitle')
    } else {
      expand.className = 'jsoneditor-button jsoneditor-invisible'
      expand.title = ''
    }

    return expand
  }
Create an expand/collapse button
@return {Element} expand
@private