How to use the slate-hotkeys.isExtendLineBackward function in slate-hotkeys

}

    // COMPAT: Certain browsers don't handle the selection updates properly. In
    // Chrome, the selection isn't properly extended. And in Firefox, the
    // selection isn't properly collapsed. (2017/10/17)
    if (Hotkeys.isMoveLineBackward(event)) {
      event.preventDefault()
      return editor.moveToStartOfBlock()
    }

    if (Hotkeys.isMoveLineForward(event)) {
      event.preventDefault()
      return editor.moveToEndOfBlock()
    }

    if (Hotkeys.isExtendLineBackward(event)) {
      event.preventDefault()
      return editor.moveFocusToStartOfBlock()
    }

    if (Hotkeys.isExtendLineForward(event)) {
      event.preventDefault()
      return editor.moveFocusToEndOfBlock()
    }

    // COMPAT: If a void node is selected, or a zero-width text node adjacent to
    // an inline is selected, we need to handle these hotkeys manually because
    // browsers won't know what to do.
    if (Hotkeys.isMoveBackward(event)) {
      event.preventDefault()

      if (!selection.isCollapsed) {