Selection.focusOffset
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
只读属性 Selection.focusOffset 返回选区终点（鼠标松开瞬间所记录的那个点）在焦点（Selection.focusNode）中的偏移量。返回值从零开始计数，如果选区（Selection）在焦点（Selection.focusNode）的第一个字符前结束，返回值为 0。