从函数内部调用 $(document).ready() 是否安全？

如果我使用$(document).ready()即使文档就绪事件在过去已经发生过，它是否仍能保证仅在文档准备好时才运行其中的代码？

Yes.

来自 jQueryready功能source.

// Catch cases where $(document).ready() is called after the
// browser event has already occurred.
if ( document.readyState === "complete" ) {
    // Handle it asynchronously to allow scripts the opportunity to delay ready
    return setTimeout( jQuery.ready, 1 );
}