Well to check if the returned object isn't undefined , simply use !caseOne : var caseOne = document.getElementById('contentSub') ... ... <看更多>

getElementById was called with an empty String argument this is ... the url doesn't contain a hash, firefox outputs a warning that document. ... <看更多>

You can remove all child nodes of an element by setting the innerHTML property of the parent node to blank: let menu = document.getElementById('menu'); menu ... ... <看更多>

Add this to your JS method to get the value it should work. do it in lightning way native way may break in future releases. ... <看更多>