Could u make javascripts in bootstrap studio link to local javascripts files.Once local javascripts files changed,the javascript files in bootstrap studio also changed at the same time.hopefully get your response. 
By local do you mean in a folder on your computer? How is this preferable to having the file saved inside the BBS project? You can edit your JS in Bootstrap Studio and have it update instantly, or if you prefer a more robust editor, you can link an external code editor like Visual Studio and when you edit your JS files there and then save them, they’ll be updated the same as if you edited them in BSS.
If you truly want to host your JS outside of BSS, you can use a CDN. In the Design panel, right click Javascript and choose Link External JS.