add a existing javascript file to netbeans project, is it possible

To add an external file to your NetBeans project:

• Select the file in the file system that you want to add to your NetBeans project using a file manager (such as File Explorer on Windows).
• Right-click and select Copy from the popup menu.
• Within NetBeans go to your project and position the mouse over the target directory to which you want to copy the file. You can do this in the Projects panel or the Files panel as appropriate.
• Right-click and select Paste.

That's all there is to it. This works on Linux and Windows. You can also do drag and drop, though on Linux the file is copied whereas on Windows the file is moved.

There is no menu option in NetBeans such as File -> Import existing file to do this. Just use an external file manager.

Update/clarification:

• The instructions above only specify how to copy an existing file into an existing project.
• There is no way to include an individual file that is external to the structure of a NetBeans project.
• However, it is possible to create a symbolic link (junction) from a NetBeans project directory to an external directory. To do that open a Command Prompt window as an administrator and enter a command similar to this:

mklink /J D:\NetBeansProjects\HTML5DemoCss\nbProject\MyLink2 C:\sftp

That will create a new directory in your NetBeans project named MyLink2 which maps to an external directory named c:\sftp. You can then process files in that external directory C:\sftp as though they were within your project directory MyLink.