Answer
The J-Web Platform package is included in the Junos OS from Release 14.1X53-D10 and is installed on your switch as part of the Junos OS installation. If you upgrade to a later version of Junos OS, you automatically get the Platform package associated with that Junos OS.
NOTE: If you have the Application package installed on your switch, and you upgrade Junos OS on your switch, then the Application package is removed, and the Platform package that is associated with Junos OS is installed on your switch. You must install the associated J-Web Application package over the Platform package.
You can view details of the J-Web packages installed on your switch in the About window in the J-Web interface, or in the output of the show version command in the Junos OS CLI.
For more information, click J-Web User Interface Overview.