I have used some packages in init.el, after setup a new environment and copy init.el to .emacs/init.el, then start emacs will report packages missing.

I can install it one by one. but I would like to find a way to install it with command. I want to add such install in init.el but do not wish it impact on emacs startup time.

How can I do it? say I need install "js2-mode,skewer-mode".