Can I add a js into firmware as a init.js?

It sounds like he's using ESP8266?

You can use E.setBootCode to write raw JS into flash - if you are so inclined you could add something to your build process to write your code into flash automatically, but you'd have to look at jswrap_flash.c for the exact format needed.

An easier way would be to add some C code yourself that calls jspEvaluate("your_code")