luarocks install jsmin

$ cat <<EOF > input.js
(function() {
   console.log("Hello, World!");
   // removes comments
   return 0;
  })()
EOF
$ jsmin <input.js >output.js
$
$ cat output.js
(function(){console.log("Hello, World!");return 0;})()

local jsmin = require("jsmin")
local js_str = [[
(function() {
   console.log("Hello, World!");
   // removes comments
   return 0;
})()]]

print(jsmin(js_str))
-- (function(){console.log("Hello, World!");return 0;})()