babel.bzl

babel(
  name = "transpiled",
  args = ["$(location :app)", "--out-file", "$@/transpiled.js"],
  data = [":app", "@npm//@babel/preset-env", "//:babel.config.json"],
  outs = ["transpiled.js"],
)

rollup.bzl

rollup_bundle(
  name = "app",
  srcs = [":index.js"],
  entry_point = "index.js",
  config_file = "//:rollup.config.js",
  format = "iife",
)