Parallel execution with find and xargs

build.sh

#! /usr/bin/env bash

find ./src/bookmarklets -name "*.js"  -print0 | xargs -I FILE -0 -P "$(nproc)" npm run rollup -- --input "FILE" --file "output/FILE"

# EDIT: Per my latest comment - I now prefer this:
for bookmarklet in ./src/bookmarklets/**/*.js
do
  npm run rollup -- --input "$bookmarklet" --file "output/$bookmarklet" &
done
wait

After some time to stew on this, and reading some alternatives, I feel that using a loop with background processes is more readable.

for thing in glob
do
  npm run doThingWithArg -- --file=thing & # & sends proc to bg
done
wait # waits for bg procs to complete