parallelize.js

/* @flow */
import glob from 'glob'

/**
 * writes a list of files matching the glob pattern to stdout
 * runs only the subset of files which fall within the job, set
 * in the environment variables.
 *
 * JOB_COUNT is the number of jobs we will be splitting across, 1-indexed
 * JOB_INDEX is the index of the job (subset of files) we should be running, 0-indexed
 */
const {
  JOB_COUNT = 1,
  JOB_INDEX = 0
}: any = process.env

/**
 * gets a list of files matching the given glob
 * @returns {string}
 */
function getFiles (): string[] {
  const allFiles = glob.sync('src/**/*.test.js')

  const filesPerJob = Math.ceil(allFiles.length / JOB_COUNT)
  const startIndex = filesPerJob * JOB_INDEX
  return allFiles
    .slice(startIndex, startIndex + filesPerJob)
}

const files = getFiles()

/**
 * Join the array of files is into a single string,
 * a new glob pattern which will match all of the
 * selected files, exclusively.
 */
process.stdout.write(files.map((str) => '**/' + str).join('|'))