https://twitter.com/kentcdodds/status/1218448397478187008

for-vs-filter-reduce.js

const duplicates = filesAndHashes.filter((file1, index1, array) => {
  return array.some(
    (file2, index2) => index1 !== index2 && file1.hash === file2.hash,
  )
})

const groups = duplicates.reduce((groups, {filepath, hash}) => {
  groups[hash] = groups[hash] || []
  groups[hash].push(filepath)
  return groups
}, {})

// vs

const filesByHash = {}
for (const {filepath, hash} of filesAndHashes) {
  filesByHash[hash] = filesByHash[hash] || []
  filesByHash[hash].push(filepath)
}

const groups = {}
for (const hash in filesByHash) {
  if (filesByHash[hash].length > 1) {
    groups[hash] = filesByHash[hash]
  }
}