Stumbled across this blog post a while ago, which covers why using spread ({...obj}) syntax in accumulators like reduce is a bad idea.

TL;DR: it causes a time complexity of O(n^2) instead of O(n) because it's creating an increasingly growing new object on every iteration. Whilst with assignment we're mutating the accumulator rather than creating a new object.

Checklist

• run npm run test and npm run benchmark
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct