function splitLineReducer(acc, line) {
      return acc.concat(line.split(/,/g));
  }
  const investigators = fileLines.reduce(splitLineReducer, []);

That’s unnecessarily quadratic. So yes, definitely use the more readable `flatMap` (with appropriate polyfill):

  const investigators = fileLines.flatMap(line => line.split(','));

(The `flatMap` in the article is not an appropriate polyfill)

The object spread example has the same problem, like c-smile mentioned. Use a Map for lookups instead.

> Some readers might point out that we could get a performance gain by mutating the accumulator. That is, we could change the object, instead of using the spread operator to create a new object every time. I code it this way because I want to keep in the habit of avoiding mutation. If it proved to be an actual bottleneck in production code, then I would change it. ↩

Avoiding (very local) mutation because it’s an FP bad word in a language like JavaScript is just a terrible way to go about things. Also, this turns out to be an actual bottleneck way more often than people notice.