Advent of code 2017 Day 1 (1) in Functional Programming
As far as functional programming goes, this looks pretty nice to me,
clear and readable.

One performance drawback is the splitting to a string array.
The size of the array will be similar to the input,
and it can be much smaller if you apply a filter directly on the input.
Although strings don't have a filter method like arrays,
since the selectTwins function would work on strings,
you could apply the filter method of arrays on the input directly:

[].filter.apply(parsedInput, [selectTwins])


Another performance drawback is that the input is scanned twice:
once to replace whitespace characters, and once more for splitting.
We could use the same technique as earlier to apply a filter on the input directly instead of using a regex replacement.

Putting the above suggestions together:

const isDigit = x =&gt; /\d/.test(x);
const solution = [].filter.apply(INPUT, [isDigit])
  .filter(selectTwins)
  .map(toNumber)
  .reduce(sum, 0);