Why does autoType mutate the input?

[akre54]

Hi there - mostly curious but what's the reason for autoType mutating the object in-place?

This caused a bug in my app since the function is not idempotent - passing the same object twice succeeds on the first time with a numeric value, and fails on the second due to the object[key].trim() line. The value is a string the first time around, and a number after it is parsed.

The fix is to always clone the result first (i.e. arr.map(r => d3.autoType({ ...r }))) but given how most d3 utility functions are pure I'm surprised to see this. It's clearly intentional given the explicit test for the behavior but am I missing a use case? Or is it just in the interest of speed / it's expected that the happy path is d3.csv(url, d3.autoType)?