With duplicates set to 'combine' (default) parseArrays: false can parse some query strings as arrays. The readme says:

To me it suggests that with this option disabled I should never get an array. Other interpretation could be that this applies only to a[] and a[0] notations, since the "Parsing arrays" section mentions only those two notations and parsing repeating keys as arrays is mentioned only in the duplicates section (counter point: the test for a=b&a=c was literally called a simple array test in one of the first commits to this repository - 622da20).

On the current main (6bdfaf5) (should be same in 16.4.1) and 16.4.0 I see the following behaviour:

This is not what I would expect based on the documentation (readme), but I think I may know the historical reason for this behaviour. arrayLimit was intended (until 16.4.1) to restrict only arrays with explicit indexes (see 4667c41, 8c5cda3, #294 and a longer analysis in #537 (comment)) and I believe parseArrays: false was meant to replace arrayLimit: -1 (#60, cc735a5) and affect the notation with indices, not repetitions (8e299ca).

This is related also to #260 and 2b94ea7, which extended parseArrays from indexed-only to also apply to [] notation, without index.

I think this is a documentation "bug" that requires clarification or maybe a combination of documentation and code problems (especially with changes to 6.14.1), but this requires answering some questions:

• Should parseArrays: false be equivalent to arrayLimit: -1?
  • If not, then how does it affect duplicates: 'combine'? (it doesn't, but should this be considered a bug?)
  • In any case, the documentation is very vague and needs clarification.
• Should arrayLimit apply to arrays without explicit index (Query string array parsing behavior changed in version 6.14.1 #537)?
  • Until GHSA-6rw7-vpxm-498p and 6.14.1 the answer was no (Query string array parsing behavior changed in version 6.14.1 #537 (comment)).
• Should a[]=b&a[]=c behave differently from a=b&a=c and create a sub-object with key '0' that contains an array/object?
  • This, I think, is a bug (likely introduced when fixing Bug Report: qs.parse when parsing a string with empty indexes [] in a key. #260, but I did not test older versions).