Unexpected change in behaviour v6/v7 for .notEmpty()

[brianweet]

Describe the bug

We're using .notEmpty() to validate an incoming request body. After upgrading major version from 6 to 7, our unit tests started to fail. Turns out v7 .notEmpty() does not return a validation error for an empty array.

I do not see this in the breaking changes, unless it's part of some of the other points described there

edit: after looking through the docs, it seems .notEmpty() should have been used for strings. However I think the changed behaviour is still unexpected?

To Reproduce

Here you can see the different results:

1. Validation error for empty array on version 6.14.0: https://runkit.com/brianweet/express-validator-issue-template/6.14.0
2. No validation error on version 7.0.1: https://runkit.com/brianweet/express-validator-issue-template/7.0.1

Expected behavior

v6 and v7 to have similar behaviour

Current behavior

v7 does not return validation error for empty array

Environment:

• Express-validator version: 7.0.1
• Express version:
• Node.js version:

[gustavohenke]

I'm guessing this is an unexpected regression from fixing #791.

Basically, the new behaviour for standard validators is: if you have an array, then every item in the array should comply with the validator.
In the case of there being no items, then it means essentially every item is complying in not being empty...

I think that changing isEmpty() to check the array's length might make sense.

In the mean time, you can fix this by instead using the isArray({ min: 1 }) validator.