[kevin9327]

isDate throws a TypeError instead of returning false when the delimiter used in format is not in delimiters:

isDate('29.02.2020', 'DD.MM.YYYY');           // TypeError: Cannot read properties of undefined (reading 'startsWith')
isDate('2020.02.29', { format: 'YYYY.MM.DD' }); // TypeError: Cannot read properties of undefined (reading 'length')
isDate('2020-02-29', { format: 'YYYY-MM-DD', delimiters: ['/'] }); // TypeError

isValidFormat accepts . as a format delimiter, but the default delimiters are ['/', '-']. So formatDelimiter is undefined, split(undefined) keeps the whole string as one part, and when the input has the same length as the format, dateObj.y / dateObj.m are never set. Inputs of other lengths already return false.

Since the format's delimiter isn't an allowed one, no input can match. The fix returns false in that case, which is what other mismatched inputs already get. Passing delimiters: ['.'] keeps working as before.

Related to #1709 (where . formats were enabled).

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

AI-assistance note: I found this bug and drafted the fix with help from an AI coding assistant, then checked the change and ran the test suite locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9ff3424) to head (bc0bcf7).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2953   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2600    +1     
  Branches       658       659    +1     
=========================================
+ Hits          2599      2600    +1     

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