[OsamaAnsar]

Fixes # (no existing issue; the problem is described below)

What was wrong

JaroWinklerDistance returns a wrong value for strings of length 1 that are equal after lowercasing:

• In distance() the match window is floor(max(len1, len2) / 2) - 1, which is -1 when both strings have length 1. No character can match, so the Jaro distance is 0.
• In JaroWinklerDistance the prefix loop while (s1[l] === s2[l] && l < 4) compares undefined === undefined past the end of the strings, so it counts a prefix of 4 characters for strings that are shorter. With Jaro distance 0 this gives 0 + 4 * 0.1 * (1 - 0).

Real output on current master:

call	before	after
JaroWinklerDistance('A', 'a', { ignoreCase: true })	0.4	1
JaroWinklerDistance('a', 'A', { ignoreCase: true })	0.4	1
JaroWinklerDistance('I', 'i', { ignoreCase: true })	0.4	1

Unchanged before and after: ('A', 'A') is 1, ('a', 'b') is 0, ('A', 'b', { ignoreCase: true }) is 0, ('a', 'ab') is 0.8500000000000001, ('ab', 'AB', { ignoreCase: true }) is 1, ('MARTHA', 'MARHTA') is 0.9611111111111111, ('DIXON', 'DICKSONX') is 0.8133333333333332, ('DWAYNE', 'DUANE') is 0.8400000000000001.

Proposed Changes

• Clamp the match window with Math.max(0, ...). This is the change that fixes the example above (the window only goes negative when both strings have length 1).
• Bound the prefix loop by the lengths of both strings. On its own this does not fix the example above (it would give 0.1); it only matters when a precomputed dj option is passed for strings shorter than 4 characters.
• Added Jasmine tests in spec/jaro-winkler_spec.ts for strings of length 1 (with and without ignoreCase) and for the prefix bound with the dj option.

Verification

• With only the change in lib/natural/distance/jaro-winkler_distance.js reverted (confirmed with git diff --stat and grep), the 2 new specs fail; with the change restored all 10 specs in jaro-winkler_spec pass.
• npm test: 860 specs, 0 failures.
• eslint . --ext .ts (the lint script): no errors. npx standard lib/natural/distance/jaro-winkler_distance.js: no errors.
• npm run test_io was not run (it needs external services).

Checklist before merging

• Added Jasmine tests
• Updates TypeScipt index.d.ts files (not applicable: no API or type change, index.d.ts is untouched)