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fix(js/ts): endsWith regression for empty pattern with non-ordinal comparison #4745

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Merged
MangelMaxime merged 4 commits into from
Jul 9, 2026
Merged

fix(js/ts): endsWith regression for empty pattern with non-ordinal comparison #4745

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deee75d
fix(js/ts): endsWith regression for empty pattern with non-ordinal co…
Thorium Jul 8, 2026
5243718
fix(beam): endsWith regression for empty pattern with non-ordinal com…
MangelMaxime Jul 9, 2026
2070e48
test: add test for empty pattern in `String.IndexOf`, `String.Contains`
MangelMaxime Jul 9, 2026
9d456d5
fix(python): `LastIndexOf(value, startIndex)` off-by-one on inclusive…
MangelMaxime Jul 9, 2026
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7 changes: 6 additions & 1 deletion src/Fable.Transforms/Python/Replacements.fs
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Expand Up @@ -1557,14 +1557,19 @@ let strings (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr opt
| [ ExprType String ] -> Helper.InstanceCall(c, "rfind", t, args, i.SignatureArgTypes, ?loc = r) |> Some
| [ ExprType Char as str; ExprType(Number(Int32, NumberInfo.Empty)) as start ]
| [ ExprType String as str; ExprType(Number(Int32, NumberInfo.Empty)) as start ] ->
// .NET's LastIndexOf(value, startIndex) searches the inclusive window [0, startIndex],
// so the exclusive end passed to Python's rfind must be startIndex + 1.
let endIdx =
makeBinOp r (Number(Int32, NumberInfo.Empty)) start (makeIntConst 1) BinaryPlus

Helper.InstanceCall(
c,
"rfind",
t,
[
str
Value(NumberConstant(NumberValue.Int32 0, NumberInfo.Empty), None)
start
endIdx
],
i.SignatureArgTypes,
?loc = r
Expand Down
10 changes: 10 additions & 0 deletions src/fable-library-beam/fable_string.erl
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Original file line number Diff line number Diff line change
Expand Up @@ -135,9 +135,13 @@ remove(Str, StartIdx, Count) ->
binary:part(Str, StartIdx + Count, byte_size(Str) - StartIdx - Count)
]).

starts_with(_Str, <<>>) ->
true;
starts_with(Str, Prefix) ->
binary:match(Str, Prefix) =:= {0, byte_size(Prefix)}.

ends_with(_Str, <<>>) ->
true;
ends_with(Str, Suffix) ->
SuffixLen = byte_size(Suffix),
StrLen = byte_size(Str),
Expand Down Expand Up @@ -262,6 +266,8 @@ filter(Fn, Str) ->
index_of(Str, Sub) ->
index_of(Str, Sub, 0).

index_of(_Str, <<>>, StartIdx) ->
StartIdx;
index_of(Str, Sub, StartIdx) ->
SearchStr = binary:part(Str, StartIdx, byte_size(Str) - StartIdx),
case binary:match(SearchStr, Sub) of
Expand Down Expand Up @@ -293,6 +299,8 @@ index_of_any_loop([C | Rest], CharSet, Idx, StartIdx) ->
last_index_of(Str, Sub) ->
last_index_of(Str, Sub, byte_size(Str) - 1).

last_index_of(Str, <<>>, MaxIdx) ->
erlang:min(MaxIdx + 1, byte_size(Str));
last_index_of(Str, Sub, MaxIdx) ->
SubLen = byte_size(Sub),
SearchLen = erlang:min(MaxIdx + SubLen, byte_size(Str)),
Expand All @@ -302,6 +310,8 @@ last_index_of(Str, Sub, MaxIdx) ->
Matches -> element(1, lists:last(Matches))
end.

contains(_Str, <<>>) ->
true;
contains(Str, Sub) ->
binary:match(Str, Sub) =/= nomatch.

Expand Down
2 changes: 1 addition & 1 deletion src/fable-library-ts/String.ts
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Expand Up @@ -81,7 +81,7 @@ export function endsWith(str: string, pattern: string, ic: boolean | StringCompa
return str.endsWith(pattern);
}
if (str.length >= pattern.length) {
return cmp(str.slice(-pattern.length), pattern, ic) === 0;
return cmp(str.slice(str.length - pattern.length), pattern, ic) === 0;
}
return false;
}
Expand Down
28 changes: 28 additions & 0 deletions tests/Beam/StringTests.fs
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Expand Up @@ -172,6 +172,16 @@ let ``test String.EndsWith with OrdinalIgnoreCase works`` () =
"ABCD".EndsWith("xabcd", StringComparison.OrdinalIgnoreCase) |> equal false
"ABCD".EndsWith("abcd", StringComparison.OrdinalIgnoreCase) |> equal true

[<Fact>]
let ``test String.StartsWith and String.EndsWith with empty pattern works`` () =
// .NET: every string starts/ends with ""
"abcd".StartsWith("") |> equal true
"abcd".EndsWith("") |> equal true
"".StartsWith("") |> equal true
"".EndsWith("") |> equal true
"abcd".StartsWith("", StringComparison.OrdinalIgnoreCase) |> equal true
"abcd".EndsWith("", StringComparison.OrdinalIgnoreCase) |> equal true

// --- Substring ---

[<Fact>]
Expand All @@ -197,6 +207,12 @@ let ``test String.Contains works`` () =
"ABC".Contains("B") |> equal true
"ABC".Contains("Z") |> equal false

[<Fact>]
let ``test String.Contains with empty pattern works`` () =
// .NET: every string contains ""
"ABC".Contains("") |> equal true
"".Contains("") |> equal true

// --- IndexOf ---

[<Fact>]
Expand All @@ -207,6 +223,12 @@ let ``test String.IndexOf works`` () =
let ``test String.IndexOf works with offset`` () =
"abcdbc".IndexOf("bc", 3) |> equal 4

[<Fact>]
let ``test String.IndexOf with empty pattern works`` () =
"abcdbc".IndexOf("") |> equal 0
"abcdbc".IndexOf("", 3) |> equal 3
"abcdbc".IndexOf("", 6) |> equal 6

[<Fact>]
let ``test String.IndexOf char works`` () =
"abcd".IndexOf('b') * 100 + "abcd".IndexOf('e') |> equal 99
Expand Down Expand Up @@ -253,6 +275,12 @@ let ``test String.LastIndexOf with index and StringComparison`` () =
"abcdbcebc".LastIndexOf("b", 3, StringComparison.Ordinal)
|> equal 1

[<Fact>]
let ``test String.LastIndexOf with empty pattern works`` () =
"abcdbc".LastIndexOf("") |> equal 6
"abcdbc".LastIndexOf("", 3) |> equal 4
"abcdbc".LastIndexOf("", 0) |> equal 1

// --- Access char by index ---

[<Fact>]
Expand Down
21 changes: 21 additions & 0 deletions tests/Js/Main/StringTests.fs
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Expand Up @@ -1041,6 +1041,27 @@ let tests = testList "Strings" [
"ABCD".EndsWith(fst arg, true, CultureInfo.InvariantCulture)
|> equal (snd arg)

testCase "String.EndsWith with empty pattern and StringComparison works" <| fun () ->
// .NET: every string ends with "" for all StringComparison values
"hello".EndsWith("", StringComparison.CurrentCulture) |> equal true
"hello".EndsWith("", StringComparison.InvariantCulture) |> equal true
"hello".EndsWith("", StringComparison.Ordinal) |> equal true
"hello".EndsWith("", StringComparison.OrdinalIgnoreCase) |> equal true
"hello".EndsWith("", StringComparison.InvariantCultureIgnoreCase) |> equal true
"".EndsWith("", StringComparison.CurrentCulture) |> equal true
"hello".StartsWith("", StringComparison.CurrentCulture) |> equal true
"hello".StartsWith("", StringComparison.Ordinal) |> equal true
"hello".StartsWith("", StringComparison.OrdinalIgnoreCase) |> equal true

testCase "String.EndsWith with non-empty pattern and StringComparison works" <| fun () ->
"hello".EndsWith("LO", StringComparison.OrdinalIgnoreCase) |> equal true
"hello".EndsWith("LO", StringComparison.InvariantCultureIgnoreCase) |> equal true
"hello".EndsWith("lo", StringComparison.Ordinal) |> equal true
"hello".EndsWith("LO", StringComparison.Ordinal) |> equal false
"hello".EndsWith("he", StringComparison.Ordinal) |> equal false
"hello".EndsWith("hello", StringComparison.CurrentCulture) |> equal true
"hello".EndsWith("xhello", StringComparison.Ordinal) |> equal false

testCase "String.Trim works" <| fun () ->
" abc ".Trim()
|> equal "abc"
Expand Down
30 changes: 24 additions & 6 deletions tests/Python/TestString.fs
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Expand Up @@ -518,6 +518,12 @@ let ``test String.Contains works`` () =
"ABC".Contains("B") |> equal true
"ABC".Contains("Z") |> equal false

[<Fact>]
let ``test String.Contains with empty pattern works`` () =
// .NET: every string contains ""
"ABC".Contains("") |> equal true
"".Contains("") |> equal true

[<Fact>]
let ``test String.Contains with StringComparison works`` () =
"ABC".Contains("b", StringComparison.Ordinal) |> equal false
Expand Down Expand Up @@ -594,6 +600,12 @@ let ``test String.IndexOf works with offset`` () =
"abcdbc".IndexOf("bc", 3)
|> equal 4

[<Fact>]
let ``test String.IndexOf with empty pattern works`` () =
"abcdbc".IndexOf("") |> equal 0
"abcdbc".IndexOf("", 3) |> equal 3
"abcdbc".IndexOf("", 6) |> equal 6

[<Fact>]
let ``test String.LastIndexOf works`` () =
"abcdbc".LastIndexOf("bc") * 100 + "abcd".LastIndexOf("bd")
Expand All @@ -604,6 +616,12 @@ let ``test String.LastIndexOf works with offset`` () =
"abcdbcebc".LastIndexOf("bc", 3)
|> equal 1

[<Fact>]
let ``test String.LastIndexOf with empty pattern works`` () =
"abcdbc".LastIndexOf("") |> equal 6
"abcdbc".LastIndexOf("", 3) |> equal 4
"abcdbc".LastIndexOf("", 0) |> equal 1

[<Fact>]
let ``test String.IndexOf with StringComparison works`` () =
"abcdbc".IndexOf("b", StringComparison.Ordinal)
Expand Down Expand Up @@ -652,42 +670,42 @@ let ``test String.EndsWith char works`` () =

[<Fact>]
let ``test String.StartsWith works`` () =
let args = [("ab", true); ("bc", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("bc", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"abcd".StartsWith(fst arg)
|> equal (snd arg)

[<Fact>]
let ``test String.StartsWith with OrdinalIgnoreCase works`` () =
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".StartsWith(fst arg, StringComparison.OrdinalIgnoreCase)
|> equal (snd arg)

[<Fact>]
let ``test String.StartsWith with ignoreCase boolean works`` () =
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".StartsWith(fst arg, true, CultureInfo.InvariantCulture)
|> equal (snd arg)

[<Fact>]
let ``test String.EndsWith works`` () =
let args = [("ab", false); ("cd", true); ("bc", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", false); ("cd", true); ("bc", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"abcd".EndsWith(fst arg)
|> equal (snd arg)

[<Fact>]
let ``test String.EndsWith with OrdinalIgnoreCase works`` () =
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true)]
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".EndsWith(fst arg, StringComparison.OrdinalIgnoreCase)
|> equal (snd arg)

[<Fact>]
let ``test String.EndsWith with ignoreCase boolean works`` () =
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true)]
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".EndsWith(fst arg, true, CultureInfo.InvariantCulture)
|> equal (snd arg)
Expand Down
30 changes: 24 additions & 6 deletions tests/Rust/tests/src/StringTests.fs
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Expand Up @@ -880,6 +880,12 @@ let ``String.Contains works`` () =
"ABC".Contains("B") |> equal true
"ABC".Contains("Z") |> equal false

[<Fact>]
let ``String.Contains with empty pattern works`` () =
// .NET: every string contains ""
"ABC".Contains("") |> equal true
"".Contains("") |> equal true

[<Fact>]
let ``String.Contains with char works`` () =
"ABC".Contains('B') |> equal true
Expand Down Expand Up @@ -1081,6 +1087,12 @@ let ``String.IndexOf works with offset`` () =
"abcdbc".IndexOf("bc", 3)
|> equal 4

[<Fact>]
let ``String.IndexOf with empty pattern works`` () =
"abcdbc".IndexOf("") |> equal 0
"abcdbc".IndexOf("", 3) |> equal 3
"abcdbc".IndexOf("", 6) |> equal 6

[<Fact>]
let ``String.LastIndexOf works`` () =
"abcdbc".LastIndexOf("bc") * 100 + "abcd".LastIndexOf("bd")
Expand All @@ -1091,6 +1103,12 @@ let ``String.LastIndexOf works with offset`` () =
"abcdbcebc".LastIndexOf("bc", 3)
|> equal 1

[<Fact>]
let ``String.LastIndexOf with empty pattern works`` () =
"abcdbc".LastIndexOf("") |> equal 6
"abcdbc".LastIndexOf("", 3) |> equal 4
"abcdbc".LastIndexOf("", 0) |> equal 1

[<Fact>]
let ``String.IndexOfAny works`` () =
"abcdbcebc".IndexOfAny([|'b'|]) |> equal 1
Expand Down Expand Up @@ -1122,42 +1140,42 @@ let ``String.EndsWith char works`` () =

[<Fact>]
let ``String.StartsWith works`` () =
let args = [("ab", true); ("bc", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("bc", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"abcd".StartsWith(fst arg)
|> equal (snd arg)

[<Fact>]
let ``String.StartsWith with OrdinalIgnoreCase works`` () =
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".StartsWith(fst arg, StringComparison.OrdinalIgnoreCase)
|> equal (snd arg)

[<Fact>]
let ``String.StartsWith with ignoreCase boolean works`` () =
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", true); ("AB", true); ("BC", false); ("cd", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".StartsWith(fst arg, true, CultureInfo.InvariantCulture)
|> equal (snd arg)

[<Fact>]
let ``String.EndsWith works`` () =
let args = [("ab", false); ("cd", true); ("bc", false); ("abcdx", false); ("abcd", true)]
let args = [("ab", false); ("cd", true); ("bc", false); ("abcdx", false); ("abcd", true); ("", true)]
for arg in args do
"abcd".EndsWith(fst arg)
|> equal (snd arg)

[<Fact>]
let ``String.EndsWith with OrdinalIgnoreCase works`` () =
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true)]
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".EndsWith(fst arg, StringComparison.OrdinalIgnoreCase)
|> equal (snd arg)

[<Fact>]
let ``String.EndsWith with ignoreCase boolean works`` () =
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true)]
let args = [("ab", false); ("CD", true); ("cd", true); ("bc", false); ("xabcd", false); ("abcd", true); ("", true)]
for arg in args do
"ABCD".EndsWith(fst arg, true, CultureInfo.InvariantCulture)
|> equal (snd arg)
Expand Down
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