Fix bit_array slices of slices on JavaScript

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Fixed `string.inspect` not formatting the `\f` form feed control character
   correctly on Erlang.
 - `dynamic.unsafe_coerce` function has been deprecated.
+- Fixed `bit_array` slices of slices sometimes being incorrect on JavaScript.
 
 ## v0.37.0 - 2024-04-19
 

src/gleam_stdlib.mjs

@@ -362,7 +362,8 @@ export function bit_array_slice(bits, position, length) {
   const start = Math.min(position, position + length);
   const end = Math.max(position, position + length);
   if (start < 0 || end > bits.length) return new Error(Nil);
-  const buffer = new Uint8Array(bits.buffer.buffer, start, Math.abs(length));
+  const byteOffset = bits.buffer.byteOffset + start;
+  const buffer = new Uint8Array(bits.buffer.buffer, byteOffset, Math.abs(length));
   return new Ok(new BitArray(buffer));
 }
 

test/gleam/bit_array_test.gleam

@@ -1,4 +1,5 @@
 import gleam/bit_array
+import gleam/result
 import gleam/should
 
 pub fn byte_size_test() {
@@ -88,6 +89,11 @@ pub fn slice_test() {
   bit_array.from_string("hello")
   |> bit_array.slice(1, 6)
   |> should.equal(Error(Nil))
+
+  bit_array.from_string("ab")
+  |> bit_array.slice(1, 1)
+  |> result.try(bit_array.slice(_, 0, 1))
+  |> should.equal(Ok(<<"b":utf8>>))
 }
 
 pub fn to_string_test() {