Fix drop_start on JavaScript for multi-byte strings

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Fixed a bug where `string.drop_start` would return incorrect results on
+  JavaScript when the string contained multi-byte characters.
+
 ## v1.0.2 - 2026-05-14
 
 - Fixed deprecation warnings when using OTP 29

src/gleam_stdlib.mjs

@@ -200,7 +200,8 @@ export function length(data) {
 }
 
 export function string_byte_slice(string, index, length) {
-  return string.slice(index, index + length);
+  const bytes = new TextEncoder().encode(string);
+  return new TextDecoder().decode(bytes.subarray(index, index + length));
 }
 
 export function string_grapheme_slice(string, idx, len) {

test/gleam/string_test.gleam

@@ -471,6 +471,14 @@ pub fn drop_start_3499_test() {
   assert string.drop_start("\r]", 1) == "]"
 }
 
+pub fn drop_start_multibyte_test() {
+  // https://github.com/gleam-lang/stdlib/issues/924
+  assert string.drop_start("广州abcdefghijklmn", 0) == "广州abcdefghijklmn"
+  assert string.drop_start("广州abcdefghijklmn", 1) == "州abcdefghijklmn"
+  assert string.drop_start("广州abcdefghijklmn", 2) == "abcdefghijklmn"
+  assert string.drop_start("广州abcdefghijklmn", 3) == "bcdefghijklmn"
+}
+
 pub fn drop_end_basic_test() {
   assert string.drop_end("gleam", up_to: 2) == "gle"
 }