fix(shared): return "" from sliceUtf16Safe when end <= start, matching native .slice

Simon-XYDT · claude · Simon-XYDT · commit dcd458949fc4 · 2026-07-05T19:20:50.000+08:00
sliceUtf16Safe silently swapped reversed bounds (to &lt; from) instead of
returning "" like String.prototype.slice, creating a subtle footgun for
callers with dynamic start/end pairs.

Caller scan across src/, extensions/, and packages/ confirmed no production
code relies on the old swap behavior — all callers use (text, 0, N) or
(text, -N) forms only.

Co-Authored-By: Claude Opus 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/packages/normalization-core/src/utf16-slice.test.ts b/packages/normalization-core/src/utf16-slice.test.ts
@@ -23,8 +23,8 @@ describe("sliceUtf16Safe", () => {
     expect(sliceUtf16Safe("hello", 0, 10)).toBe("hello");
   });
 
-  it("swaps start and end when start > end", () => {
-    expect(sliceUtf16Safe("hello", 3, 1)).toBe("el");
+  it("returns empty when start > end, matching String.prototype.slice", () => {
+    expect(sliceUtf16Safe("hello", 3, 1)).toBe("");
   });
 
   it("preserves emoji with surrogate pairs", () => {
diff --git a/packages/normalization-core/src/utf16-slice.ts b/packages/normalization-core/src/utf16-slice.ts
@@ -19,10 +19,8 @@ export function sliceUtf16Safe(input: string, start: number, end?: number): stri
   let from = start < 0 ? Math.max(len + start, 0) : Math.min(start, len);
   let to = end === undefined ? len : end < 0 ? Math.max(len + end, 0) : Math.min(end, len);
 
-  if (to < from) {
-    const tmp = from;
-    from = to;
-    to = tmp;
+  if (to <= from) {
+    return "";
   }
 
   if (from > 0 && from < len) {
