fix: uneval grows sparse arrays by one slot when first hole is past index 0 (#160)

* fix: do not grow sparse arrays by one slot in uneval

When the first hole in an array literal was at an index greater than 0,
uneval emitted two commas for it: one from `if (i > 0) result += ','` at
the top of the iteration, and a second after `i -= 1` re-processed the
same index. So `uneval([1, , 3])` produced `[1,,,3]` (length 4) instead
of `[1,,3]` (length 3), and the round-tripped array gained a slot.

The comma at the top of the iteration already represents the hole, so
drop the `i -= 1` re-processing.

* chore: add changeset

* Apply suggestion from @elliott-with-the-longest-name-on-github

---------

Co-authored-by: Elliott Johnson <hello+github@ell.iott.dev>

Author: spokodev

.changeset/uneval-sparse-array-extra-comma.md

@@ -0,0 +1,5 @@
+---
+'devalue': patch
+---
+
+fix: do not grow sparse arrays by one slot in uneval

src/uneval.js

@@ -261,9 +261,7 @@ export function uneval(value, replacer) {
 							return `Object.assign(Array(${thing.length}),{${entries}})`;
 						}
 
-						// Re-process this index as a hole in the array literal
 						has_holes = true;
-						i -= 1;
 					}
 					// else: already decided on array literal, hole is just an empty slot
 					// (the comma separator is all we need — no content for this position)

test/index.test.js

@@ -1422,6 +1422,17 @@ uvu.test('ignores non-numeric array properties in dense encoding', () => {
 	assert.ok(!(0 in parsed));
 });
 
+uvu.test('uneval round-trips sparse arrays whose first hole is not at index 0', () => {
+	for (const arr of [[1, , 3], [1, ,], [1, , , 4], [1, 2, , 4]]) {
+		const evaled = (0, eval)(uneval(arr));
+		assert.is(evaled.length, arr.length, `length for keys ${Object.keys(arr).join(',')}`);
+		assert.equal(Object.keys(evaled), Object.keys(arr));
+		for (const k of Object.keys(arr)) {
+			assert.is(evaled[k], arr[k]);
+		}
+	}
+});
+
 uvu.test('ignores non-numeric array properties in sparse encoding', () => {
 	// Sparse path (very sparse — Object.assign / SPARSE encoding wins)
 	const arr = [];