fix: skip empty array values in stringify

Empty array values caused malformed query strings with leading or
trailing `&` separators because the separator was appended unconditionally
for every key while the inner loop produced no output. For example,
`stringify({a: [], b: "1"})` produced `"&b=1"` and
`stringify({a: "1", b: []})` produced `"a=1&"`.

Skip empty arrays entirely and only emit the separator when the current
key actually contributes a key=value pair, matching the behavior of the
native `querystring` module.

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: DeepView Autofix <276251120+deepview-autofix@users.noreply.github.com>
Co-Authored-By: Nikita Skovoroda <chalkerx@gmail.com>
Signed-off-by: Nikita Skovoroda <chalkerx@gmail.com>

Author: 3 people

lib/stringify.js

@@ -39,12 +39,14 @@ function stringify(input) {
     const value = input[key];
     const encodedKey = encodeString(key) + "=";
 
-    if (i) {
-      result += separator;
-    }
-
     if (Array.isArray(value)) {
       valueLength = value.length;
+      if (valueLength === 0) {
+        continue;
+      }
+      if (result.length > 0) {
+        result += separator;
+      }
       for (let j = 0; j < valueLength; j++) {
         if (j) {
           result += separator;
@@ -56,6 +58,9 @@ function stringify(input) {
         result += getAsPrimitive(value[j]);
       }
     } else {
+      if (result.length > 0) {
+        result += separator;
+      }
       result += encodedKey;
       result += getAsPrimitive(value);
     }

test/stringify.test.ts

@@ -97,6 +97,13 @@ test("should coerce numbers to string", () => {
   assert.strictEqual(qs.stringify({ foo: Number.POSITIVE_INFINITY }), "foo=");
 });
 
+test("should skip empty array values without leaving stray separators", () => {
+  assert.strictEqual(qs.stringify({ a: [], b: "1" }), "b=1");
+  assert.strictEqual(qs.stringify({ a: "1", b: [] }), "a=1");
+  assert.strictEqual(qs.stringify({ a: "1", b: [], c: "2" }), "a=1&c=2");
+  assert.strictEqual(qs.stringify({ a: [] }), "");
+});
+
 test("should return empty string on certain inputs", () => {
   assert.strictEqual(qs.stringify(undefined as never), "");
   assert.strictEqual(qs.stringify(0 as never), "");