fix: correct prePush ordering for ReadableString in object properties

When a ReadableString (created from a Readable stream) appeared as a
non-first property in an object, _push() would prepend prePush (the
opening quote) before the objectItem prefix (e.g. ',"key":'), producing
invalid JSON like {"key":"value"","data":hello"}.

Fix: separate the objectItem prefix from the data, so that prePush is
inserted between the prefix and data rather than before the prefix.

Before: '",key:value' — quote lands before comma
After:  ',key:"value' — quote lands after colon, before value

Added two test cases covering ReadableStream values as non-first object
properties.

Author: Henrik Blum

src/JsonStreamStringify.ts

@@ -454,12 +454,14 @@ export class JsonStreamStringify extends Readable {
   /** if set, this string will be prepended to the next _push call, if the call output is not empty, and set to undefined */
   prePush?: string;
   private _push(data) {
-    const out = (this.objectItem ? this.objectItem.write() : '') + data;
-    if (this.prePush && out.length) {
-      this.buffer += this.prePush;
+    const prefix = this.objectItem ? this.objectItem.write() : '';
+    if (this.prePush && (prefix.length || data.length)) {
+      this.buffer += prefix + this.prePush;
       this.prePush = undefined;
+    } else {
+      this.buffer += prefix;
     }
-    this.buffer += out;
+    this.buffer += data;
     if (this.buffer.length >= this.bufferSize) {
       this.pushCalled = !this.push(this.buffer);
       this.buffer = '';

test-src/JsonStreamStringify.spec.ts

@@ -274,6 +274,17 @@ describe('JsonStreamStringify', function () {
     a: readableStream(1, 2, 3),
   },                                                                 '{"a":[1,2,3]}'));
 
+  it('{key:"value",data:readableStream("hello")} should be {"key":"value","data":"hello"}', createTest({
+    key: 'value',
+    data: readableStream('hello'),
+  }, '{"key":"value","data":"hello"}'));
+
+  it('{a:"b",c:readableStream("d"),e:"f"} should be {"a":"b","c":"d","e":"f"}', createTest({
+    a: 'b',
+    c: readableStream('d'),
+    e: 'f',
+  }, '{"a":"b","c":"d","e":"f"}'));
+
   it('readableStream(\'a\', \'b\', \'c\') should be "abc"', createTest(readableStream('a', 'b', 'c'), '"abc"'));
 
   it('readableStream(\'a\', \'b\', \'c\') should be "abc"', () => {