ReadableString values produce invalid JSON when used as non-first object properties

[hbl18]

Description

When a non-objectMode Readable stream is used as a value in an object passed to JsonStreamStringify, the resulting JSON is invalid. The opening quote (") for the stream content is placed before the object key separator (,"key":) instead of after it.

This was introduced in v3.1.0 (the complete rewrite) and affects all versions from 3.1.0 through 3.1.6.

We faced this issue in a pipeline of our project and used GitHub Copilot to find and fix the Issue.

Minimal Reproduction

const { PassThrough } = require('node:stream');
const { JsonStreamStringify } = require('json-stream-stringify');

async function test() {
  const stream = new PassThrough();
  const obj = { key: 'value', data: stream };
  const jss = new JsonStreamStringify(obj);

  setImmediate(() => {
    stream.write(Buffer.from('hello'));
    stream.end();
  });

  const chunks = [];
  for await (const chunk of jss) {
    chunks.push(chunk.toString());
  }
  const result = chunks.join('');
  console.log('Output:', result);

  try {
    JSON.parse(result);
    console.log('Valid JSON');
  } catch (e) {
    console.log('INVALID JSON:', e.message);
  }
}

test();

Expected output:

Output: {"key":"value","data":"hello"}
Valid JSON

Actual output (v3.1.6):

Output: {"key":"value"","data":hello"}
INVALID JSON: Expected ',' or '}' after property value in JSON at position 14 (line 1 column 15)

Root Cause

The bug is in the _push method in src/JsonStreamStringify.ts:

private _push(data) {
    const out = (this.objectItem ? this.objectItem.write() : '') + data;
    if (this.prePush && out.length) {
      this.buffer += this.prePush;   // ← prePush is placed BEFORE objectItem prefix + data
      this.prePush = undefined;
    }
    this.buffer += out;

When setReadableStringItem sets this.prePush = '"', and then _push is called:

1. objectItem.write() returns the key prefix (e.g. ,"data":)
2. These are concatenated into out = ',"data":hello'
3. prePush (") is appended to buffer before out

Result: buffer += '"' then buffer += ',"data":hello' → ..."value"","data":hello"

Suggested Fix

Separate the object prefix from data, and insert prePush between them:

private _push(data) {
    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 += data;

This ensures the key prefix (,"data":) is emitted first, then the opening quote ("), then the stream data.

Notes

• ReadableString as the first property in an object is unaffected (no objectItem prefix to misorder)
• ReadableObject values are unaffected (they use _push('[') directly, not prePush)
• Tested on Node.js v24.x

Affected versions

3.1.0 – 3.1.6 (all versions since the v3.1.0 rewrite)