Extra data: two JSON documents where one was expected

The parser read one complete JSON value and then found more text after it. JSON.parse and json.loads accept exactly one value. The usual reason is a JSON Lines file (also called NDJSON), where each line is its own document.

What each runtime prints

We ran this input through JSON.parse in each browser and Node version, and through json.loads in Python. These are the messages exactly as printed.

{"a": 1}
{"b": 2}

• Chrome, Node 20+, Firefox and Python all point at line 2, column 1: the start of the second document.
• Safari says only "Unable to parse JSON string", with no position.
• Node 18 says "Unexpected token { in JSON at position 9". The { is the start of the second object.

Common causes

1. 1

   A JSON Lines file

   Log files, event exports, and batch files for many AI and data APIs use one JSON object per line, often with the extension .jsonl or .ndjson.

2. 2

   json.dump() called more than once on the same file

   Appending a new object to a file in a loop writes several documents side by side, not one list.

3. 3

   Two responses or chunks joined together

   Reading a streaming response and concatenating the chunks, or a retry that appended a second body to the first.

How to fix it

If the data is JSON Lines, parse each line on its own.

JavaScript

const rows = text
  .split('\n')
  .filter((line) => line.trim())
  .map((line) => JSON.parse(line));

Python

import json

with open("events.jsonl", encoding="utf-8") as f:
    rows = [json.loads(line) for line in f if line.strip()]

• If the documents are joined with no newline between them, Python can still split them: json.JSONDecoder().raw_decode(text, i) returns one value and the index where it ends, so you can loop until the end of the text.
• If you write the file yourself and want one document, collect the objects in a list and call json.dump() once.

Related

• Trailing comma in JSON: "Expected double-quoted property name"
• Unexpected end of JSON input: an empty or cut-off body
• JSON Repair
• JSON Viewer