Unexpected Non-Whitespace Character After JSON Data: Causes & Fix

What Does "Unexpected Non-Whitespace Character After JSON" Mean?

In JavaScript engines (Chrome V8, Node.js), calling JSON.parse() on a string containing extra characters after a valid JSON root throws:

SyntaxError: Unexpected non-whitespace character after JSON at position 15 (line 1 column 16)

In Firefox / SpiderMonkey:

SyntaxError: JSON.parse: unexpected non-whitespace character after JSON data at line 1 column 16

In Python:

json.decoder.JSONDecodeError: Extra data: line 1 column 16 (char 15)

This error means the JSON parser successfully parsed a complete, valid JSON value (like an object or array), but then found additional characters instead of reaching the end of the string.

Common Causes & Examples

Cause 1: Two JSON Objects Back-to-Back (NDJSON / JSON Lines)

When reading data from streaming APIs (like OpenAI ChatGPT stream, Docker logs, or Elasticsearch dumps), each line is an independent JSON object:

{"id": 1, "status": "ok"}{"id": 2, "status": "pending"}

Passing this entire string to JSON.parse() fails at character position 26 (the second {).

Cause 2: Appended Log Messages or Server Output

If a backend script emits a debug statement before or after JSON output:

{"status": "success", "count": 4}Connected to database.

The text Connected to database. sits after the valid closing brace }, causing an immediate parse error.

Cause 3: Duplicate API Payloads in Network Buffers

When reading raw TCP streams or improperly delimited HTTP chunks, two responses can concatenate into a single buffer.

How to Fix It

Fix 1: Parsing NDJSON / JSON Lines Properly

If your data consists of newline-delimited JSON, split the string by newlines and parse each line independently:

const rawNDJSON = '{"id": 1, "name": "Alpha"}\n{"id": 2, "name": "Beta"}';

// ✅ Split by line, filter empty lines, and parse each
const records = rawNDJSON
  .split('\n')
  .map(line => line.trim())
  .filter(line => line.length > 0)
  .map(line => JSON.parse(line));

console.log(records);
// [{ id: 1, name: 'Alpha' }, { id: 2, name: 'Beta' }]

To test and inspect newline-delimited payloads interactively, use the JSON Lines tool.

Fix 2: Extracting JSON from Messy Output with Regex

If server logs or trailing text are attached to your JSON payload:

function extractFirstJSON(str) {
  const firstBrace = str.indexOf('{');
  const lastBrace = str.lastIndexOf('}');
  
  if (firstBrace !== -1 && lastBrace !== -1 && lastBrace > firstBrace) {
    const cleanJSON = str.substring(firstBrace, lastBrace + 1);
    return JSON.parse(cleanJSON);
  }
  
  throw new Error("No valid JSON object boundaries found in input");
}

// Example:
const messy = '{"user": "dev123", "token": "abc"} Execution time: 12ms';
const result = extractFirstJSON(messy);
console.log(result.user); // "dev123"

Fix 3: Handling Python Extra Data with JSONDecoder.raw_decode

In Python, the JSONDecoder class provides raw_decode(), which returns the parsed object along with the character index where it stopped:

import json

raw_stream = '{"id": 101, "event": "click"} trailing text...'

decoder = json.JSONDecoder()
obj, end_pos = decoder.raw_decode(raw_stream)

print("Parsed object:", obj)       # {'id': 101, 'event': 'click'}
print("Ended at character:", end_pos) # 29
print("Remaining text:", raw_stream[end_pos:]) # " trailing text..."

Diagnostic Summary

Use JSONKit's JSON Fixer to automatically inspect and repair malformed strings directly in your browser.