Common JSON Parse Errors and How to Fix Them

JSON parse errors can be cryptic. This guide covers every common error message with the exact cause and fix for each one.

SyntaxError: Unexpected token

The most common JSON error. It means the parser encountered a character it didn't expect at that position. The character after "Unexpected token" tells you what was wrong.

❌ Unexpected token ' in JSON at position 1

Cause: Single quotes used instead of double quotes

Wrong: {'name': 'James'}

Fix: {"name": "James"}

❌ Unexpected token u in JSON at position 0

Cause: Variable is undefined — no JSON was returned

Wrong: JSON.parse(undefined)

Fix: if (response) JSON.parse(response)

❌ Unexpected token < in JSON at position 0

Cause: Server returned HTML instead of JSON (error page)

Wrong: <!DOCTYPE html><html>...

Fix: Check response.ok before calling response.json()

❌ Unexpected token , in JSON at position 45

Cause: Trailing comma after last item

Wrong: {"name": "James","age": 30,}

Fix: {"name": "James","age": 30}

SyntaxError: Unexpected end of JSON input

This error means the JSON string ended before the structure was complete. Usually caused by truncated responses or unclosed brackets.

// Truncated JSON — missing closing brackets
{"users": [
  {"id": 1, "name": "James"},
  {"id": 2, "name": "Anna"   ← missing ] and }

// Fix — ensure your API returns complete responses
// Check timeout settings
// Check maximum response size settings

JSON parse errors by language

JavaScript

try {
  const data = JSON.parse(jsonString)
} catch (e) {
  if (e instanceof SyntaxError) {
    console.error('JSON parse failed:', e.message)
    // Log the problematic string
    console.error('Input was:', jsonString?.substring(0, 100))
  }
}

Python

import json

try:
    data = json.loads(json_string)
except json.JSONDecodeError as e:
    print(f"JSON error: {e.msg}")
    print(f"Line: {e.lineno}, Column: {e.colno}")
    print(f"Position: {e.pos}")

Java

try {
    JsonNode node = objectMapper.readTree(jsonString);
} catch (JsonProcessingException e) {
    System.err.println("JSON error: " + e.getMessage());
    System.err.println("Location: " + e.getLocation());
}

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→ Debugging Malformed JSON API Responses → Fixing JSON in Python and Java APIs → Why JSON Breaks in Fetch Responses