5 Common JSON Syntax Errors and How to Fix Them

# 5 Common JSON Syntax Errors and How to Fix Them

JSON (JavaScript Object Notation) is everywhere in modern web development—from API responses to configuration files. Despite its simple syntax, developers constantly encounter frustrating JSON errors that break applications.

This guide covers the 5 most common JSON syntax errors you'll encounter, how to identify them instantly, and how to fix them permanently.

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Why JSON Errors Are So Common

JSON's strict syntax requirements mean even a single misplaced character can invalidate an entire document. Unlike JavaScript, JSON:

• ✅ Requires double quotes for strings (single quotes are invalid)
• ✅ Prohibits trailing commas
• ✅ Mandates exact formatting for keys and values
• ✅ Allows no comments whatsoever

Let's dive into the errors that trip up developers most frequently.

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Error #1: Trailing Commas (The Silent Killer)

The Problem

Trailing commas—extra commas after the last item in an array or object—are perfectly valid in JavaScript but completely forbidden in JSON.

❌ Invalid JSON

{
  "username": "alice123",
  "email": "alice@example.com",
  "active": true,
}

{
  "products": [
    { "id": 1, "name": "Laptop" },
    { "id": 2, "name": "Mouse" },
    { "id": 3, "name": "Keyboard" },
  ]
}

✅ Valid JSON

{
  "username": "alice123",
  "email": "alice@example.com",
  "active": true
}

{
  "products": [
    { "id": 1, "name": "Laptop" },
    { "id": 2, "name": "Mouse" },
    { "id": 3, "name": "Keyboard" }
  ]
}

Error Message You'll See

Unexpected token } in JSON at position 78
SyntaxError: Trailing comma in JSON

How to Fix It

Manual Fix:

Scan for commas before closing braces } or brackets ]

Remove any trailing commas

Validate with a JSON linter

Prevention Tip: Use a JSON formatter like BigJSON that automatically detects and highlights trailing commas. Pro Tip for Developers:

// In your code editor, search for these patterns:
,\s}   // Comma before closing brace

,\s]   // Comma before closing bracket

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Error #2: Single Quotes Instead of Double Quotes

The Problem

JavaScript allows both single and double quotes for strings, but JSON only accepts double quotes ("). Using single quotes is one of the most common mistakes when converting JavaScript objects to JSON.

❌ Invalid JSON

{
  'name': 'John Doe',
  'age': 30,
  'city': 'New York'
}

{
  "name": 'John Doe',
  "age": 30
}

✅ Valid JSON

{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}

Error Message You'll See

Unexpected token ' in JSON at position 2
SyntaxError: Expected property name or '}' in JSON

How to Fix It

Quick Fix:

// JavaScript: Convert single to double quotes
const fixedJson = invalidJson.replace(/'/g, '"');

Better Approach: Use proper JSON serialization:

// Instead of manually writing JSON with quotes:
const wrong = "{ 'name': 'John' }";  // ❌

// Use JSON.stringify():
const obj = { name: 'John' };
const correct = JSON.stringify(obj);  // ✅ "{"name":"John"}"

When This Error Happens

• Copying JavaScript object literals and forgetting to convert quotes
• Manually typing JSON without a proper editor
• Using template literals incorrectly

---

Error #3: Unquoted Keys

The Problem

In JavaScript, object keys can be unquoted if they're valid identifiers. JSON requires all keys to be strings wrapped in double quotes.

❌ Invalid JSON

{
  name: "Alice",
  age: 28,
  isActive: true
}

{
  user_id: 42,
  "email": "test@example.com"
}

✅ Valid JSON

{
  "name": "Alice",
  "age": 28,
  "isActive": true
}

{
  "user_id": 42,
  "email": "test@example.com"
}

Error Message You'll See

Unexpected token n in JSON at position 4
SyntaxError: Expected property name enclosed in double quotes

How to Fix It

Visual Studio Code Users:

Install the "JSON" language extension

Format your document (Shift + Alt + F)

VS Code will highlight unquoted keys in red

Find and Replace Pattern:

Find:    (\\w+):\\s
Replace: "$1":

Real-World Example

// Common mistake when logging:
console.log({ status: 'success', data: result });  // Valid JS ✅

// Trying to save this as JSON:
{
  status: "success",  // ❌ Invalid JSON
  data: { ... }
}

// Correct JSON:
{
  "status": "success",  // ✅
  "data": { ... }
}

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Error #4: Invalid Escape Sequences

The Problem

JSON supports limited escape sequences. Using invalid ones or forgetting to escape special characters causes parsing errors.

❌ Invalid JSON

{
  "path": "C:\\Users\\Alice\\Documents",
  "message": "Line 1
Line 2"
}

{
  "quote": "He said "hello" to me"
}

✅ Valid JSON

{
  "path": "C:\\\\Users\\\\Alice\\\\Documents",
  "message": "Line 1\nLine 2"
}

{
  "quote": "He said \"hello\" to me"
}

Valid Escape Sequences in JSON

| Character | Escape Sequence | Description |

|-----------|----------------|-------------|

| " | \" | Double quote |

| \ | \\\\ | Backslash |

| / | \/ | Forward slash |

| Newline | \n | Line feed |

| Tab | \t | Horizontal tab |

| Carriage return | \r | Carriage return |

| Backspace | \b | Backspace |

| Form feed | \f | Form feed |

| Unicode | \uXXXX | Unicode character |

Error Message You'll See

Unexpected token in JSON at position 25
SyntaxError: Invalid escape sequence

How to Fix It

JavaScript Escaping:

// Automatically handle escaping:
const data = {
  path: "C:\\Users\\Alice",
  message: "Line 1\nLine 2"
};

const json = JSON.stringify(data);
// Result: {"path":"C:\\\\Users\\\\Alice","message":"Line 1\\nLine 2"}

Manual Path Escaping:

const windowsPath = "C:\\Users\\Alice";
const jsonPath = windowsPath.replace(/\\/g, "\\\\\\\\");

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Error #5: Comments in JSON (JSON ≠ JavaScript!)

The Problem

Developers often add comments to JSON files for documentation, but JSON does not support comments of any kind. No //, no / /, nothing.

❌ Invalid JSON

{
  // User configuration
  "username": "admin",
  "role": "superuser",
  / This is temporary

     Remove in production /
  "debugMode": true
}

✅ Valid JSON (No Comments Allowed)

{
  "username": "admin",
  "role": "superuser",
  "debugMode": true
}

Error Message You'll See

Unexpected token / in JSON at position 4
SyntaxError: JSON does not support comments

Workarounds

Option 1: Use a Comment Field

{
  "_comment": "User configuration - update before production",
  "username": "admin",
  "role": "superuser",
  "debugMode": true
}

Option 2: Use JSON5 or JSONC

For configuration files, consider:

• JSON5: Supports comments, trailing commas, and more
• JSONC: JSON with Comments (used by VS Code)

// config.json5
{
  // User configuration
  username: "admin",  // No quotes needed
  role: "superuser",
  debugMode: true,    // Trailing comma OK!
}

Option 3: Separate Documentation File

project/
├── config.json          # Pure JSON
├── config.README.md     # Documentation with comments

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Bonus: How to Catch JSON Errors Early

1. Use Online Validators

BigJSON Validator - Instant validation with helpful error messages:

• Paste your JSON
• Get line-by-line error detection
• See exactly where the problem is

2. IDE Extensions

VS Code:

• Built-in JSON validation
• Install "Error Lens" for inline error messages

Sublime Text:

• JSONLint package

Atom:

• linter-jsonlint

3. Pre-Commit Hooks

# .git/hooks/pre-commit
#!/bin/bash
for file in $(git diff --cached --name-only | grep -E '\.json$'); do
  if ! python -m json.tool "$file" > /dev/null 2>&1; then
    echo "Invalid JSON in $file"
    exit 1
  fi
done

4. Automated Testing

// Jest test
test('config.json is valid', () => {
  const fs = require('fs');
  const config = fs.readFileSync('config.json', 'utf8');

  expect(() => {
    JSON.parse(config);
  }).not.toThrow();
});

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Quick Reference: Error Detection Checklist

Before submitting or deploying JSON files, check:

• [ ] No trailing commas after last items
• [ ] All strings use double quotes (")
• [ ] All object keys are quoted
• [ ] Special characters are properly escaped
• [ ] No comments anywhere
• [ ] Proper nesting of brackets and braces
• [ ] Valid data types (string, number, boolean, null, object, array)
• [ ] Numbers don't have leading zeros (except 0.x)

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Tools to Prevent JSON Errors

1. BigJSON Online Tools

Visit BigJSON for:

• Formatter: Auto-fix formatting issues
• Validator: Instant syntax checking
• Minifier: Remove whitespace for production

2. Command-Line Tools

# Node.js built-in validator
node -e "JSON.parse(require('fs').readFileSync('file.json'))"

# Python validator
python -m json.tool file.json

# jq (JSON processor)
jq . file.json

3. Browser Developer Tools

// Console validation
try {
  JSON.parse(jsonString);
  console.log('✅ Valid JSON');
} catch (e) {
  console.error('❌ Invalid:', e.message);
}

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Common Error Messages Decoded

| Error Message | Likely Cause | Fix |

|--------------|-------------|-----|

| Unexpected token , | Trailing comma | Remove comma before } or ] |

| Unexpected token ' | Single quotes | Replace with double quotes |

| Expected property name | Unquoted key | Quote all object keys |

| Unexpected token } | Missing comma | Add comma between items |

| Unexpected end of JSON | Unclosed brace/bracket | Check nesting |

| Invalid escape sequence | Wrong escape character | Use valid escapes (\\, \n, etc.) |

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Real-World Example: Debugging an API Response

Let's fix a real API response with multiple errors:

❌ Before (Invalid)

{
  success: true,              // Error: Unquoted key
  'message': 'Data retrieved', // Error: Single quotes
  data: [
    {
      id: 1,
      name: "Product A",
      price: 29.99,
    },                         // Error: Trailing comma
    {
      id: 2,
      name: "Product B",
      // Note: Special offer  // Error: Comment
      price: 19.99
    }
  ],                           // Error: Trailing comma
}

✅ After (Valid)

{
  "success": true,
  "message": "Data retrieved",
  "data": [
    {
      "id": 1,
      "name": "Product A",
      "price": 29.99
    },
    {
      "id": 2,
      "name": "Product B",
      "price": 19.99
    }
  ]
}

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Preventing Errors in Your Workflow

For Developers

// Always use JSON.stringify() instead of manual concatenation
const config = {
  apiKey: process.env.API_KEY,
  timeout: 5000
};

// ❌ Manual (error-prone)
const json = '{ "apiKey": "' + config.apiKey + '", "timeout": ' + config.timeout + ' }';

// ✅ Automatic (safe)
const json = JSON.stringify(config);

For Configuration Files

Use strict validation in your editor

Set up linting rules

Add validation to CI/CD pipeline

Test JSON files as part of build process

For API Responses

// Express.js example
app.get('/api/data', (req, res) => {
  const data = { status: 'ok', results: [...] };

  // ❌ Manual string building
  res.send('{"status":"' + data.status + '"}');

  // ✅ Automatic JSON serialization
  res.json(data);  // Handles escaping, quotes, etc.
});

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Conclusion

The 5 most common JSON syntax errors are:

Trailing commas - Remove commas before } and ]

Single quotes - Always use double quotes (")

Unquoted keys - Quote all object keys

Invalid escapes - Use proper escape sequences

Comments - Remove all comments (use JSON5 if needed)

Remember: JSON is not JavaScript! Its strict syntax leaves no room for flexibility.

Next Steps

Validate your JSON: Use BigJSON Validator

Format automatically: Use BigJSON Formatter

Learn more: Read our Complete JSON Guide

Got questions about JSON errors? Drop a comment below!

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Related Resources

• Common JSON Errors and How to Fix Them
• How to Format JSON Files
• JSON Validator Tool
• JSON Formatter Online

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Last updated: February 15, 2026