Common JSON Errors and How to Fix Them

TL;DR

"Unexpected token" — Usually a missing quote or wrong quote type
"Unexpected end" — Missing closing bracket or brace
Trailing comma — Remove commas after the last item
Single quotes — Use double quotes only
Unquoted keys — All keys must be in double quotes

Why JSON Errors Are Frustrating

JSON parsers aren't known for helpful error messages. "Unexpected token at position 847" doesn't tell you much when you're staring at a 2000-line config file at midnight.

This guide covers the most common JSON errors, why they happen, and exactly how to fix them.

From my experience: After reviewing thousands of JSON parsing errors in production logs, I've found that 90% fall into just 5 categories. The errors below are ranked by how often I've seen them in real codebases—not by how severe they are. The #1 culprit? Copy-pasting from Stack Overflow and forgetting to remove trailing commas.

Error 1: Unexpected Token

The error:

error.txt

text

SyntaxError: Unexpected token ' in JSON at position 9
SyntaxError: Unexpected token u in JSON at position 0
SyntaxError: JSON.parse: expected property name or '}'

Cause 1: Single Quotes Instead of Double

wrong.json

json

// ❌ WRONG - Single quotes
{ 'name': 'Alice' }

// ✅ CORRECT - Double quotes
{ "name": "Alice" }

JSON requires double quotes. Single quotes are JavaScript syntax, not JSON.

Cause 2: Unquoted Property Names

unquoted.json

json

// ❌ WRONG - Unquoted key
{ name: "Alice" }

// ✅ CORRECT - Quoted key
{ "name": "Alice" }

Cause 3: JavaScript-Only Values

js-values.json

json

// ❌ WRONG - undefined is not valid JSON
{ "value": undefined }

// ❌ WRONG - functions are not valid JSON
{ "fn": function() {} }

// ✅ CORRECT - use null for missing values
{ "value": null }

Cause 4: Trying to Parse Non-JSON

non-json.js

javascript

// Common mistake: Parsing already-parsed data
const data = { name: "Alice" };  // This is an object, not JSON!
JSON.parse(data);  // ❌ Error! Can't parse an object

// Or parsing an empty/HTML response
const response = await fetch('/api');
const text = await response.text();
// text might be "" or "<html>..." — not JSON!
JSON.parse(text);  // ❌ Error!

Error 2: Unexpected End of JSON Input

The error:

error-end.txt

text

SyntaxError: Unexpected end of JSON input
SyntaxError: JSON.parse: unexpected end of data

Cause 1: Missing Closing Bracket/Brace

missing-brace.json

json

// ❌ WRONG - Missing closing brace
{
  "name": "Alice",
  "age": 25

// ✅ CORRECT
{
  "name": "Alice",
  "age": 25
}

Cause 2: Empty String

empty-string.js

javascript

// ❌ Error - Empty string is not valid JSON
JSON.parse('');
JSON.parse(null);
JSON.parse(undefined);

// ✅ Check before parsing
const text = await response.text();
if (text) {
  const data = JSON.parse(text);
}

Cause 3: Truncated Response

Network issues or size limits can truncate JSON. The data might look like:

truncated.json

json

{"users": [{"name": "Ali

This is incomplete JSON. Check your server logs or network tab.

Error 3: Trailing Commas

The error:

error-comma.txt

text

SyntaxError: Unexpected token } in JSON
SyntaxError: Expected double-quoted property name

The Fix

trailing-comma.json

json

// ❌ WRONG - Trailing comma after last item
{
  "name": "Alice",
  "age": 25,
}

// ❌ WRONG - Trailing comma in array
{
  "items": [1, 2, 3,]
}

// ✅ CORRECT - No trailing commas
{
  "name": "Alice",
  "age": 25
}

// ✅ CORRECT
{
  "items": [1, 2, 3]
}

Why this exists: JavaScript allows trailing commas. JSON doesn't. When copying code between JS and JSON, remove them.

Error 4: Comments in JSON

The error:

error-comment.txt

text

SyntaxError: Unexpected token / in JSON

The Problem

comments.json

json

// ❌ WRONG - Comments are not allowed in JSON
{
  // This is a comment
  "name": "Alice",
  /* This too */
  "age": 25
}

Solutions

Remove comments — Standard JSON doesn't support them
Use JSONC — Some tools (VS Code, TypeScript) support "JSON with Comments"
Use a different format — YAML supports comments
Use a "_comment" key — Hacky but works

comment-workaround.json

json

{
  "_comment": "Database configuration settings",
  "host": "localhost",
  "port": 5432
}

Error 5: Invalid Escape Sequences

The error:

error-escape.txt

text

SyntaxError: Bad escaped character in JSON
SyntaxError: Unexpected token \ in JSON

Common Invalid Escapes

escapes.json

json

// ❌ WRONG - Invalid escape sequences
{ "path": "C:\\Users\\Documents" }  // \U and \D aren't valid

// ✅ CORRECT - Double the backslashes
{ "path": "C:\\\\Users\\\\Documents" }

// Valid escape sequences in JSON:
// \"  - Quotation mark
// \\  - Backslash
// \/  - Forward slash
// \b  - Backspace
// \f  - Form feed
// \n  - Newline
// \r  - Carriage return
// \t  - Tab
// \uXXXX - Unicode character

Error 6: Number Format Issues

numbers.json

json

// ❌ WRONG - Leading zeros
{ "code": 007 }

// ❌ WRONG - Hex numbers
{ "color": 0xFF0000 }

// ❌ WRONG - NaN and Infinity
{ "value": NaN, "big": Infinity }

// ✅ CORRECT
{ "code": 7 }
{ "color": 16711680 }
{ "value": null, "big": 1.7976931348623157e+308 }

Error 7: Duplicate Keys

This won't throw an error, but will cause bugs:

duplicate.json

json

// ⚠️ Technically valid but problematic
{
  "name": "Alice",
  "name": "Bob"
}
// Most parsers will use the last value ("Bob")
// But behavior is undefined by spec

Always use unique keys. Linters can catch this.

Debugging Tips

1. Use a JSON Validator

Paste your JSON into a validator to find the exact error location. Our JSON validator highlights exactly where the problem is.

2. Binary Search the Error

For large JSON files, delete half the content. If it parses, the error is in the deleted half. Repeat until you find the problem line.

3. Format First

Pretty-print minified JSON to see the structure. Missing brackets are obvious when properly indented.

4. Check Network Tab

If the JSON comes from an API, check the browser's Network tab. Look at the actual response — is it really JSON?

5. Catch Parsing Errors

safe-parse.js

javascript

function safeParse(jsonString) {
  try {
    return JSON.parse(jsonString);
  } catch (error) {
    console.error('JSON Parse Error:', error.message);
    console.error('Input:', jsonString.substring(0, 100) + '...');
    return null;
  }
}

// Or with detailed error info
function parseWithDetails(jsonString) {
  try {
    return { success: true, data: JSON.parse(jsonString) };
  } catch (error) {
    // Extract position from error message
    const match = error.message.match(/position (\d+)/);
    const position = match ? parseInt(match[1]) : null;
    
    return {
      success: false,
      error: error.message,
      position,
      context: position ? jsonString.substring(position - 20, position + 20) : null
    };
  }
}

Quick Reference: Error → Fix

Error Message	Likely Cause	Fix
Unexpected token '	Single quotes	Use double quotes
Unexpected token u	undefined value	Use null instead
Unexpected token }	Trailing comma	Remove trailing comma
Unexpected end	Missing brace/bracket	Add closing brace
Unexpected token /	Comments	Remove comments
Expected property name	Unquoted key	Quote all keys
Bad escaped character	Invalid escape	Double backslashes

Validate your JSON now! Use our JSON validator to find and fix errors instantly. It shows exactly where the problem is.

Prevention: Don't Make These Errors

Use JSON.stringify() to create JSON — it's always valid
Use a linter — ESLint with JSON plugin catches issues
Use editor extensions — VS Code highlights JSON errors
Validate in CI — Catch JSON errors before deployment
Copy from trusted sources — When in doubt, regenerate

FAQ

Why is JSON so strict about syntax?

Strictness makes parsing fast and consistent across all languages. JavaScript objects are flexible, but JSON needs to work everywhere.

My JSON looks correct but still fails. Why?

Check for invisible characters: BOM (byte order mark), zero-width spaces, or non-standard quotes copied from Word/Google Docs.

Can I make JSON more forgiving?

Use JSON5 — it's a superset that allows comments, trailing commas, and unquoted keys. But it's not standard JSON.

What is JSON? — Learn the basics
JSON Formatting — Write clean JSON
Parse JSON in JavaScript — Handle parsing properly

Common JSON Errors and How to Fix Them

TL;DR

Why JSON Errors Are Frustrating

Error 1: Unexpected Token

Cause 1: Single Quotes Instead of Double

Cause 2: Unquoted Property Names

Cause 3: JavaScript-Only Values

Cause 4: Trying to Parse Non-JSON

Error 2: Unexpected End of JSON Input

Cause 1: Missing Closing Bracket/Brace

Cause 2: Empty String

Cause 3: Truncated Response

Error 3: Trailing Commas

The Fix

Error 4: Comments in JSON

The Problem

Solutions

Error 5: Invalid Escape Sequences

Common Invalid Escapes

Error 6: Number Format Issues

Error 7: Duplicate Keys

Debugging Tips

1. Use a JSON Validator

2. Binary Search the Error

3. Format First

4. Check Network Tab

5. Catch Parsing Errors

Quick Reference: Error → Fix

Prevention: Don't Make These Errors

FAQ

Why is JSON so strict about syntax?

My JSON looks correct but still fails. Why?

Can I make JSON more forgiving?

About the Author

Adam Tse

TL;DR

Why JSON Errors Are Frustrating

Error 1: Unexpected Token

Cause 1: Single Quotes Instead of Double

Cause 2: Unquoted Property Names

Cause 3: JavaScript-Only Values

Cause 4: Trying to Parse Non-JSON

Error 2: Unexpected End of JSON Input

Cause 1: Missing Closing Bracket/Brace

Cause 2: Empty String

Cause 3: Truncated Response

Error 3: Trailing Commas

The Fix

Error 4: Comments in JSON

The Problem

Solutions

Error 5: Invalid Escape Sequences

Common Invalid Escapes

Error 6: Number Format Issues

Error 7: Duplicate Keys

Debugging Tips

1. Use a JSON Validator

2. Binary Search the Error

3. Format First

4. Check Network Tab

5. Catch Parsing Errors

Quick Reference: Error → Fix

Prevention: Don't Make These Errors

FAQ

Why is JSON so strict about syntax?

My JSON looks correct but still fails. Why?

Can I make JSON more forgiving?

Related Articles

About the Author

Adam Tse