JSON Errors

Why Is My JSON Invalid? 10 Common JSON Errors Explained

Explore the 10 most common JSON syntax errors with examples, learn why parsers reject them, and discover how to find and fix them in seconds.

PlayWithJSON Team

Oct 5, 2026

8 min read

“This looks completely correct. Why is it still failing?”

Have you ever looked at a JSON document and been 100% sure it was valid, only to have your backend, browser, or API client crash with a cryptic SyntaxError: Unexpected token?

JSON has a simple grammar, but it has no room for forgiveness. A single missing comma, an accidental trailing comma, single quotes from a copy-paste, or smart typographic quotes will cause a JSON parser to reject the entire payload.

In this guide, we break down the 10 most common JSON errors, explain the exact parser mechanics behind them, and show you how to find and fix them in seconds.

10 Errors Covered in This Guide:

1Missing Commas 2Using Single 3Unquoted Property 4Trailing Commas 5Missing or 6Invalid Backslash 7Using JavaScript-Specific 8Capitalized Booleans 9Accidentally Using 10Extra Text

Missing Commas Between Properties

Forgetting a comma between keys or array items causes unexpected token errors.

Invalid JSONSyntax Error

JSON

1{
2  "name": "John"
3  "age": 25
4}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "age": 25
4}

Rule to Remember:

A comma separates key-value pairs and array items, but never put one after the final item.

SyntaxError: Unexpected token '"age"' at line 3

Using Single Quotes (') Instead of Double Quotes (")

Common when copying JavaScript or Python code. Standard JSON strictly requires double quotes.

Invalid JSONSyntax Error

JSON

1{
2  'name': 'John',
3  'age': 25
4}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "age": 25
4}

Rule to Remember:

Strings and property keys must always be enclosed in standard double quotes (").

SyntaxError: Unexpected token ''' at line 2

Unquoted Property Names

In JavaScript object literals, key quotes are optional. In JSON, unquoted keys are invalid.

Invalid JSONSyntax Error

JSON

1{
2  name: "John",
3  age: 25
4}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "age": 25
4}

Rule to Remember:

Every key name must be wrapped in double quotes.

SyntaxError: Expected double-quoted property name at line 2

Trailing Commas After Final Items

Leaving a comma after the last property in an object or array breaks RFC 8259 syntax.

Invalid JSONSyntax Error

JSON

1{
2  "name": "John",
3  "age": 25,
4}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "age": 25
4}

Rule to Remember:

Never place a comma after the final property or array element.

SyntaxError: Unexpected token } in JSON at position 32

Missing or Extra Brackets ({ } and [ ])

Deeply nested structures often leave unclosed brackets or mismatched curly/square braces.

Invalid JSONSyntax Error

JSON

1{
2  "user": {
3    "name": "John",
4    "age": 25
5  }

Correct JSONValid RFC 8259

JSON

1{
2  "user": {
3    "name": "John",
4    "age": 25
5  }
6}

Rule to Remember:

Every opening { must match a closing }, and every [ must match a closing ].

SyntaxError: Unexpected end of JSON input

Invalid Backslash Escape Sequences

Backslashes are reserved for escapes (\n, \t, \", \\). Naked backslashes like \w fail.

Invalid JSONSyntax Error

JSON

1{
2  "message": "Hello\world"
3}

Correct JSONValid RFC 8259

JSON

1{
2  "message": "Hello\\world"
3}

Rule to Remember:

To render a literal backslash, escape it with a double backslash (\\).

SyntaxError: Bad escaped character at line 2

Using JavaScript-Specific Values (undefined, NaN)

JSON does not support undefined, functions, Symbols, or Date objects.

Invalid JSONSyntax Error

JSON

1// JavaScript (INVALID in JSON)
2{
3  "name": "John",
4  "email": undefined
5}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "email": null
4}

Rule to Remember:

Use null to represent empty or missing values instead of undefined.

SyntaxError: Unexpected token u in JSON at position 28

Capitalized Booleans and Null (True, FALSE, NULL)

Python or SQL developers often write True or NULL. JSON literals are strictly lowercase.

Invalid JSONSyntax Error

JSON

1{
2  "active": True,
3  "verified": FALSE,
4  "address": NULL
5}

Correct JSONValid RFC 8259

JSON

1{
2  "active": true,
3  "verified": false,
4  "address": null
5}

Rule to Remember:

Always write booleans and null in strict lowercase: true, false, null.

SyntaxError: Unexpected token T in JSON at line 2

Accidentally Using JSON5 Syntax in Standard JSON

Comments (//, /* */) and single quotes work in JSON5 and tsconfig, but crash JSON.parse().

Invalid JSONSyntax Error

JSON

1// Valid in JSON5, INVALID in Standard JSON:
2{
3  // User comments are allowed
4  name: 'John',
5  age: 25,
6}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John",
3  "age": 25
4}

Rule to Remember:

Standard JSON forbids comments, unquoted keys, and single quotes.

SyntaxError: Unexpected token / in JSON at line 3

Extra Text Outside the JSON Document

Pasting log prefixes ("Response: { ... }") or markdown code fences into parsers.

Invalid JSONSyntax Error

JSON

1Here is the response:
2
3{
4  "name": "John"
5}

Correct JSONValid RFC 8259

JSON

1{
2  "name": "John"
3}

Rule to Remember:

The document must start with { or [ and contain nothing else before or after.

SyntaxError: Unexpected token H in JSON at line 1

Bonus: The “Looks Fine” Problem (Smart Quotes)

Some JSON errors are invisible to the naked eye because the characters look nearly identical. The most notorious example is typographic curly quotes (smart quotes):

❌ Smart / Typographic Quotes (Copied from Docs/Word/Slack):

Text

1{
2  “name”: “John”
3}

Contains unicode characters “ (U+201C) and ” (U+201D).

✅ Standard ASCII Double Quotes (Code Editor):

JSON

1{
2  "name": "John"
3}

Contains standard ASCII character " (U+0022).

Debugging Advice: When copying JSON from chat apps (Slack, Microsoft Teams, Skype) or blog posts, word processors often auto-replace straight quotes with curly quotes. Always inspect with a code editor or JSON validator.

Why Does JSON Fail So Easily?

JSON's strictness is completely intentional. Douglas Crockford designed JSON in 2001 to be a universal, language-agnostic data interchange format.

In a programming language like JavaScript or Python, flexibility allows for faster coding. However, for machine-to-machine data exchange where a Python server communicates with a Go microservice and a React client, ambiguity is disastrous.

By enforcing rigid rules—double quotes only, no trailing commas, no unquoted keys—every programming language on Earth can parse JSON using the exact same predictable parser algorithm without guessing the developer's intent.

How to Debug Invalid JSON in 6 Fast Steps

When a parser crashes, never scan blindly through thousands of lines. Follow this systematic diagnostic routine:

Format the JSON

Turn single-line minified JSON into an indented hierarchy. Formatting reveals structure and missing punctuation instantly.

Validate Syntax with a Tool

Don’t guess. Paste the JSON into a dedicated validator to pinpoint the exact line, column, and character causing the error.

Check Right Before the Error Line

Parsers often flag the token AFTER the real problem (e.g., flagging line 4 because line 3 was missing a comma).

Inspect Quotation Marks

Check for single quotes ('), backticks (`), or typographic “smart quotes” copied from chat/docs.

Scan Commas and Trailing Commas

Verify that every property has a comma except the very last one before the closing } or ].

Match Brackets from Outside-In

Ensure every { has a matching } and every [ has a matching ]. Nested formatting makes this obvious.

Practical Challenge: Spot the 2 Hidden Bugs

This snippet looks clean and readable, but running JSON.parse() on it will immediately fail. Can you spot both errors?

The Problematic Payload:

JSON

1{
2  "user": {
3    "name": 'John',
4    "age": 25,
5    "skills": [
6      "JavaScript",
7      "Python",
8    ]
9  }
10}

Click Reveal Solution above to inspect:

Look closely at line 3 and line 7.

JSON Syntax Error vs. JSON Data Error

Many developers confuse structural syntax errors with application semantic data errors. Here is why the distinction matters:

Syntax Error (Invalid JSON)Parser Crash

JSON

1{
2  "name": "John"
3  "age": 25
4}

The file violates RFC 8259 grammar (missing comma). The parser halts immediately before any data can be extracted or read.

Data Error (Valid JSON, Invalid Semantics)Schema / Logic Issue

JSON

1{
2  "age": "twenty-five" // Valid syntax, but application expects number!
3}

The JSON itself parses cleanly into memory. However, the application crashes later when doing mathematical calculations because age was a string rather than a number.

Interactive Pre-Send JSON Syntax Checklist

Before pushing a payload to an external API or saving a configuration file, run through this quick 11-point inspection:

JavaScript Object vs. JSON: Don't Mix Them Up

A massive percentage of daily JSON errors happen because frontend developers confuse JavaScript code with JSON strings. JavaScript allows unquoted keys, single quotes, trailing commas, and comments; JSON permits none of these.

JavaScript Object Literal (Code):

JavaScript

1// JavaScript Object Literal
2const user = {
3  name: 'John',
4  age: 25,
5  isActive: true,
6};

Standard JSON (Data Format):

JSON

1{
2  "name": "John",
3  "age": 25,
4  "isActive": true
5}

Fast Resolution Workflow

Format → Validate → Locate → Fix → Validate Again

When JSON looks right but still fails, let an automated validator do the heavy lifting instead of burning mental energy searching for a missing comma.

Format & Validate JSON Now Try JSON5 Formatter (For Comments & Trailing Commas)

“This looks completely correct. Why is it still failing?”

Have you ever looked at a JSON document and been 100% sure it was valid, only to have your backend, browser, or API client crash with a cryptic SyntaxError: Unexpected token?

In this guide, we break down the 10 most common JSON errors, explain the exact parser mechanics behind them, and show you how to find and fix them in seconds.