JSON Format Validation and How to Fix the Most Common Parse Errors

What is JSON: Data Exchange Format Basics

JSON (JavaScript Object Notation) is the most widely used data exchange format in web development. It appears everywhere—API responses, configuration files, database exports, and more.

Basic JSON Structure

JSON consists of two data structures:

1. Object: {key: value} format
2. Array: [value1, value2, ...] format

Basic JSON example:

{  "name": "Taro Tanaka",  "age": 30,  "isActive": true,  "hobbies": ["reading", "travel", "programming"],  "address": {    "city": "Tokyo",    "postalCode": "100-0001"  }}

Data Types Available in JSON

• String: Enclosed in double quotes "text"
• Number: 123, 45.67, -10, etc.
• Boolean: true or false
• null: null
• Object: {}
• Array: []

Important: JSON does not support undefined, functions, or Date objects.

6 Common Parse Errors and Their Causes

Here are typical error patterns encountered when parsing JSON.

Error 1: Trailing Comma

Error Message

SyntaxError: Unexpected token } in JSON at position 45

Problematic Code

{  "name": "Taro Tanaka",  "age": 30,  // ← This trailing comma is NG}

Solution

Remove the comma after the last property.

{  "name": "Taro Tanaka",  "age": 30}

While JavaScript allows trailing commas in object literals, JSON strictly prohibits them.

Error 2: Single Quotes Used

Error Message

SyntaxError: Unexpected token ' in JSON at position 2

Problematic Code

{  'name': 'Taro Tanaka'  // ← Single quotes are NG}

Solution

Change all strings to double quotes.

{  "name": "Taro Tanaka"}

In JSON, both keys and values must always use double quotes.

Error 3: undefined / NaN / Infinity Mixed In

Error Message

SyntaxError: Unexpected token u in JSON at position 10

Problematic Code

{  "name": "Taro Tanaka",  "email": undefined  // ← undefined cannot be used in JSON}

Solution

Replace undefined with null, or remove the property entirely.

{  "name": "Taro Tanaka",  "email": null}

Or

{  "name": "Taro Tanaka"}

Similarly, NaN and Infinity are also not allowed in JSON.

Error 4: Comments Written

Error Message

SyntaxError: Unexpected token / in JSON at position 15

Problematic Code

{  // This is a comment ← Comments not allowed in JSON  "name": "Taro Tanaka"}

Solution

Remove all comments.

{  "name": "Taro Tanaka"}

JSON has no comment syntax. If comments are needed, create a dedicated key.

{  "_comment": "This is user information",  "name": "Taro Tanaka"}

Error 5: Missing Key Quotes

Error Message

SyntaxError: Unexpected token n in JSON at position 3

Problematic Code

{  name: "Taro Tanaka"  // ← Key has no quotes}

Solution

Always enclose keys in double quotes.

{  "name": "Taro Tanaka"}

While JavaScript objects allow key quotes to be omitted, JSON requires them.

Error 6: Numeric Key Mixed In

Error Message

SyntaxError: Unexpected number in JSON at position 5

Problematic Code

{  123: "value"  // ← Key is a number}

Solution

Convert numeric keys to strings.

{  "123": "value"}

JSON keys must always be strings.

Validation Methods: Browser, VS Code, Online Tools

There are several ways to verify if JSON is in the correct format.

Method 1: Validate in Browser Console

Open browser developer tools (F12), go to Console tab, and run:

JSON.parse('{"name": "Taro Tanaka", "age": 30}');

If no error appears, the JSON is in correct format. If an error occurs, it shows where the problem is.

Method 2: Use VS Code Extensions

In Visual Studio Code, opening a .json file automatically enables syntax highlighting and validation.

When errors exist:

• Red wavy underlines appear on relevant lines
• Error details display in Problems panel (Ctrl+Shift+M / Cmd+Shift+M)

Additionally, Alt+Shift+F (Windows) or Option+Shift+F (Mac) formats indentation and line breaks automatically.

Method 3: Online JSON Validator

Browser-based JSON formatter tools allow validation + formatting without installation.

Main features:

• Real-time syntax checking
• Error location highlighting
• Auto-formatting (indentation alignment)
• Compression (minify)
• Tree display

Simply paste your JSON, and error types and locations are instantly identified.

Method 4: Automated Validation with Node.js Script

For automated JSON file validation in CI/CD pipelines, use Node.js scripts.

const fs = require('fs');try {  const data = fs.readFileSync('config.json', 'utf8');  JSON.parse(data);  console.log('✅ JSON is correctly formatted');} catch (error) {  console.error('❌ JSON parse error:', error.message);  process.exit(1);}

Integrating this script into the build process enables automatic pre-deployment checks.

Debugging Techniques for Deeply Nested Cases

Finding errors in complex nested JSON is difficult. These techniques improve efficiency.

Technique 1: Parse Partially

To identify error locations, divide and parse JSON sections.

// Test parts instead of the wholeJSON.parse('{"name": "Taro Tanaka"}');  // OKJSON.parse('{"hobbies": ["reading", "travel",]}');  // Error detected

Technique 2: Format with JSON Formatter

Minified JSON without line breaks or indentation is hard to read, so format it first.

Before formatting:

{"name":"Taro Tanaka","age":30,"address":{"city":"Tokyo","postalCode":"100-0001"}}

After formatting:

{  "name": "Taro Tanaka",  "age": 30,  "address": {    "city": "Tokyo",    "postalCode": "100-0001"  }}

Visibility improves, making error locations easier to spot.

Technique 3: Compare with Diff Tools

Comparing correct and incorrect JSON with diff tools (like VS Code's compare feature) makes differences obvious.

In VS Code:

1. Select two files
2. Right-click → "Compare Selected"

Differences are then highlighted.

Differences Between JSON and JavaScript Objects

JSON and JavaScript objects are similar but strictly different.

Aspect	JSON	JavaScript Object
Key quotes	Required (double only)	Optional
Value quotes	Double only	Single also OK
Trailing commas	Not allowed	Allowed (ES5+)
Comments	Not allowed	Allowed
undefined	Not allowed	Allowed
Functions	Not allowed	Allowed
Date objects	Not allowed (convert to string)	Allowed

Conversion Considerations

When converting JavaScript objects to JSON, JSON.stringify() automatically adjusts, but some data is lost.

const obj = {  name: "Taro Tanaka",  age: 30,  greet: function() { return "Hello"; },  // Functions are removed  undefined: undefined,  // undefined is removed  date: new Date()  // Converted to ISO 8601 string};console.log(JSON.stringify(obj));// Output: {"name":"Taro Tanaka","age":30,"date":"2026-05-26T00:00:00.000Z"}

Automating JSON Validation in API Development

When developing web APIs, automating JSON validation for requests and responses prevents errors in advance.

Validation with JSON Schema

JSON Schema is a standard for defining JSON data structure.

Schema example:

{  "type": "object",  "properties": {    "name": { "type": "string" },    "age": { "type": "number", "minimum": 0 }  },  "required": ["name", "age"]}

Validating against this schema ensures data integrity.

Implementation Example in Node.js (using ajv library)

const Ajv = require('ajv');const ajv = new Ajv();const schema = {  type: "object",  properties: {    name: { type: "string" },    age: { type: "number", minimum: 0 }  },  required: ["name", "age"]};const validate = ajv.compile(schema);const data = { name: "Taro Tanaka", age: 30 };if (validate(data)) {  console.log('✅ Validation succeeded');} else {  console.log('❌ Validation failed:', validate.errors);}

Integrating as middleware in web frameworks like Express automatically rejects invalid requests.

Summary: Knowing Error Patterns Makes It Easy

JSON parse errors are simple to solve once you know the patterns.

• Remove trailing commas
• Unify to double quotes (no single quotes)
• Convert undefined to null
• Remove comments
• Key quotes mandatory

Daily-use tools:

• Browser console (quick validation)
• VS Code (real-time checks during development)
• JSON formatter tool (instant online verification)
• JSON schema (automated API validation)

Looking at position X in error messages shows where the problem occurred. Stay calm and check one by one.

Use JSON formatter tools to debug efficiently.