JSON Validator: Real-Time JSON Validation & Error Checking

Validate JSON instantly with real-time error checking. See syntax errors, line numbers, and error messages. Free, no registration required.

Last updated: March 2026 • Real-time validation: Instant error detection

What is JSON Validation?

JSON validation checks whether a JSON string follows the correct syntax rules. Valid JSON has proper structure: matching braces, correct key-value formatting, valid data types, and proper quotation marks.

When JSON validation fails, you get clear error messages showing exactly where and why the syntax is invalid. JSON Visualiser validates your JSON as you type, highlighting errors with squigglies and displaying detailed error messages.

Common JSON Validation Errors

Missing Comma

{  "name": "John"  "age": 30  // Missing comma after previous line}

Error: Expected comma or } after property value at line 3, column 2

Fix: Add a comma after "name": "John"

Trailing Comma

{  "name": "John",  "age": 30 // Trailing comma (invalid in strict JSON)}

Error: Trailing comma at line 3, column 14

Fix: Remove the trailing comma after 30

Unquoted Keys

{  "name": "John" // Keys must be in double quotes}

Error: Expected property name or } at line 2, column 2

Fix: Add double quotes around the key: "name"

Single Quotes Instead of Double Quotes

{  "name": "John" // JSON requires double quotes, not single}

Error: Unexpected token ' at line 2, column 2

Fix: Replace single quotes with double quotes: "name"

Missing Closing Brace

{  "name": "John",  "age": 30// Missing closing brace

Error: Expected } at end of input at line 4, column 1

Fix: Add } at the end

Invalid Data Types

{  "name": undefined // undefined is not a valid JSON value}

Error: Unexpected token u at line 2, column 11

Fix: Use valid JSON types: string, number, boolean, array, object, or null

How to Validate JSON

Step 1: Open JSON Visualiser

Navigate to the JSON Visualiser homepage. The editor loads instantly with no sign-up required.

Step 2: Paste or Type JSON

Paste your JSON into the editor pane, or type directly. The Monaco editor provides syntax highlighting.

Step 3: View Validation Results

As you type, JSON Visualiser validates in real-time:

• Valid JSON: No errors, green indicator
• Invalid JSON: Errors highlighted with red squigglies
• Error panel shows line numbers and messages

Step 4: Fix Errors

Click on error squigglies to jump to the error location. Read the error message for guidance. Fix the issue and validation updates instantly.

Validation Features

Real-Time Validation

Validation happens as you type, not when you click a button. See errors immediately and fix them before they become problems.

Error Highlighting

Red squigglies under invalid code show exact error locations. Hover over squigglies to see detailed error messages.

Clear Error Messages

Understand exactly what's wrong:

• Expected comma after property value
• Trailing comma not allowed
• Unexpected token in JSON
• Unclosed string literal
• Mismatched brackets

Line and Column Numbers

Each error shows the precise line and column. Navigate to errors instantly by clicking in the error panel.

Syntax Highlighting

Color-coded JSON helps you spot errors visually:

• Keys in one color
• String values in another
• Numbers, booleans, and null each have distinct colors
• Brackets and punctuation clearly visible

Tree View Verification

Even valid JSON may have unexpected structure. Use tree view to:

• Verify object nesting
• Check array contents
• Confirm data types
• Navigate through complex structures

JSON vs. JavaScript Object

Common confusion: JSON looks like JavaScript objects but has stricter rules:

Example - Invalid JSON:

{  name: "John",        // Unquoted key (invalid)  'age': 30,           // Single quotes (invalid)  greet: function() {} // Function (invalid)}

Example - Valid JSON:

{  "name": "John",  "age": 30,  "greet": "Hello" // String value (valid)}

Advanced Validation

Schema Validation (Coming Soon)

Validate JSON against a JSON Schema definition:

• Ensure required fields exist
• Check data type constraints
• Validate value ranges
• Verify string patterns

Circular Reference Detection

JSON cannot have circular references. Our tree view can detect when objects reference themselves, which would make the JSON non-serializable.

Size Limits

Large JSON files can cause issues:

• Browser memory limits (typically 100-500MB)
• Performance degradation with very deep nesting
• Timeout issues on extremely large files

JSON Visualiser handles files up to several MB comfortably.

API Integration

Validating API Responses

Test your API endpoints and validate responses:

fetch("https://api.example.com/data")  .then((response) => response.json())  .then((data) => {    console.log("Valid JSON:", data);  })  .catch((error) => {    console.error("Invalid JSON:", error);  });

Paste the API response into JSON Visualiser to validate and explore the structure.

Debugging Failed JSON Parsing

If your application throws "Unexpected token" or "JSON.parse failed":

1. Copy the failing JSON
2. Paste into JSON Visualiser
3. See exact error location and message
4. Fix the JSON
5. Copy back to your application

Validate your JSON instantly with real-time error checking. No registration, instant load, and clear error messages that help you fix issues quickly.

References: JSON.org specification, ECMA-404 standard, MDN JSON documentation.