How to Format and Validate JSON Online

JSON (JavaScript Object Notation) is the world's most used data exchange format. Created by Douglas Crockford in the early 2000s, it became the de facto standard for web APIs, configuration files, NoSQL databases, and structured data storage.

Its popularity comes from its simplicity: human-readable, easy to parse for machines, lightweight in size, and natively supported by JavaScript (the language of the web). Today, virtually every modern web application uses JSON to communicate between frontend and backend.

But this simplicity hides a trap: a single misplaced comma, a missing quote, or an unclosed bracket makes the entire file invalid. That's why a good JSON formatter/validator is an essential tool for every developer.

Trailing comma. The number 1 error. In JavaScript, {"a": 1, "b": 2,} is valid. In strict JSON, this trailing comma is forbidden.

Single quotes. JSON only accepts double quotes. {'name': 'John'} is invalid — use {"name": "John"}.

Comments. JSON does not support comments. No // or /* */. If you need comments, use JSON5 or JSONC.

Unquoted keys. {name: "John"} is JavaScript, not JSON. Keys must be in double quotes.

Control characters. Literal newlines in strings are forbidden. Use \n instead.

NaN, Infinity, undefined. These JavaScript values are not valid JSON values. Use null instead.

Step 1: Paste your JSON. Copy-paste your raw JSON into the editor. The code is automatically analyzed in real time.

Step 2: Error detection. Syntax errors are highlighted with an explanatory message. The tool indicates the exact line and position of each error.

Step 3: Auto formatting. Click "Format" to neatly indent your JSON. Choose between 2 or 4 spaces of indentation.

Step 4: Copy or download. Retrieve your formatted JSON in one click. Everything happens in your browser.

Name your keys in camelCase. firstName rather than first_name or FirstName. This is the most common convention in APIs.

Use consistent types. If a field is a number, don't put it in quotes. "age": 25 not "age": "25".

Avoid excessive nesting levels. More than 4-5 levels of depth makes JSON difficult to read and maintain.

Use arrays for lists. Even if the list contains only one element, prefer an array for consistency.

Document your structure. Use JSON Schema to define and validate the expected structure of your data.

REST APIs. JSON is the default format for nearly all modern APIs. API responses are JSON, POST/PUT request bodies too.

Configuration. package.json (Node.js), tsconfig.json (TypeScript), .eslintrc.json — modern configuration files massively use JSON.

Databases. MongoDB stores BSON documents (binary JSON). PostgreSQL has a native JSONB type. Firebase Realtime Database is essentially a giant JSON tree.

Conversion. CSV ↔ JSON is one of the most common conversions. Allplix offers dedicated tools for these conversions in both directions.