JSON Validator & Formatter

What is JSON?

JSON (JavaScript Object Notation) is a lightweight text format for storing and exchanging data. Proposed by Douglas Crockford in the early 2000s, JSON became the most widely used data interchange format on the internet thanks to its simple, human-readable syntax that machines can parse with equal ease.

Unlike XML or CSV, JSON can represent complex nested data structures — objects containing arrays containing other objects — without verbose markup. This makes JSON the default choice for REST APIs, application configuration files, NoSQL databases like MongoDB, and settings storage in modern web applications.

JSON is standardized under RFC 8259 and ECMA-404. These standards define exactly what is valid — from how strings and numbers are written, to how objects and arrays are structured. Any deviation from the standard makes the JSON invalid and unparseable.

JSON Syntax Rules

JSON supports six data types:

• String: Must be enclosed in double quotes. Example: "hello". Single quotes are not valid in JSON.
• Number: Integer or decimal, positive or negative, supports scientific notation. Examples: 42, -3.14, 1.5e10.
• Boolean: Only true and false — always lowercase.
• Null: The keyword null (lowercase) represents an empty/absent value.
• Object: A collection of key-value pairs inside curly braces {}. Every key must be a string in double quotes.
• Array: An ordered list of values inside square brackets [], separated by commas.

What JSON does not allow: comments (neither // nor /* */), trailing commas after the last item, unquoted keys, and special JavaScript values like undefined, NaN, or Infinity.

Common JSON Errors and Fixes

1. Trailing Comma

The most common error among developers who write JSON by hand after working with JavaScript. JSON strictly forbids commas after the last element of an object or array. Prevention: always double-check the last item before } or ].

2. Unquoted Object Keys

In JavaScript you can write { name: 'Alice' }, but in JSON every key must have double quotes: { "name": "Alice" }. This is the source of many bugs when developers copy JavaScript object literals directly into a JSON file.

3. Comments in JSON

Standard JSON does not support comments of any kind. If you need comments in configuration files, consider YAML or TOML instead. Some editors support JSONC (JSON with Comments) as a non-standard extension, but it is not valid JSON.

4. Unterminated Strings

Occurs when a string value is cut off or missing its closing quote. Most often encountered when pasting content that contains unescaped double quotes inside a string — the fix is to escape inner quotes with a backslash: \".

JSON vs XML vs YAML

Summary: JSON wins for REST APIs and web data exchange; YAML wins for configuration files (Kubernetes, Docker Compose, CI/CD pipelines); XML remains common in legacy enterprise systems (SOAP, RSS, XSLT).

Best Practices for Valid JSON

• Always use an editor with JSON syntax highlighting (VS Code, JetBrains IDEs) — errors are visually flagged immediately.
• Add a JSON linter step in your CI/CD pipeline — catch errors before they reach production.
• When generating JSON programmatically, use JSON.stringify() (JavaScript) or json.dumps() (Python) instead of manually concatenating strings.
• Validate JSON after every manual edit — especially when adding or removing elements.
• Use consistent key naming: either camelCase or snake_case throughout a document, never mixed.
• Avoid special characters in keys — while technically valid, they require bracket notation to access in code and are a common source of bugs.