JSON Syntax Cheat Sheet & Quick Reference

Overview

JavaScript Object Notation (JSON), specified by RFC 8259, is the ubiquitous text-based data interchange format for modern REST APIs, configuration files, and document databases. While lightweight and human-readable, JSON enforces strict syntax rules where even a single misplaced comma or unescaped character causes catastrophic parser exceptions.

This reference sheet organizes core data types, strict formatting rules, code examples, common pitfalls, and API best practices for immediate lookup during coding and code reviews.

Syntax Tables & Core Data Types

JSON supports exactly six primitive and structured data types. Every value in a valid JSON document must conform strictly to one of these types:

Practical Examples

Below is a production-grade API response payload combining nested objects, arrays of objects, ISO 8601 timestamp strings, and nullable fields:

{
  "status": "success",
  "data": {
    "user": {
      "id": "usr_9b8c7d6e",
      "username": "jaytech",
      "isActive": true,
      "deletedAt": null,
      "roles": ["developer", "reviewer"],
      "metadata": {
        "lastLogin": "2026-07-03T10:30:00Z",
        "loginCount": 142
      }
    }
  }
}

Common Mistakes & How to Avoid Them

• Trailing Commas: Placing a comma after the final key/value pair (`{"id": 1,}`) causes immediate parser failures in strict JSON parsers. Always verify list terminations.
• Single Quotes: Using single quotes for keys or values (`{'user': 'alice'}`) violates RFC 8259. Use double quotes consistently.
• Unescaped Control Characters: Literal line breaks, tabs, or unescaped double quotes inside string values break lexical scanning. Escape them (`\n`, `\t`, `\"`).
• Comments: Adding `//` or `/* */` comments inside JSON files fails standard parsing. Remove all comments before sending payloads over the wire.

Best Practices for APIs

• Consistent Naming Conventions: Standardize on `camelCase` or `snake_case` across all API endpoints (e.g., `createdAt` vs `created_at`).
• ISO 8601 Timestamps: Serialize dates in UTC ISO format (`YYYY-MM-DDTHH:mm:ssZ`) rather than ambiguous locale strings or arbitrary epoch integers.
• Enveloping Responses: Wrap lists or root payloads inside a structured object (`{"data": [...]}`) to allow future extensibility without breaking client parsing.

Related Interactive Tools

Related Engineering Guides

Frequently Asked Questions