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What Is JSON Validation
JSON validation verifies that a string conforms to the JSON (JavaScript Object Notation) specification defined in RFC 8259. A valid JSON document must follow strict syntax rules: objects use curly braces with double-quoted keys, arrays use square brackets, strings must be double-quoted, and values must be one of six types (object, array, string, number, boolean, null). Even a single syntax error—a missing comma, unquoted key, or trailing comma—makes the entire document invalid.
JSON parsing errors are among the most common issues in web development, API integration, and configuration management. An API that returns malformed JSON breaks every client consuming it. A misconfigured JSON file prevents an application from starting. JSON validation catches these issues before they cause runtime failures, providing specific error messages that point to the exact location and nature of the problem.
How JSON Validation Works
A JSON validator parses the input string according to the JSON grammar, checking for:
Structural rules:
- Objects:
{ "key": value }— keys must be double-quoted strings, separated by colons, pairs separated by commas - Arrays:
[value, value]— values separated by commas - No trailing commas after the last element
- No single quotes (only double quotes)
- No comments (// or /* */ are invalid in JSON)
- No undefined, NaN, or Infinity values
Common validation errors:
| Error | Invalid JSON | Correct JSON |
|---|---|---|
| Single quotes | {'name': 'John'} | {"name": "John"} |
| Trailing comma | {"a": 1, "b": 2,} | {"a": 1, "b": 2} |
| Unquoted key | {name: "John"} | {"name": "John"} |
| Single value | undefined | null |
| Comments | {"a": 1} // comment | {"a": 1} |
| No quotes on string | {"name": John} | {"name": "John"} |
Schema validation goes beyond syntax to verify that JSON data conforms to an expected structure. JSON Schema (draft-07 or later) defines required fields, data types, value ranges, and patterns. For example, a schema can require that an "age" field is a positive integer and an "email" field matches an email pattern.
Common Use Cases
- API development: Validate request and response payloads before processing to catch malformed data early
- Configuration management: Verify JSON config files (package.json, tsconfig.json) before deployment
- Data pipeline validation: Check JSON data quality at ingestion points in ETL pipelines
- Testing: Validate API responses in automated test suites against expected schemas
- Debugging: Identify the exact location and type of syntax errors in large JSON documents
Best Practices
- Validate early in the pipeline — Check JSON validity at API entry points, file upload handlers, and data ingestion stages
- Use JSON Schema for structural validation — Syntax validation alone doesn't catch wrong data types or missing required fields
- Provide meaningful error messages — Include line number, column, and expected vs. actual token in error output
- Handle encoding correctly — JSON must be UTF-8 encoded (RFC 8259); reject documents with BOM markers or other encodings
- Test with edge cases — Validate handling of deeply nested objects, very large numbers, Unicode escape sequences, and empty documents
References & Citations
- Ecma International. (2017). JSON Data Interchange Syntax (ECMA-404). Retrieved from https://www.ecma-international.org/publications-and-standards/standards/ecma-404/ (accessed January 2025)
- JSON Schema. (2024). JSON Schema Specification. Retrieved from https://json-schema.org/specification (accessed January 2025)
Note: These citations are provided for informational and educational purposes. Always verify information with the original sources and consult with qualified professionals for specific advice related to your situation.
Frequently Asked Questions
Common questions about the JSON Validator & Formatter
JSON syntax rules: (1) Data in name/value pairs: {"name":"value"}. (2) Strings use double quotes: "text" not 'text'. (3) Numbers: no quotes: 42, 3.14. (4) Booleans: true, false (lowercase). (5) Null: null (lowercase). (6) Arrays: [1, 2, 3]. (7) Objects: {"key":"value"}. (8) No trailing commas: [1,2,] invalid. (9) Keys must be strings: {name:"John"} invalid. Common errors: single quotes instead of double, trailing commas, unquoted keys, comments (not allowed in JSON), control characters unescaped. This tool identifies exact error location and suggests fixes.