How to Validate JSON Syntax & Fix Common Errors

JSON validation is the process of verifying that a text payload conforms strictly to the JSON data format specification (RFC 8259 / ECMA-404). Validating checks lexical grammar, token nesting, bracket pairing, and escape sequences. There are two levels of JSON validation: 1. Syntax Validation: Ensures the document is well-formed JSON that can be deserialized by any standard parser. 2. Schema Validation: Ensures the JSON structure, data types, required fields, and ranges match a predetermined JSON Schema contract.

- "Unexpected token ', ... is not valid JSON": You used single quotes instead of double quotes around keys or strings. - "Unexpected token }, ... expected property name": A trailing comma was left after the final key-value pair. - "Unexpected end of JSON input": An opening curly brace { or square bracket [ was never closed. - "Unexpected token / in JSON": You added comments (// or /* */). JSON does not permit comments.

// INVALID (Will fail validation):
{
  'name': "John", // Single quotes & comment
  "age": 30,
}

// VALID (Passes validation):
{
  "name": "John",
  "age": 30
}

Trailing commas are valid in ECMAScript object literals, but strictly forbidden in JSON. When generating JSON manually or copying code from JavaScript files, automated linters and pre-commit hooks can catch these before deployment. BrowserTools JSON Validator and JSON Repair Tool instantly detect trailing commas and highlight the exact line and character position.