What is JSON Validator?
JSON Validator checks whether a text string conforms to the JSON specification defined in RFC 8259. Valid JSON must use double-quoted keys and strings, forbid trailing commas, and structure data exclusively with objects, arrays, strings, numbers, booleans, and null.
Developers use validators at integration boundaries — before posting to a REST endpoint, after receiving a webhook, or when a CI pipeline rejects a config file. A quick validation pass saves hours of debugging cryptic server errors.
Validation vs Formatting
Both operations call JSON.parse() under the hood, but they serve different workflows:
- Validator — Reports valid or invalid with the parser's error message. Output is not rewritten.
- Formatter — Parses and re-serializes with indentation or minification. Invalid input produces the same error but also blocks formatted output.
If you already know the JSON is broken and want readable output after fixing it, use JSON Formatter. If you need a fast gate before a deploy script runs, use JSON Validator.
Common JSON Syntax Rules
- Keys must be double-quoted strings:
{"name": "value"}not{name: "value"}. - String values use double quotes only:
"hello"not'hello'. - No trailing commas:
[1, 2, 3]not[1, 2, 3,]. - Numbers are decimal; leading zeros on integers are invalid except
0. - Root can be an object
{}or array[], not a bare string or number.
Worked Examples: Valid vs Invalid JSON
Example 1 — Invalid JavaScript object literal pasted as JSON
{
userId: 42,
roles: ['admin', 'billing',], // active roles
}
- Why it fails: Unquoted keys (
userId,roles), single-quoted strings ('admin'), a trailing comma after'billing', and a//comment.
Corrected valid JSON:
{
"userId": 42,
"roles": ["admin", "billing"]
}
Example 2 — Unescaped quotes inside a string value
- Invalid:
{"message": "Click "Submit" to continue"} - Valid:
{"message": "Click \"Submit\" to continue"}
Debugging Invalid API Responses
When an API returns unexpected data, paste the raw response body into the validator. Parser errors often point to the first offending character — a common culprit is an HTML 502 Bad Gateway page starting with <!DOCTYPE html> returned instead of JSON (Unexpected token '<').
Once your payload validates, pretty-print or minify it with the JSON Formatter, or decode encoded fields first using Base64 Decode.