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JSON Validator — Validate JSON Syntax Online

JSON Validator checks whether your input is valid JSON and reports the precise error location if not. It also shows the total character and byte count for valid JSON.

What Makes JSON Invalid?

The JSON specification (ECMA-404 / RFC 8259) is unambiguous: there is exactly one correct way to write each value. The most common sources of invalid JSON in practice are: trailing commas after the last element of an array or object, single-quoted strings instead of double-quoted strings, unquoted keys, JavaScript comments (// or /* */), undefined values, NaN, Infinity, and unescaped control characters inside strings.

JavaScript developers often write "JSON-like" object literals in their code and are surprised when the same syntax fails JSON.parse(). The difference: JavaScript allows single quotes, trailing commas, and unquoted identifiers as keys. JSON does not. If you are copying an object literal from JS code into a JSON file, you need to apply the stricter rules.

SmartDevBox's validator uses the native JSON.parse() function, which provides a descriptive error message that includes the position of the first syntax error. Position means character offset from the start of the string; the validator converts this to a line and column number to make it easy to find the offending character in a large document.

Validating JSON in CI/CD Pipelines

Beyond the browser, JSON validation belongs in automated workflows. Configuration files (package.json, tsconfig.json, .eslintrc.json), API contract fixtures, and seed data files should all be validated before deployment. A malformed config file that reaches production can cause silent failures or full service outages.

For command-line validation, the built-in Python tool works well: python3 -m json.tool input.json > /dev/null exits with code 0 for valid JSON and prints the error with line number for invalid JSON. Node.js users can run node -e "require('./file.json')" for a quick sanity check. Both approaches can be wired into a pre-commit hook or CI step.

Common Use Cases

  • Verifying that a config file is valid JSON before use
  • Debugging malformed API responses
  • Checking JSON in CI/CD pipeline validation steps

Frequently Asked Questions

How does the JSON validator report errors?

If the JSON is invalid, the validator shows the error message from the JavaScript JSON.parse() engine, which includes the position information.

Privacy & Security

This tool runs entirely in your browser using client-side JavaScript. No data is sent to a server — your input never leaves your machine. SmartDevBox has no account system, no usage tracking, and no paid tier. See the Privacy & Security page for full details.

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