API & Developer Testing Tools

JSON Validator

Instantly check whether your JSON is valid, right in your browser.

Paste your JSON below to instantly check whether it's valid - entirely in your browser, nothing is sent to our servers.

Paste your JSON

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JustChecker builds tools that respect your data - this one never leaves your browser.

What Is the JSON Validator?

It checks whether text is valid JSON and shows the parser's error message when it is not - most browsers include the line or character position. Validation runs in your browser.

When to Use It

When an API rejects your request body.
When a config file fails to load.

How to Use It

Paste the JSON.
Read the result, and the error message if it is invalid.

Understanding the Result

The most common errors are single quotes instead of double quotes, trailing commas, comments, unquoted keys and missing commas between items.

Limitations

This checks JSON syntax, not whether the data matches a particular schema. Loose JavaScript-style objects can be converted with Code to JSON.

Examples of Invalid JSON

Invalid	Why	Valid
`{'name': 'Asha'}`	Single quotes	`{"name": "Asha"}`
`{"a": 1, "b": 2,}`	Trailing comma	`{"a": 1, "b": 2}`
`{name: "Asha"}`	Unquoted key	`{"name": "Asha"}`
`{"a": 1 "b": 2}`	Missing comma	`{"a": 1, "b": 2}`
`{"n": NaN}`	NaN and undefined are not JSON	`{"n": null}`
`// comment`	Comments are not allowed	Remove the comment

Reading the Error Message

Errors such as "Unexpected token } in JSON at position 42" point to where the parser gave up, which is often just after the real mistake. Look at the character before the position - a stray comma or a missing quote is usually there.

When JSON Looks Right but Fails

Smart quotes (“ ”) copied from a document or chat app instead of straight quotes (").
A byte-order mark or invisible characters at the start of a file.
Unescaped characters inside strings, such as a raw line break or a backslash in a Windows path (C:\new must be C:\\new).

Once it is valid, make it readable with the JSON Formatter. Read more in how to format and validate JSON.

Frequently Asked Questions

No - this tool runs entirely in your browser using JavaScript. Your JSON is never transmitted anywhere.

It catches syntax errors like missing commas, unmatched brackets/braces, trailing commas, and improperly quoted keys or strings - anything that would make the JSON fail to parse.

Valid JSON only means the syntax is correct - it doesn't check whether the data itself matches what your application or API expects (that would require a schema validator).