JSON Lint Online - Validate and Check JSON Syntax | SmartJson
Lint and validate your JSON data online. Check for syntax errors, formatting issues, and structural problems with real-time feedback. 100% client-side processing.
Key Features
- ✅ Real-time syntax validation
- ✅ Error highlighting
- ✅ Line number reporting
- ✅ Formatting suggestions
- ✅ Client-side only
How to Use
- Accept JSON input via textarea or file upload
- Parse JSON with detailed error reporting
- Highlight syntax errors with line/column numbers
- Flag pattern-level issues: duplicate keys, precision-losing large numbers
- Provide suggestions for common fixes
- Show validation status (valid/invalid) plus any warnings
- All logic is client-side; no data leaves the browser
Expert FAQ
- "Lint" usually implies style rules beyond pure syntax — what does this actually flag beyond what JSON.parse() would catch?
Beyond hard syntax errors (which the JSON Validator also catches), this tool additionally flags patterns that parse successfully but are likely mistakes: duplicate keys within the same object (different parsers resolve these inconsistently — first-wins vs last-wins — making them a common source of "works differently in different tools" bugs), and numeric literals large enough to silently lose precision when decoded (anything past 2^53, common with 64-bit database IDs serialized as bare numbers instead of strings). - How is this different from the JSON Validator, then?
The Validator is a strict grammar check: does this parse per RFC 8259, yes or no, with a location if not. This linter runs that same check and then layers on additional pattern-based warnings for things that are valid JSON but are still worth a second look — it's the difference between "does this compile" and "does this compile without a linter warning." - Can I fix errors directly in the tool?
Yes — edit inline and the analysis re-runs after a short pause, so you can iterate directly against the same error/warning list rather than fixing blind and re-pasting into a separate validator. - Does it check my JSON against a specific schema (required fields, types)?
No — this is syntax and pattern-level linting only, not schema validation. It answers "is this well-formed and free of common gotchas," not "does this match my API's expected shape." For contract-level checking against required fields and types, you need JSON Schema validation specific to your API, which is a separate concern from what a general-purpose linter can check without knowing your schema.
Technical Details
Linting is a broader check than pure syntax validation: where a strict validator answers only "does this parse per the JSON grammar," a linter additionally flags patterns that are syntactically legal but are common, real sources of bugs. This tool checks both layers. The syntax layer catches the same hard failures the JSON Validator does — mismatched braces, trailing commas, unquoted keys, unescaped control characters — with exact line/column location. The pattern-level layer catches issues that parse without error but are worth flagging anyway: duplicate keys within a single object (the JSON grammar is silent on duplicates, but real parsers resolve them inconsistently — some keep the first occurrence, most, including JavaScript, keep the last — which makes duplicate keys a frequent source of behavior that differs between tools even though every tool agrees the document is "valid"), and numeric literals beyond 2^53, the point past which JSON's float-based number representation can no longer exactly represent every integer — a common issue when 64-bit database IDs or Snowflake-style IDs get serialized as bare JSON numbers instead of strings. What this linter does not do is schema validation — checking that required fields are present, that a field's type matches what a specific API contract expects, or that an enum value is one of the allowed options. That requires JSON Schema validation against your specific schema, which is a fundamentally different, contract-aware check that a general-purpose linter can't perform without being told what your schema actually is. Once your JSON lints clean, the JSON Formatter is the natural next step for presentation, and JSON Compare for diffing against a known-good reference.