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SparkJSONOnline JSON Validator
Online JSON Validator
Paste JSON to check whether it is valid. SparkJSON reports the first syntax failure with line and column, and warns when duplicate keys would be silently overwritten by JSON.parse.
Free SparkJSON tool — runs 100% in your browser. No signup, no upload, no paywall. This page opens in validate mode.
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What an online JSON validator actually checks
A validator asks one question: does this string conform to JSON grammar? It is not the same as JSON Schema or OpenAPI checks. SparkJSON focuses on syntax plus duplicate-key warnings.
The current JSON interchange format is defined by RFC 8259 (and aligned with ECMA-404). SparkJSON’s syntax checks follow that grammar — it does not claim formal certification, only a practical parse against the rules developers expect.
Many “almost JSON” snippets fail because they were copied from JavaScript: single quotes, bare keys, comments, or trailing commas. The validator exists to catch those before your backend does.
- Line + column — errors point at the failing token, not just “Unexpected token”.
- Duplicate keys — warnings when an object redefines the same key.
- Private — validation never leaves your browser.
Common JSON syntax errors (with examples)
Trailing commas
Illegal in JSON, common in JS. Remove the comma before } or ].
{ "a": 1, } → { "a": 1 }Unquoted keys
Object keys must be double-quoted strings.
{ name: "Ada" } → { "name": "Ada" }Single-quoted strings
JSON strings use double quotes only.
{ 'ok': true } → { "ok": true }Comments
JSON has no // or /* */ comments. Strip them or use a JSONC-aware toolchain elsewhere.
{ "a": 1 /* note */ } → invalidFrequently asked questions
Is valid JSON the same as a correct API schema?
No. Syntax validity only means the document parses. Field types, required keys, and enums need JSON Schema or OpenAPI validation separately.
Why does SparkJSON warn about duplicate keys if the file still parses?
JSON.parse keeps the last value and drops earlier ones. The warning exists so you notice accidental overwrites like {"id":1,"id":2}.
Does validation upload my payload?
No. SparkJSON validates in the browser. There is no upload endpoint for editor input.