@jsonlint/core

The JSON engine behind jsonlint.com. Every error in one pass — with locations, hints, and recovery — at 25-40x the speed of the legacy jsonlint package. Zero dependencies. No install scripts.

import { validate, tryParse, parse } from "@jsonlint/core";

const { ok, diagnostics } = validate(text, { mode: "jsonc" });
// diagnostics: [{ code: "W060", message: 'duplicate object key "name"',
//                 severity: "warning", start: 41, end: 47,
//                 related: { start: 12, end: 18 },
//                 hint: "the last occurrence wins; ..." }]

Why

• Every error, one pass. Recovery-based parsing reports all problems, not just the first.
• Errors that teach. 'True' is not valid JSON — this looks like a Python literal. Smart quotes, trailing commas, unquoted keys, concatenated documents: named, located, and hinted.
• Catches what parsers can't say. Duplicate keys (with the first occurrence's location), integer precision loss (1111111111258928239 becomes 1111111111258928300), UTF-16-encoded files, BOMs.
• Safe by default. __proto__ never pollutes; depth limits; duplicate-key policy.
• JSON.parse compatible. parse() matches native output (reviver included) and throws SyntaxError.
• Fast. Pure-JS today (validates faster than JSON.parse allocates); Rust/WASM core lands behind the same API.

API

validate(input, opts) → { ok, diagnostics } — fastest; powers jsonlint.com tryParse(input, opts) → { ok, value, diagnostics } — never throws; value recovered even on errors parse(input, opts) → value or throws — drop-in for JSON.parse lineColumn(src, offset) → { line, column }

Input: string, Uint8Array, or ArrayBuffer (UTF-16/32 bytes detected and reported cleanly). Options: mode: "strict" | "jsonc", duplicateKeys: "warn" | "error" | "allow", protoKeys, maxDepth, reviver.

Conformance

JSONTestSuite 95/95 valid accepted, 188/188 invalid rejected; the complete historical test corpora of zaach/jsonlint and @prantlf/jsonlint; regression tests named for the GitHub issues they resolve (see the engine repo's ISSUE_AUDIT.md).

About

By Todd Garland, who has run jsonlint.com for many years. Engineered in collaboration with Claude Fable 5 (Anthropic). Every claim here is backed by tests running in public CI: github.com/toddynho/jsonlint-core — including the full audit of 411 issues across six predecessor projects (ISSUE_AUDIT.md) and the hardening report (HARDENING.md).

MIT