is-json

is-json

Check whether a value is a valid JSON object/array (as a string, or a plain object) — without writing a try/catch on the caller side.

Install

npm install is-json
# or
pnpm add is-json
# or
yarn add is-json

Size

Zero runtime dependencies. What ships in the npm tarball:

What	Raw	Gzipped
ESM runtime (index.mjs)	996 B	407 B
CJS runtime (index.cjs)	1 006 B	418 B
Types (.d.mts / .d.cts)	2 398 B	678 B
Sourcemaps (debug-only, not loaded)	6 598 B	1 360 B

Only one of the two runtime files is loaded by your bundler / Node, so the real cost in your app is a single sub-KB module.

Usage

import isJSON from "is-json";              // ESM
// const isJSON = require("is-json");      // CJS — same function

isJSON('{"a":1,"b":[1,2,3]}');             // true
isJSON('[{"a":1}, {"b":2}]');              // true
isJSON('{"a":"contains } brace"}');        // true (v2 used to fail here)

isJSON('not json');                        // false
isJSON('{a:1}');                           // false (invalid JSON)
isJSON("");                                // false
isJSON(null);                              // false
isJSON(42);                                // false (default rejects non-strings)

passObjects — accept plain objects directly

isJSON({ a: 12, b: [1, 2, 3] });           // false (string required)
isJSON({ a: 12, b: [1, 2, 3] }, true);     // true  (plain object accepted)

isJSON([1, 2, 3], true);                   // false (arrays are not plain objects)
isJSON(new Date(), true);                  // false (Date is not a plain object)
isJSON(new Map(), true);                   // false

Only objects whose prototype is Object.prototype (or null) qualify. Arrays, class instances, Date, Map, etc. are rejected.

isJSON.strict — accept any valid JSON token

The default isJSON only accepts object/array shapes (a deliberate v3 choice — most callers want "is this a JSON payload"). For full RFC 8259 validity (scalars at the top level), use strict:

isJSON("42");           // false  — scalar tokens rejected by default
isJSON.strict("42");    // true   — any valid JSON value
isJSON.strict("true");  // true
isJSON.strict("null");  // true
isJSON.strict('"hi"');  // true

isJSON.strict({ a: 1 }); // true  — plain objects also accepted
isJSON.strict(123);      // false — non-string, non-plain-object inputs rejected

API

interface IsJSON {
  (value: unknown, passObjects?: boolean): boolean;
  strict: (value: unknown) => boolean;
}

declare const isJSON: IsJSON;
export default isJSON;

Migrating from v2

isJSON(str, passObjects?) and isJSON.strict(str) keep the same shape and the same require('is-json') / default-import ergonomics. Behaviour changes that may affect you:

Scenario	v2	v3
isJSON('[1,2,3]')	false (regex bug)	true
isJSON('{"a":"}"}')	false (regex bug)	true
isJSON('{"msg":"hello world"}')	brittle	true
isJSON.strict({a:1})	true	true (preserved)
isJSON.strict('null')	null (falsy)	true
isJSON.strict(123)	true (parse coercion)	false
isJSON({...}, true) accepting class instances	true	false

The regex-based default validator from v2 has been replaced with a JSON.parse-based check, eliminating false positives and false negatives around braces in strings, whitespace-in-values, and nested arrays.

License

ISC @joaquimserafim