validating-json-data

Validating JSON Data with z-schema

z-schema validates JSON data against JSON Schema (draft-04, draft-06, draft-07, draft-2019-09, draft-2020-12). Default draft: draft-2020-12.

Quick start

import ZSchema from 'z-schema';

const validator = ZSchema.create();

const schema = {
  type: 'object',
  properties: {
    name: { type: 'string' },
    age: { type: 'integer', minimum: 0 },
  },
  required: ['name'],
};

// Throws on invalid data
validator.validate({ name: 'Alice', age: 30 }, schema);

Install: npm install z-schema

Choosing a validation mode

z-schema has four modes based on two toggles: async and safe. Pick the one that fits the use case.

Mode	Factory call	Returns	Use when
Sync throw	ZSchema.create()	true or throws ValidateError	Default — simple scripts and middleware
Sync safe	ZSchema.create({ safe: true })	{ valid, err? }	Need to inspect errors without try/catch
Async throw	ZSchema.create({ async: true })	Promise<true> or rejects	Using async format validators
Async safe	ZSchema.create({ async: true, safe: true })	Promise<{ valid, err? }>	Async + non-throwing

Sync throw (default)

import ZSchema from 'z-schema';

const validator = ZSchema.create();

try {
  validator.validate(data, schema);
} catch (err) {
  // err is ValidateError
  console.log(err.details); // SchemaErrorDetail[]
}

Sync safe

const validator = ZSchema.create({ safe: true });

const result = validator.validate(data, schema);
if (!result.valid) {
  console.log(result.err?.details);
}

Or call .validateSafe() on a regular (throwing) validator for the same result shape:

const validator = ZSchema.create();
const { valid, err } = validator.validateSafe(data, schema);

Async throw

Required when using async format validators.

const validator = ZSchema.create({ async: true });

try {
  await validator.validate(data, schema);
} catch (err) {
  console.log(err.details);
}

Async safe

const validator = ZSchema.create({ async: true, safe: true });

const { valid, err } = await validator.validate(data, schema);

Inspecting errors

ValidateError has .details — an array of SchemaErrorDetail:

interface SchemaErrorDetail {
  message: string; // "Expected type string but found type number"
  code: string; // "INVALID_TYPE"
  params: (string | number | Array<string | number>)[];
  path: string | Array<string | number>; // "#/age" or ["age"]
  keyword?: string; // "type", "required", "minLength", etc.
  inner?: SchemaErrorDetail[]; // sub-errors from anyOf/oneOf/not
  schemaPath?: Array<string | number>;
  schemaId?: string;
}

Example: walking nested errors

Combinators (anyOf, oneOf, not) produce nested inner errors:

const { valid, err } = validator.validateSafe(data, schema);
if (!valid && err) {
  for (const detail of err.details) {
    console.log(`${detail.path}: [${detail.code}] ${detail.message}`);
    if (detail.inner) {
      for (const sub of detail.inner) {
        console.log(`  └─ ${sub.path}: [${sub.code}] ${sub.message}`);
      }
    }
  }
}

Filtering errors

Pass ValidateOptions as the third argument to include or exclude specific error codes:

// Only report type errors
validator.validate(data, schema, { includeErrors: ['INVALID_TYPE'] });

// Suppress string-length errors
validator.validate(data, schema, { excludeErrors: ['MIN_LENGTH', 'MAX_LENGTH'] });

For the full error code list, see references/error-codes.md.

Schema pre-compilation

Compile schemas at startup for better runtime performance and to resolve cross-references:

const validator = ZSchema.create();

const schemas = [
  {
    id: 'address',
    type: 'object',
    properties: { city: { type: 'string' }, zip: { type: 'string' } },
    required: ['city'],
  },
  {
    id: 'person',
    type: 'object',
    properties: {
      name: { type: 'string' },
      home: { $ref: 'address' },
    },
    required: ['name'],
  },
];

// Compile all schemas (validates them and registers references)
validator.validateSchema(schemas);

// Validate data using a compiled schema ID
validator.validate({ name: 'Alice', home: { city: 'Paris' } }, 'person');

Remote references

Manual registration

ZSchema.setRemoteReference('http://example.com/schemas/address.json', addressSchema);
// or per-instance:
validator.setRemoteReference('http://example.com/schemas/person.json', personSchema);

Automatic loading via schema reader

import fs from 'node:fs';
import path from 'node:path';

ZSchema.setSchemaReader((uri) => {
  const filePath = path.resolve(__dirname, 'schemas', uri + '.json');
  return JSON.parse(fs.readFileSync(filePath, 'utf8'));
});

Diagnosing missing references

const { valid, err } = validator.validateSafe(data, schema);
if (!valid && err) {
  const missing = validator.getMissingReferences(err);
  const remote = validator.getMissingRemoteReferences(err);
}

Custom format validators

Global (shared across all validator instances)

ZSchema.registerFormat('postal-code', (value) => {
  return typeof value === 'string' && /^\d{5}(-\d{4})?$/.test(value);
});

Instance-scoped

const validator = ZSchema.create();
validator.registerFormat('postal-code', (value) => {
  return typeof value === 'string' && /^\d{5}(-\d{4})?$/.test(value);
});

Via options at creation time

const validator = ZSchema.create({
  customFormats: {
    'postal-code': (value) => typeof value === 'string' && /^\d{5}(-\d{4})?$/.test(value),
  },
});

Async format validators

Return Promise<boolean>. Requires { async: true }.

const validator = ZSchema.create({ async: true });
validator.registerFormat('user-exists', async (value) => {
  if (typeof value !== 'number') return false;
  const user = await db.findUser(value);
  return user != null;
});

Listing registered formats

const formats = ZSchema.getRegisteredFormats();

Choosing a draft version

Set the draft explicitly if the schema targets a specific version:

const validator = ZSchema.create({ version: 'draft-07' });

Valid values: 'draft-04', 'draft-06', 'draft-07', 'draft2019-09', 'draft2020-12' (default), 'none'.

For a feature comparison across drafts, see references/draft-comparison.md.

Common options

Option	Default	Purpose
breakOnFirstError	false	Stop validation at the first error
noEmptyStrings	false	Reject empty strings as type string
noEmptyArrays	false	Reject empty arrays as type array
strictMode	false	Enable all strict checks at once
ignoreUnknownFormats	false	Suppress unknown format errors (modern drafts always ignore)
formatAssertions	null	null=always assert, true=respect vocabulary, false=annotation-only
reportPathAsArray	false	Report error paths as arrays instead of JSON Pointer strings

For the full options reference, see references/options.md.

Validating sub-schemas

Target a specific path within a schema:

validator.validate(carData, fullSchema, { schemaPath: 'definitions.car' });

Browser usage (UMD)

<script src="node_modules/z-schema/umd/ZSchema.min.js"></script>
<script>
  var validator = ZSchema.create();
  try {
    validator.validate({ name: 'test' }, { type: 'object' });
  } catch (err) {
    console.log(err.details);
  }
</script>

TypeScript types

All types are exported from the package:

import type {
  JsonSchema, // Schema type (all drafts union)
  ZSchemaOptions, // Configuration options
  ValidateOptions, // Per-call options (schemaPath, includeErrors, excludeErrors)
  ValidateResponse, // { valid: boolean, err?: ValidateError }
  SchemaErrorDetail, // Individual error detail
  ErrorCode, // Error code string literal type
  FormatValidatorFn, // (input: unknown) => boolean | Promise<boolean>
  SchemaReader, // (uri: string) => JsonSchema
} from 'z-schema';

import { ValidateError } from 'z-schema';

Reference files

• references/error-codes.md — Full error code list with descriptions and examples
• references/options.md — Complete options reference with defaults
• references/draft-comparison.md — Feature comparison across JSON Schema drafts

Important conventions

• Always use ZSchema.create(options?) — never new ZSchema(). The factory returns the correctly typed variant.
• Error details are on .details (not .errors).
• Import types with import type { ... } and values with import { ValidateError }.
• Default draft is draft2020-12. Specify explicitly if targeting an older draft.