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zaggino/z-schema/validating-json-data

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PublishedAugust 22, 2026 at 12:07 PM

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version: "1.0.1" name: validating-json-data description: Validates JSON data against JSON Schema using the z-schema library. Use when the user needs to validate JSON, check data against a schema, handle validation errors, use custom format validators, work with JSON Schema drafts 04 through 2020-12, set up z-schema in a project, compile schemas with cross-references, resolve remote $ref, configure validation options, or inspect error details. Covers sync/async modes, safe error handling, schema pre-compilation, remote references, TypeScript types, and browser/UMD usage.

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

typescript

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)

typescript

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

typescript

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:

typescript

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

Async throw

Required when using async format validators.

typescript

const validator = ZSchema.create({ async: true });
try {
  await validator.validate(data, schema);
} catch (err) {
  console.log(err.details);
}

Async safe

typescript

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:

typescript

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:

typescript

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:

typescript

// 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:

typescript

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

typescript

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

typescript

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

typescript

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)

typescript

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

Instance-scoped

typescript

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

Via options at creation time

typescript

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 }.

typescript

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

typescript

const formats = ZSchema.getRegisteredFormats();

Choosing a draft version

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

typescript

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:

typescript

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

Browser usage (UMD)

html

<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:

typescript

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.

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