cereale/README.md

Cereale

Validated domain objects, not validated data.

Cereale maps JSON onto your own classes and gives you back real instances — with your methods, your inheritance, your instanceof checks — and type-checks the validation rules against the fields they are attached to. Zero runtime dependencies.

class User {
  @JsonProperty('display_name')
  @IsString() @MinLength(2)
  displayName!: string;

  @IsInt() @Min(0)
  age!: number;

  @IsString()
  age2!: number;   // ← compile error: Type 'number' is not assignable to type 'string'

  greet() { return `Hi ${this.displayName}`; }
}

const user = fromJsonSync(User, body);   // a real User
user.greet();                            // your methods are still there

docs/index.html is a self-contained page with an interactive playground that runs this library in the browser. Build its assets with npm run build:docs and open the file — it loads nothing from the network.

Where it fits

The stack Cereale replaces is class-validator + class-transformer:

	class-validator + class-transformer	Cereale
Packages to install	2, plus reflect-metadata	1, no runtime dependencies
Decorators	legacy (experimentalDecorators)	TC39 standard
Rules checked against the field	no — @IsInt() name: string compiles	yes, at compile time
Mapping and validation	two libraries that must agree	one model

The comparison people ask about is Zod, and it is worth being precise about, because Cereale is not a drop-in for it:

	Zod	Cereale
Result of parsing	an anonymous object matching a schema	an instance of your class
Methods, getters, inheritance	none — data only	preserved
Where the type comes from	inferred from the schema	your class declaration
Bidirectional mapping (renaming both ways)	not the focus	first-class

Cereale does not infer your type from a schema. You write the field type and the rule, and what it guarantees is that the two cannot disagree — @IsInt() name!: string does not compile. If you want z.infer, you want Zod; that is a different design, not a missing feature.

Reach for Cereale when your domain model is already a class — a NestJS provider, a TypeORM entity, anything with behaviour attached. Reach for Zod when you just want the data.

Features

• Strongly typed decorators: a rule that does not fit its field is a compile error.
• Real instances: nested objects, polymorphic subtypes and arrays all come back as classes.
• Field-name mapping: @JsonProperty, @JsonAlias and naming strategies, both directions.
• Access control: keep passwords out of responses and server-owned ids out of requests.
• Nothing fails quietly: a misconfigured compiler, a cycle, or a value JSON cannot carry raises an error that names the cause — never an empty object.
• Sync and async: every entry point has a synchronous twin.
• Zero dependencies, ESM + CJS, Node 20+.

Installation

npm install cereale

Cereale uses TC39 standard decorators (since 0.2.0), so no experimentalDecorators flag:

{
  "compilerOptions": {
    "target": "ES2022",
    "lib": ["ESNext", "ESNext.Decorators"]
  }
}

Requires TypeScript 5.2+ and Node 20+. reflect-metadata is not needed and emitDecoratorMetadata is not read.

experimentalDecorators must be off. The two decorator systems cannot coexist in one program, so a project that still needs legacy decorators for another library cannot use Cereale yet. If yours is configured for them, you get an error saying exactly that rather than a TypeError from somewhere inside the engine.

Toolchain support

Whether Cereale works at all depends on your compiler emitting standard decorators, so the three ✅ rows are checked by a test rather than asserted here — each compiles a decorated class with that tool and asserts the metadata arrived. The ❌ row cannot be: oxc ships inside a native binary with no standalone transform API.

Transformer	Status	Notes
tsc	✅	With experimentalDecorators: false and target: ES2022+
esbuild	✅	experimentalDecorators: false via tsconfigRaw, plus esbuild's own top-level target: es2022. Its default esnext target leaves decorator syntax in the output
swc	✅	jsc.transform.decoratorVersion: "2022-03"
oxc	❌	Used by Vite 8 and Vitest 4 — see below

If you are on Vite 8 or Vitest 4, oxc leaves decorator syntax in the output without reporting anything: vitest prints 0 test next to a bare SyntaxError, and vite build reports success while emitting a bundle that throws the moment it is imported. Cereale ships the plugin that fixes it:

// vite.config.ts / vitest.config.ts
import { defineConfig } from 'vite';
import { standardDecorators } from 'cereale/vite';

export default defineConfig({
  plugins: [standardDecorators()],
});

It transforms .ts, .mts and .cts outside node_modules with esbuild, falling back to the TypeScript compiler if esbuild is not installed — Cereale depends on neither. Pass include to widen or narrow the set (decorated classes in .tsx files need this), transformer: 'esbuild' | 'typescript' to pin one, or target to change the output level from the default es2022. Nothing in the plugin is specific to Cereale; delete it once oxc implements the transform.

Quick Start

1. Define your model

import {
  IsString, IsDate, IsInt, Min, ValidateNested, JsonType,
  JsonProperty, JsonWriteOnly, JsonPolymorphic,
} from 'cereale';

class Address {
  @IsString() street!: string;
  @IsString() city!: string;

  format() { return `${this.street}, ${this.city}`; }
}

abstract class Media {
  // Standard decorators cannot decorate an `abstract` member, so declare it concretely.
  @IsString() type: string = '';
  @IsString() title: string = '';
}

class Book extends Media {
  @IsString() override type = 'book';
  @IsString() author!: string;

  @JsonProperty('published_at')
  @IsDate()
  publishedAt!: Date;
}

class Library {
  @IsString() name!: string;

  @ValidateNested() @JsonType(() => Address)
  address!: Address;              // the class must match the field

  @ValidateNested({ each: true })
  @JsonPolymorphic<Media>('type', [{ value: Book, name: 'book' }])
  items!: Media[];
}

Constraints accumulate down an inheritance chain: Book is checked against Media's rules as well as its own, and re-stating a rule on an override does not report it twice.

2. Map JSON, synchronously or not

import { fromJsonSync, toJsonSync, JsonValidationError, flattenErrors } from 'cereale';

try {
  const library = fromJsonSync(Library, json);
  library.address.format();                   // your method, on a real Address
  library.items[0] instanceof Book;           // true
  console.log(toJsonSync(library));
} catch (error) {
  if (error instanceof JsonValidationError) {
    console.error(flattenErrors(error.errors));
    // { "items[0].title": ["title must be a string"] }
  }
}

Every function has an async form too (fromJson, toJson, …) for when a serializer, deserializer or validator of yours returns a Promise.

3. Modern Web Frameworks (Request Integration)

Cereale is compatible with Fetch-based frameworks like Hono, Next.js, and Remix. Use the fromRequest async helper.

import { fromRequest, toPlain } from 'cereale';

// Hono Example
app.post('/books', async (c) => {
  const book = await fromRequest(Book, c.req.raw);
  return c.json(await toPlain(book));
});

Field-name Mapping

JSON rarely uses the same names as your classes.

import { JsonProperty, JsonAlias } from 'cereale';

class User {
  @JsonProperty('first_name')
  firstName: string;        // <-> {"first_name": "Ada"}

  @JsonProperty('surname')
  @JsonAlias('last_name')   // also accepted on input, never emitted
  lastName: string;
}

Or convert every property at once with a naming strategy:

import { configure, toPlain } from 'cereale';

// once, for the whole application
configure({ namingStrategy: 'snake_case' });

// or per call
await toPlain(user, { namingStrategy: 'snake_case' });

Built-in strategies: identity (default), camelCase, PascalCase, snake_case, SCREAMING_SNAKE_CASE, kebab-case. You can also pass your own (propertyKey: string) => string. An explicit @JsonProperty always wins.

Acronyms split where a reader expects them to: parseHTTPResponse becomes parse_http_response, not parse_h_t_t_p_response.

Access Control

import { JsonIgnore, JsonReadOnly, JsonWriteOnly } from 'cereale';

class Account {
  @JsonReadOnly()      // sent to clients, never settable by them
  id: number;

  @IsString()
  email: string;

  @JsonWriteOnly()     // accepted from clients, never echoed back
  @IsString()
  password: string;

  @JsonIgnore()        // never crosses the boundary in either direction
  internalNotes: string;
}

Options

Every mapping function takes an optional trailing options argument, and configure() sets defaults for the whole application. Per-call options win.

Option	Values	Default	Meaning
validate	boolean	true	Validate the result; throw JsonValidationError on failure.
namingStrategy	strategy name or function	identity	JSON naming convention for properties without @JsonProperty.
unknownKeys	allow | strip | error	allow	What to do with incoming keys matching no declared property.
maxDepth	number	64	Nesting depth before a JsonMappingError is raised, bounding hostile payloads.

// lenient parse: build the instance, inspect the damage yourself
const draft = await fromJson(Order, body, { validate: false });
const problems = flattenErrors(await validate(draft));

// strict intake: reject anything you did not declare
const order = await fromJson(Order, body, { unknownKeys: 'error' });

Synchronous API

Nothing on the default path is genuinely asynchronous — only a serializer, deserializer or validator you supply can be — so every mapping function has a synchronous twin.

import { fromJsonSync, toJsonSync, validateSync } from 'cereale';

const user = fromJsonSync(User, body);      // no await
const errors = validateSync(user);
const payload = toJsonSync(user);

validateSync, validateOrRejectSync, toPlainSync, toJsonSync, toInstanceSync, toInstanceArraySync, fromJsonSync, fromJsonArraySync.

If one of your hooks does return a Promise, the synchronous call raises a JsonMappingError naming the async function to use instead, rather than handing back a half-built object. fromRequest has no synchronous form, since reading a request body is inherently async.

API Reference

Mapping Decorators

• @JsonProperty(name: string): Renames the property in JSON, both directions.
• @JsonAlias(...names: string[]): Extra names accepted on input only.
• @JsonIgnore(): Excludes the property from mapping entirely.
• @JsonReadOnly(): Serialized, but never populated from incoming JSON.
• @JsonWriteOnly(): Populated from incoming JSON, but never serialized.
• @JsonSerialize(serializer: ClassConstructor<JsonSerializer>): Custom serializer for a property. Skipped when the value is null/undefined.
• @JsonDeserialize(deserializer: ClassConstructor<JsonDeserializer>): Custom deserializer for a property.
• @JsonType(typeFunction: () => ClassConstructor<any>): Explicitly sets the type for nested transformations. Applies element-wise to arrays.
• @JsonPolymorphic(discriminator, subTypes, options?): Polymorphic transformation based on a discriminator field. options accepts { onUnknown: 'keep' | 'error' } (default keep, which preserves the raw value) and { fallback: ClassConstructor }.

Validation Decorators

Most validation decorators accept an optional ValidationOptions object:

• each: boolean: Apply validation to each element of an array.
• message: string | ((args: ValidationArguments) => string): Custom error message, reported verbatim.

Decorator	Description
@IsString()	Checks if value is a string.
@IsNumber()	Checks if value is a number (and not NaN).
@IsInt()	Checks if value is an integer.
@IsBoolean()	Checks if value is a boolean.
@IsBigInt()	Checks if value is a bigint.
@IsObject()	Checks if value is an object (not null/array).
@IsDate()	Checks if value is a valid Date object.
@IsDefined()	Checks if value is not null or undefined.
@IsOptional()	Skips other validations if value is null/undefined.
@IsNotEmpty()	Checks if value is not null/undefined/empty string.
@IsEmpty()	Checks if value is null/undefined/''/[]/{}.
@Equals(value) / @NotEquals(value)	Strict equality against a fixed value.
@IsEnum(enumObject)	Checks membership of a TypeScript enum.
@IsInstance(Class)	Checks value instanceof Class.
@Min(value) / @Max(value)	Numeric bounds.
@Positive() / @Negative()	Checks sign.
@IsDivisibleBy(n)	Checks value % n === 0.
@IsPort()	Integer in 0–65535, as number or numeric string.
@IsLatitude() / @IsLongitude()	Geographic bounds.
@MinLength(len) / @MaxLength(len)	String length bounds.
@Length(min, max?)	Both bounds in one rule.
@IsAlpha() / @IsAlphanumeric()	Character-class checks.
@IsLowercase() / @IsUppercase()	Case checks.
@IsNumberString()	String that parses as a finite number.
@Contains(s) / @NotContains(s)	Substring checks.
@StartsWith(s) / @EndsWith(s)	Affix checks.
@Email()	Checks if string is a valid email.
@IsUrl()	Checks if string is a valid URL.
@IsUUID(version?)	Checks if string is a valid UUID.
@IsIP(version?)	Checks if string is a valid IPv4/IPv6 address.
@IsJSON()	Checks if string parses as JSON.
@IsDateString()	Checks if string is a parseable date.
@IsSemVer()	Checks if string is a semantic version.
@IsHexColor()	Checks #rgb, #rrggbb, #rrggbbaa.
@Matches(regex)	Checks if string matches a regular expression.
@MinDate(d) / @MaxDate(d)	Date bounds. Accepts () => Date for a moving bound.
@IsArray()	Checks if value is an array.
@ArrayNotEmpty()	Checks if array is not empty.
@ArrayMinSize(n) / @ArrayMaxSize(n)	Array size bounds.
@ArrayUnique(keyFn?)	Checks for duplicate elements.
@ArrayContains(vals) / @ArrayNotContains(vals)	Membership checks.
@IsIn(values) / @IsNotIn(values)	Allow/deny lists.
@ValidateNested(options?)	Recursively validates nested objects/arrays.
@ValidateIf(o => boolean)	Skips this property's rules when the condition is false.
@Allow()	Declares a property with no rules of its own.
@Validate(validator, constraints?, options?)	Applies a custom validator class or function.

Write your own with registerDecorator({ name, target, propertyName, validator }).

Utilities

• toJson(obj, options?): Validates and serializes an instance to a JSON string (Promise<string>).
• toPlain(obj, options?): Validates and transforms an instance to a plain object (Promise<any>).
• fromJson(clazz, json, options?): Parses JSON to a validated class instance (Promise<T>).
• fromJsonArray(clazz, json, options?): Same, for a JSON array (Promise<T[]>).
• toInstance(clazz, plain, options?): Transforms a plain object to a validated class instance (Promise<T>).
• toInstanceArray(clazz, plain, options?): Same, for an array (Promise<T[]>).
• fromRequest(clazz, request, options?): Extracts JSON from a Fetch Request (Promise<T>).
• validate(obj, options?): Full validation, returning Promise<ValidationError[]>.
• validateOrReject(obj, options?): As above, but throws JsonValidationError.
• Synchronous twins of all of the above except fromRequest: toPlainSync, toJsonSync, fromJsonSync, fromJsonArraySync, toInstanceSync, toInstanceArraySync, validateSync, validateOrRejectSync.
• configure(options) / getConfig() / resetConfig(): Library-wide defaults.

Error Handling

JsonValidationError carries a nested ValidationError[]. Three helpers turn it into something you can return to a client:

import { flattenErrors, formatErrors, collectErrorMessages } from 'cereale';

flattenErrors(errors);        // { "items[0].qty": ["qty must be at least 1"] }
formatErrors(errors);         // "items[0].qty: qty must be at least 1"
collectErrorMessages(errors); // ["qty must be at least 1"]

Values of properties that never leave the process — @JsonWriteOnly and @JsonIgnore — are replaced with REDACTED in ValidationError.value, so a rejected password does not travel into your logs inside an error object. The property name and message are unaffected.

JsonMappingError is raised when a value cannot be mapped at all — a body that is not JSON, a circular reference, an unknown discriminator under { onUnknown: 'error' } — as distinct from mapping fine and failing validation.

Framework Integrations

Hono / Next.js / Cloudflare Workers

Use fromRequest for seamless integration with the Fetch Request API.

NestJS

Use Cereale inside your controllers for explicit mapping and validation without needing reflect-metadata.

import { toInstance } from 'cereale';

@Post()
async create(@Body() body: any) {
  const user = await toInstance(User, body);
  return this.userService.create(user);
}

Express / Fastify

import { toInstance, toPlain, JsonValidationError, flattenErrors } from 'cereale';

app.post('/user', async (req, res) => {
  try {
    const user = await toInstance(User, req.body);
    res.json(await toPlain(user));
  } catch (err) {
    if (err instanceof JsonValidationError) {
      return res.status(400).json({ errors: flattenErrors(err.errors) });
    }
    throw err;
  }
});

Performance

Decorator metadata is fixed once your classes are declared, so cereale resolves each class's validation, serialization and deserialization plans once and memoizes them per prototype. A version counter invalidates the caches if metadata is registered late, so registerDecorator after first use still behaves correctly. The engines are synchronous internally, so the async entry points do not pay for a microtask per property.

Indicative throughput for a customer record with a nested address and 10 orders, measured against JSON.parse + JSON.stringify (5.8 us) on the same machine:

Operation	Time
toInstance (deserialize + validate)	~8 us
toInstance with { validate: false }	~3 us
validate on an existing instance	~4.5 us
toPlain (validate + serialize)	~12 us

If you validate at the edge and map internally afterwards, { validate: false } skips the dominant cost.

Serialization also checks every value it walks against the set JSON cannot represent. That costs a few percent on toPlain, which is the price of never emitting {} where a Map used to be; primitives are handled inline and the check is skipped for arrays and dates, so it is one Symbol.toStringTag read per object.

Notes and Limitations

• Rules are checked, types are not inferred. You write both the field type and the rule; cereale guarantees they agree. If you want the type derived from a schema, that is Zod's model, not this one.

• abstract fields cannot be decorated. Standard decorators do not apply to abstract members. Declare the field concretely in the base class instead.

• accessor fields cannot be decorated. Their value lives in a private slot that mapping and validation cannot reach. Applying a decorator to one is an error, not a silent no-op.

• oxc does not transform standard decorators yet. tsc, esbuild and swc do — see Toolchain support for the Vite/Vitest plugin.

• Values JSON cannot carry are rejected, not quietly dropped. Map, Set, RegExp, Error, typed arrays, bigint, symbol and functions all raise a JsonMappingError naming the property path:

  JsonMappingError: lines[1].tags[0] is a Set, which cannot be serialized to JSON.
  Give the property a @JsonSerialize() serializer that converts it, or drop it from the
  output with @JsonIgnore().
  

  Serializing a populated Map to {} and returning success is the failure mode this library exists to prevent, so it does not do it either.

• Circular references are rejected during serialization with a JsonMappingError that names where the cycle closed. Break it with @JsonIgnore() on the back-reference.

• validate() on a plain object returns no errors: rules live on the class, so validate the instance you get back from toInstance, not the raw payload.

• Renaming is not backwards-compatible by itself. Once a property carries @JsonProperty, its original name no longer reaches it — and is refused rather than copied onto the instance behind the rename's back. Add @JsonAlias to keep older clients working. Under unknownKeys: 'error' the stale name is reported along with what the property is called now:

  JsonMappingError: "ref" is not a JSON name for Order: property "ref" is mapped to
  "order_ref". Send that name, or add @JsonAlias("ref") to keep accepting this one.
  

Contributing

Please see CONTRIBUTING.md for details on how to contribute to this project.

License

Cereale is licensed under the MIT License.