# 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. ```typescript 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 ``` **[avalon-vanguard.github.io/cereale](https://avalon-vanguard.github.io/cereale/)** — an interactive playground that runs this library in your browser, the full decorator reference, and the toolchain matrix. The page has no third-party dependencies — no CDN, no analytics, no webfonts; the only thing it fetches is its own vendored compiler, and only when you first press Run. It is served from `docs/` on `main`, and `npm run build:docs` rebuilds its assets to open locally. ## 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 ```bash npm install cereale ``` Cereale uses **TC39 standard decorators** (since 0.2.0), so no `experimentalDecorators` flag: ```json { "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](src/toolchain.test.ts) 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 | ### Frameworks **[FRAMEWORKS.md](FRAMEWORKS.md)** has a setup recipe for each, every one of them run before it was written. The short version: | | | | | --- | --- | --- | | **Angular** 21 | ✅ | Flip the scaffolded `experimentalDecorators` to `false`. Angular does not need it — `ngtsc` erases its own decorators itself | | **React, Vue, Svelte, Solid, Astro, Nuxt** | ✅ | Any Vite 8 app: add the `cereale/vite` plugin below | | **Bun** | ✅ | No configuration | | **Node** + `tsc` | ✅ | Just the flag | | **Next.js** 16 | ⚠️ | Not inline: it derives both the SWC parser *and* the transform from one flag, so decorators either compile legacy or fail to parse. Keep your models in a package compiled by `tsc` | | **NestJS** 11 | ⚠️ | Not inline: its DI needs `emitDecoratorMetadata`. Same precompiled-package route — verified working alongside `@Injectable()` in a program with both legacy flags on | **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: ```ts // 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 ```typescript 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('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 ```typescript 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. ```typescript 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. ```typescript 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: ```typescript 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 ```typescript 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. | ```typescript // 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. ```typescript 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)`: Custom serializer for a property. Skipped when the value is `null`/`undefined`. - `@JsonDeserialize(deserializer: ClassConstructor)`: Custom deserializer for a property. - `@JsonType(typeFunction: () => ClassConstructor)`: 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`). - `toPlain(obj, options?)`: Validates and transforms an instance to a plain object (`Promise`). - `fromJson(clazz, json, options?)`: Parses JSON to a validated class instance (`Promise`). - `fromJsonArray(clazz, json, options?)`: Same, for a JSON array (`Promise`). - `toInstance(clazz, plain, options?)`: Transforms a plain object to a validated class instance (`Promise`). - `toInstanceArray(clazz, plain, options?)`: Same, for an array (`Promise`). - `fromRequest(clazz, request, options?)`: Extracts JSON from a Fetch `Request` (`Promise`). - `validate(obj, options?)`: Full validation, returning `Promise`. - `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: ```typescript 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`. ```typescript import { toInstance } from 'cereale'; @Post() async create(@Body() body: any) { const user = await toInstance(User, body); return this.userService.create(user); } ``` ### Express / Fastify ```typescript 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. ## Bundle size Cereale tree-shakes. Every rule is declared so that a bundler can drop the ones you did not import, which matters for a library with 68 decorators — you pay for what you name and nothing else. Minified bytes, measured through esbuild, rollup and webpack. The table was measured by hand; what is [pinned by a test](src/treeshake.test.ts) is the property behind it — that a given import drops the parts of the library it does not reach — checked through esbuild on both the source and the published bundle: | What you import | esbuild | rollup | webpack | | --- | ---: | ---: | ---: | | `flattenErrors` | 394 | 367 | 394 | | one decorator | 1,837 | 1,823 | 1,818 | | `validateSync` | 3,722 | 3,554 | 3,738 | | `toPlainSync` | 7,744 | 7,769 | 7,771 | | `toInstanceSync` | 7,900 | 7,942 | 7,944 | | a typical DTO — 5 decorators, map, validate, format errors | 10,395 | 10,402 | 10,360 | | the whole library | 26,266 | 25,671 | 26,879 | The serializer and the deserializer drop independently: read JSON and you do not pay for writing it. The validator is kept by both, because `validate` defaults to `true` and the entry points reference it whatever a given call site passes. The floor is about a hundred bytes: cereale installs `Symbol.metadata` if the runtime lacks it, and that install has to survive tree-shaking or a `tsc`-compiled consumer decorates its classes with no metadata at all. It is why `sideEffects` names `index.js` as well as `metadata.js` — marking only the latter leaves the barrel itself droppable, so the edge to it is pruned before its own marking is ever read. That cost lands only on the first row; every import that touches a model was already carrying it. This did not come for free. Thirty of the rules are declared as top-level calls — `export const IsString = rule(…)` — and rollup can prove such a call side-effect-free by reading the factory, but esbuild and webpack will not. Without a `/*#__PURE__*/` annotation on each of them, importing one decorator pulled in the message and validator of all 68: **4,909 bytes instead of 1,837**. Nothing failed; the library was simply three times heavier in every consumer's bundle, and the only way to find out was to measure. Note what that means for measuring: rollup alone would have shown nothing wrong. `cereale/min` tree-shakes too, which took a second fix — esbuild's `minify` strips comments, annotations included, so the flat bundle was silently reproducing the same bug (5,066 bytes for one decorator). It is now minified for syntax and identifiers but not whitespace: 33.9 KB raw, 9.6 KB gzipped, about a kilobyte over the wire more than full minification would give. Still, if you are using a bundler, import from `cereale` rather than `cereale/min`. ## 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](#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](CONTRIBUTING.md) for details on how to contribute to this project. ## License Cereale is licensed under the [MIT License](LICENSE).