@youneed/schema

class-validator-style DTO validation, but on standard TC39 decorators — no reflect-metadata, no emitDecoratorMetadata, no experimentalDecorators. The exact same decorated class runs in TypeScript and plain JS.

import { IsEmail, IsNotEmpty, MinLength, IsOptional, IsInt, Min, validate } from "@youneed/schema";

class CreateUserDTO {
  @IsEmail() email!: string;
  @IsNotEmpty() @MinLength(8) password!: string;
  @IsOptional() @IsInt() @Min(18) age?: number;
}

const errors = validate(CreateUserDTO, await req.json());
// [] when valid, else:
// [{ property: "email", value: "nope", constraints: { isEmail: "email must be an email" } }]

Validate

validate(CreateUserDTO, plainObject) // ValidationError[] (class + plain object)
validate(dtoInstance)                // ValidationError[] (an instance)
isValid(CreateUserDTO, plainObject)  // boolean
validateOrThrow(CreateUserDTO, body) // throws SchemaError (carries .errors)
plainToInstance(CreateUserDTO, body) // build an instance from a plain object

ValidationError mirrors class-validator: { property, value, constraints }, where constraints is { [ruleName]: message }.

Constraints

| decorator | passes when | | --- | --- | | @IsDefined() | not null/undefined | | @IsNotEmpty() | not null/undefined and not "" | | @IsOptional() | modifier — skip the field's other rules when it's null/undefined | | @IsString() · @IsNumber() · @IsInt() · @IsBoolean() · @IsArray() | type matches | | @IsEmail() · @IsUrl() | valid email / http(s) URL | | @MinLength(n) · @MaxLength(n) | string/array length in range | | @Min(n) · @Max(n) | number in range | | @Matches(/re/) | string matches the regexp | | @IsIn([...]) | value is one of the list | | @Custom(name, test) | your own predicate |

Every decorator takes an optional { message } to override the default text:

class LoginDTO {
  @IsNotEmpty({ message: "password is required" }) password!: string;
}

Compose freely — multiple decorators on one field all apply, and subclasses inherit their parent's constraints.

Why TC39 decorators

class-validator relies on reflect-metadata + the legacy experimental decorators (experimentalDecorators + emitDecoratorMetadata), which only exist under TypeScript. @youneed/schema uses the Stage-3 standard decorators that ship in modern engines and transpile without any metadata runtime — so a compiled-to-JS DTO validates identically.

Implementation note: TS/esbuild only attach Symbol.metadata to a class that also carries a class decorator, so a fields-only DTO would lose it. Each field decorator instead registers its rules via context.addInitializer into a constructor-keyed WeakMap; validate(Class, …) constructs one throwaway instance to trigger that, then checks your plain object.

Use it in a handler / guard

import { validateOrThrow } from "@youneed/schema";

app.post("/users", (ctx) => {
  validateOrThrow(CreateUserDTO, ctx.body); // throws SchemaError → map to 422
  // …ctx.body is now known-good
});