Validation Pipe

validatorPipe(opts?) is the main entry point in @atscript/moost-validator.

It checks a handler argument's runtime type. If that type is an Atscript annotated type, it runs the model's validator before your handler executes.

The Most Common Setup

Register it globally:

import { Moost } from 'moost'
import { validatorPipe } from '@atscript/moost-validator'

const app = new Moost()
app.applyGlobalPipes(validatorPipe())

That is the best default for most apps.

Validate Request Bodies

This is the most common use case:

import { Controller } from 'moost'
import { Body, Post } from '@moostjs/event-http'
import { CreateUserDto } from './create-user.dto.as'

@Controller('users')
export class UsersController {
  @Post()
  async create(@Body() dto: CreateUserDto) {
    // dto has already been validated
    return this.users.create(dto)
  }
}

If validation fails, the pipe throws ValidatorError. For HTTP apps, pair it with Error Handling.

Validate PATCH Payloads

For partial updates, pass Atscript validator options to the pipe:

import { Controller } from 'moost'
import { Patch, Body } from '@moostjs/event-http'
import { UseValidatorPipe } from '@atscript/moost-validator'
import { UpdateUserDto } from './update-user.dto.as'

@Controller('users')
export class UsersController {
  @Patch(':id')
  @UseValidatorPipe({ partial: true })
  async patch(@Body() dto: UpdateUserDto) {
    return this.users.patch(dto)
  }
}

Use partial: 'deep' when nested objects should also allow missing fields.

Strip Unknown Properties

This is useful for request bodies that may contain extra fields:

app.applyGlobalPipes(
  validatorPipe({
    unknownProps: 'strip',
  })
)

Options:

• 'error' — reject unknown props
• 'ignore' — keep them
• 'strip' — remove them from the value

Apply It Per Controller Or Handler

Global registration is usually best, but the decorator form is useful when only part of the app uses Atscript DTOs.

Per Controller

import { Controller } from 'moost'
import { UseValidatorPipe } from '@atscript/moost-validator'

@UseValidatorPipe()
@Controller('users')
export class UsersController {}

Per Handler

import { Controller } from 'moost'
import { Body, Post } from '@moostjs/event-http'
import { UseValidatorPipe } from '@atscript/moost-validator'
import { CreateUserDto } from './create-user.dto.as'

@Controller('users')
export class UsersController {
  @Post()
  @UseValidatorPipe()
  async create(@Body() dto: CreateUserDto) {}
}

UseValidatorPipe(opts?) is sugar for @Pipe(validatorPipe(opts)).

Validate Params And Query Values Carefully

The pipe validates the value it receives. It does not coerce strings into numbers, booleans, or dates.

That means:

• body payloads are usually a good fit because JSON parsing already gives you numbers, booleans, arrays, and objects
• raw HTTP params and query values are often strings, so string-based Atscript types fit best there

Good example:

export type EmailQuery = string.email

import { Controller } from 'moost'
import { Get, Query } from '@moostjs/event-http'
import { EmailQuery } from './queries.as'

@Controller('users')
export class UsersController {
  @Get('search')
  async search(@Query('email') email: EmailQuery) {
    return this.users.searchByEmail(email)
  }
}

If a param or query needs numeric validation, parse it before it reaches this pipe or validate it as a string-shaped contract instead.

Optional Params

Parameters marked with Moost's @Optional() decorator are skipped by the pipe when their value is undefined or null. This matters for slots that are genuinely missing at the framework level — optional query strings, optional CLI flags — where the underlying Atscript type itself is not nullable.

import { Controller, Optional } from 'moost'
import { Get, Query } from '@moostjs/event-http'
import { WikiName } from './types.as'

@Controller('search')
export class SearchController {
  @Get()
  async search(@Query('wiki') @Optional() wiki?: WikiName) {
    // `wiki` is undefined when the query string is absent — no validation runs.
    // When present, `WikiName` constraints (minLength, pattern, ...) are enforced.
  }
}

The pipe still validates the value whenever one is provided, so ?wiki= (empty) or ?wiki=bad name will fail with ValidatorError as usual.

For request bodies, this is rarely needed — bodies are usually required at the framework level, and individual properties can be marked optional inside the Atscript interface itself.

Use Reusable Validated Primitive Types

This is one of the nicest patterns in Atscript + Moost.

Define a validated primitive once:

export type Email = string.email

Then use those types directly in handlers:

import { Controller } from 'moost'
import { Body, Post } from '@moostjs/event-http'
import { Email } from './types.as'

@Controller('newsletter')
export class NewsletterController {
  @Post('subscribe')
  async subscribe(@Body() email: Email) {
    return this.newsletter.subscribe(email)
  }
}

That is hard to model with class-validator, because its validation model is centered on decorated classes and their properties. Atscript validates the type itself, so standalone reusable primitives work naturally.

Options At A Glance

validatorPipe(opts?) accepts the same Partial<TValidatorOptions> object as Atscript's runtime validator.

Most useful options:

• partial — allow missing properties (PATCH)
• unknownProps — 'strip' | 'ignore' | 'error'
• errorLimit — stop after N errors

Advanced options

These are available but rarely needed in HTTP request validation. Reach for them only when you understand the underlying validator behavior:

• skipList — paths to skip during validation
• replace — value-replacement hook used by validator plugins
• plugins — custom validator plugin functions

If you already know the TypeScript validator API, the same options work here. See Validation Reference for the full low-level option details.

Beyond HTTP

The pipe is not tied to HTTP. It works anywhere Moost resolves handler arguments.

What is HTTP-specific is the built-in error transform, because that part returns HttpError(400).

Next Steps

• Error Handling — convert ValidatorError into HTTP responses
• Validation Guide — deeper validator behavior and options
• Why Atscript In Moost? — when this package is the right fit