Custom Pipeline Elements

Preview — 3 of 10 questions

Which interface must a custom pipe implement, and what method does it provide?

javascript

import { PipeTransform, Injectable, ArgumentMetadata } from '@nestjs/common';

@Injectable()
export class TrimPipe implements PipeTransform<string, string> {
  transform(value: string, _metadata: ArgumentMetadata): string {
    return typeof value === 'string' ? value.trim() : value;
  }
}

APipeTransform with transform()

BCanActivate with canActivate()

CNestInterceptor with intercept()

DExceptionFilter with catch()

In transform(value, metadata), what does the second argument metadata describe?

javascript

import { PipeTransform, ArgumentMetadata, Injectable } from '@nestjs/common';

@Injectable()
export class InspectPipe implements PipeTransform {
  transform(value: unknown, metadata: ArgumentMetadata): unknown {
    // metadata.type === 'body' | 'query' | 'param' | 'custom'
    // metadata.metatype === e.g. CreateUserDto
    // metadata.data === e.g. 'id'
    return value;
  }
}

AThe argument being processed: its type, metatype, and optional data key

BThe HTTP response headers

CThe list of other pipes in the chain

DThe authenticated user

A pipe should only validate when it runs on the request body, not on params/query. Which metadata field do you check?

javascript

import { PipeTransform, ArgumentMetadata, Injectable } from '@nestjs/common';

@Injectable()
export class BodyOnlyPipe implements PipeTransform {
  transform(value: unknown, metadata: ArgumentMetadata): unknown {
    if (metadata.type !== 'body') return value; // skip params/query
    // validate body here
    return value;
  }
}

Ametadata.data === 'body'

Bmetadata.metatype === 'body'

Cmetadata.type === 'body'

Dmetadata.source === 'body'

Answer all 10 questions (7 more), see explanations for every answer, and track your score.