PartialBody decorator for NestJS

param-type.enhancer.ts

import { ParamDecoratorEnhancer } from '@nestjs/common';
import { METADATA__PARAM_TYPE } from './constants'; // METADATA__PARAM_TYPE = 'partialBodyType'

/**
 * This enhancer grabs the Typescript type of the parameter and shoves it into
 * the metadata. We do this because the CustomParamFactory does not have access
 * to the type of param.
 */
export const paramTypeEnhancer: ParamDecoratorEnhancer = (
  target: Record<string, unknown>,
  propertyKey: string,
  parameterIndex: number,
): void => {
  // Typescript adds the "design:paramtypes" metadata with an array of class
  // types where the keys are the method argument index and the value is the
  // class type.
  const paramTypes = Reflect.getOwnMetadata('design:paramtypes', target, propertyKey);
  // We can use the parameterIndex to retrieve the specific type we want.
  const metatype = paramTypes[parameterIndex];
  // Now, we assign the parameter type to the metadata at a key we know.
  Reflect.defineMetadata(METADATA__PARAM_TYPE, metatype, target[propertyKey]);
};

partial-body.decorator.ts

import { ArgumentMetadata, createParamDecorator, ExecutionContext } from '@nestjs/common';
import { METADATA__PARAM_TYPE } from './constants'; // METADATA__PARAM_TYPE = 'partialBodyType'
import { paramTypeEnhancer } from './param-type.enhancer';

/**
 * Just like @Body() but allows partials. You must have
 * validateCustomDecorators=false in your global validation pipe.
 */
export const PartialBody = createParamDecorator(
  // Someone may want to explicitly choose the type to validate against, so if
  // they pass a parameter like @PartialBody(MyType), then data will be MyType
  // and we will use that for validation instead of the metadata type found.
  async (data: unknown, ctx: ExecutionContext): Promise<any> => {
    // Determine the type, either explicitly provided or from the metadata.
    const metatype = data !== undefined ? data : Reflect.getOwnMetadata(METADATA__PARAM_TYPE, ctx.getHandler());

    // Validate partially.
    const request = ctx.switchToHttp().getRequest();
    const transformedValue = plainToClassFromExist(metatype, request.body);
    const errors = await validate(transformedValue, {
      skipUndefinedProperties: true
    });

    // TODO: throw errors
  },
  [paramTypeEnhancer],
);