Allow property to be an array or plain type

Question

I was trying to be able to pass the property as an array of strings or just a string value.

class GetUsersDTO {
  // @IsArray() | @IsString()
  readonly status;
}

But it's not possible even though it's a common use case. Can we do it with a plain class-validator package?

Answer 1

You'll need to make your own custom validator for this. Typescript doesn't reflect generics (yes, a union type is a special generic) and because of that, class-validator won't be able to properly figure out which of the two decorators to use.

import { ValidatorConstraint, ValidatorConstraintInterface, ValidationArguments } from 'class-validator';

@ValidatorConstraint({ name: 'customText', async: false })
export class IsArrayOrString implements ValidatorConstraintInterface {
  validate(val: string | string[], args: ValidationArguments) {
    const isArray = Array.isArray(val);
    const isString = typeof val === 'string';
    return isArray || isString
  }
}

And now you can use it like

class GetUsersDTO {
  @Validate(IsArrayOrString)
  readonly status: string | string[];
}

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My personal word of warning: using multiple allowed inputs to the same endpoint usually brings nothing but trouble and headache. If possible, keep them separate.