custom validation for uuid and null

src/shared/uuid.validation.ts

+import {
+  ValidatorConstraint,
+  ValidatorConstraintInterface,
+  ValidationArguments,
+  Validator,
+} from 'class-validator';
+
+// check if input is null or valid uuid
+@ValidatorConstraint({ name: 'uuid', async: true })
+export class Uuid implements ValidatorConstraintInterface {
+  validate(uuid: string, args: ValidationArguments) {
+    const validator = new Validator();
+    return validator.isUUID(uuid, '4') || uuid == null; // for async validations you must return a Promise<boolean> here
+  }
+
+  defaultMessage(args: ValidationArguments) {
+    return 'Not valid uuid';
+  }
+}