function for extracting validation error messages recursively from the class-validator library ValidationError array

formatClassValidatorError.ts

  function formatClassValidatorError(
    validationErrors: ValidationError[],
    accumulator: string[] = [],
  ): string[] {
    for (const error of validationErrors) {
      if (!error.children.length && !Object.keys(error.constraints).length) {
        return accumulator;
      }

      if (error.children.length) {
        return this.formatClassValidatorError(error.children, accumulator);
      }

      accumulator.push(...Object.values(error.constraints));
    }

    return accumulator;
  }