How to use the gulp-amphtml-validator.format function in gulp-amphtml-validator

gulp.task('amphtml:validate', () => {
  return gulp.src('../../testdata/feature_tests/*.html')
  // Valide the input and attach the validation result to the "amp" property
  // of the file object.
      .pipe(gulpAmpHtmlValidator.validate())
  // Print the validation results to the console.
      .pipe(gulpAmpHtmlValidator.format())
  // Exit the process with error code (1) if an AMP validation error
  // occurred.
      .pipe(gulpAmpHtmlValidator.failAfterError());
});

return () => {
    return gulp.src(page)
      // Validate the input and attach the validation result to the "amp" property
      // of the file object.
      .pipe(gulpAmpValidator.validate())
      // Print the validation results to the console.
      .pipe(gulpAmpValidator.format())
      // Exit the process with error code (1) if an AMP validation error
      // occurred.
      .pipe(gulpAmpValidator.failAfterError());
      // .pipe(gulpAmpValidator.failAfterWarningOrError());
  };
};

gulp.task('validate', function () {
  return gulp.src('dist/**/*.html')
    .pipe(gulpAmpValidator.validate())
    .pipe(gulpAmpValidator.format())
    .pipe(gulpAmpValidator.failAfterError());
});