Running validations imperatively

express-validator favors the declarative way of doing things that express middlewares bring. This means most of the APIs look and work better when simply passed into an express route handler.

You can, however, give control of running these validations to your own middleware/route handler.
This is possible with the use of the declarative method run(req), available on both validation chain and sanitization chains.

Check the examples below to understand how this method can help you:

Example: standardized validation error response

// can be reused by many routes

// parallel processing
const validate = validations => {
  return async (req, res, next) => {
    await Promise.all(validations.map(validation => validation.run(req)));

    const errors = validationResult(req);
    if (errors.isEmpty()) {
      return next();
    }

    res.status(400).json({ errors: errors.array() });
  };
};

// sequential processing, stops running validations chain if the previous one have failed.
const validate = validations => {
  return async (req, res, next) => {
    for (let validation of validations) {
      const result = await validation.run(req);
      if (result.errors.length) break;
    }

    const errors = validationResult(req);
    if (errors.isEmpty()) {
      return next();
    }

    res.status(400).json({ errors: errors.array() });
  };
};

app.post('/api/create-user', validate([
  body('email').isEmail(),
  body('password').isLength({ min: 6 })
]), async (req, res, next) => {
  // request is guaranteed to not have any validation errors.
  const user = await User.create({ ... });
});

Example: validating with a condition

app.post(
  '/update-settings',
  body('email').isEmail(),
  body('password').optional().isLength({ min: 6 }),
  async (req, res, next) => {
    // if a password has been provided, then a confirmation must also be provided.
    if (req.body.password) {
      await body('passwordConfirmation')
        .equals(req.body.password)
        .withMessage('passwords do not match')
        .run(req);
    }

    // Check the validation errors, and update the user's settings.
  },
);