express-validator returns validation errors twice

I don't know why when req.body is a empty object - {}, the validator will run through all node of validation chain. You can check again, add each message for each condition, like as follow:

class UserRequestValidator extends RequestValidator {  public createUser = [    body('username')      .isString().withMessage('username must be a string') // you can see both error messages in the response      .exists().withMessage('username must be exist'),    body('password') // the same for this field      .isString()      .exists(),    this.validate,  ];  public fetchUserById = [    param('id') // because id is exist in `req.params`, then only one test has been executed.      .isString().withMessage('id must be a string')      .isUUID()      .exists(),    this.validate,  ];}

I found a solution for your case in https://github.com/express-validator/express-validator/issues/638 , stop chain in the first error with .bail() function.

Then your validator class will be like:

class UserRequestValidator extends RequestValidator {  public createUser = [    body('username')       // always check exists() first      .exists().withMessage('username must be exist').bail()      .isString().withMessage('username must be a string').bail(),    body('password')      .exists().bail()      .isString().bail(),    this.validate,  ];  public fetchUserById = [    param('id')      .isString()      .isUUID()      .exists(),    this.validate,  ];}

You can also set onlyFirstError to true when retrieving the error array.From the documentation:

If the option onlyFirstError is set to true, then only the first errorfor each field will be included

Example usage:

function validateRequestParams (req, res, next) {    const errors = validationResult(req)    if (errors.isEmpty()) {        return next()    } else {        return res.status(400).json({            bodyValidationErrors: errors.array({ onlyFirstError: true })        })    }}