Express-validator validation causes typescript error: request.query Object is possibly 'undefined'

Question

I am building a Node.js app using express, which I wanted to improve with express-validator. This is my first time with express-validator and I don't understand the warnings it causes. Its documentation is also not really verbose.

Here is a simplified version of a segment before I added the validation, the database calls have been replaced by sending a response back with the entry_id from the GET request.

router.get(
  '/list_entries',
  function (req, res, next) {
    res.send(req.query.entry_id)
  }
)

I added validation to check if entry_id is given in req.query and is in the valid range.:

router.get(
  '/list_entries',
  query('entry_id').isInt({ min: 1 }),
  function (req, res, next) {
    const errors = validationResult(req)
    if (!errors.isEmpty()) {
      return res.status(400).json({ errors: errors.array() })
    }
    res.send(req.query.entry_id)
  }
)

It seems to function well, in case of out of range values the errors do get displayed. However, this also triggered a warning in typescript stating:Object is possibly 'undefined' referring to req.query in the response. I don't have a clear understanding how does the validation cause this, and how can I overcome it (other than using optional chaining like req.query?.entry_id). I wonder where I can find some documentation or working examples to enlighten me. Thanks in advance!

Answer 1

Your problem has little to do with Express Validator and is just Typescript being careful.

It has reasoned that it cannot prove (at compile-time) that req.query is defined, so it is warning you req.query.entry_id might cause a TypeError at run-time.

The simplest solution is to write this:

res.send(req.query?.entry_id)

which is simply shorthand for:

res.send(req.query === undefined ? undefined : req.query.entry_id)