ipfs.dht.findProvs() can't catch error with fake hash

[Dendiom]

We want to implement a 'exist' function to judge if provided hash is exist in the private ipfs network. We use ipfs.dht.findProvs() to achieve this logic like this:

ipfs.dht.findProvs(hash)
.then(resp => {...})
.catch(e => {...})

but we found that if the hash is valid but not exist, it will throw an error that can't catch in promise:
Uncaught TypeError: Cannot read property 'Type' of undefined
after see the source code, we found this:

const handleResult = (res, callback) => {
      // Inconsistent return values in the browser vs node
      if (Array.isArray(res)) {
        res = res[0]
      }

      // Type 4 keys
      if (res.Type !== 4) {
        const errMsg = `key was not found (type 4)`

        return callback(errcode(new Error(errMsg), 'ERR_KEY_TYPE_4_NOT_FOUND'))
      }
.....

It means that the res is undefined when hash is not exist?
or would you please give us a better way to achieve the 'exist' function

[alanshaw]

@Dendiom would you be willing to submit a PR to callback with an empty array if res is null?

[KrishnaPG]

Looks like this still does not handle the case where res is empty array. For example, in one case we are receiving empty array (for whatever reason), and it is causing error:

TypeError: Cannot read property 'Type' of undefined
    at handleResult (\node_modules\ipfs-http-client\src\dht\findprovs.js:38:15)
    at streamToValueWithTransformer (\node_modules\ipfs-http-client\src\utils\stream-to-value-with-transformer.js:14:5)
    at send (\node_modules\ipfs-http-client\src\dht\findprovs.js:68:7)
    at f (\node_modules\once\once.js:25:25)
    at concat (\node_modules\ipfs-http-client\src\utils\stream-to-value.js:12:22)
    at ConcatStream.<anonymous> (\node_modules\concat-stream\index.js:32:43)
    at ConcatStream.emit (events.js:187:15)
    at finishMaybe (\node_modules\readable-stream\lib\_stream_writable.js:620:14)
    at endWritable (\node_modules\readable-stream\lib\_stream_writable.js:629:3)
    at ConcatStream.Writable.end (\node_modules\readable-stream\lib\_stream_writable.js:567:22)

When res = [] it turns out at line 34 res = res[0] causes it become undefined leading to the above exception.

[alanshaw]

@KrishnaPG are you able to submit a fix for this?

[KrishnaPG]

Thank you - yes, array is being checked and sampled after the individual element checks line 33.

Moving the array sampling to before fixes this. PR: #1003