Uncaught error in getUserFromMention

[gnprice]

One of our more widespread issues reported by Sentry is this one:

undefined is not an object (evaluating 'o.split')

with stack

    const [userFullName, userIdRaw] = unformattedMessage.split('|');

in getUserFromMention < MentionWarnings.

So it seems that unformattedMessage here is undefined. The fuller source listng is

  getUserFromMention = (completion: string): UserOrBot | void => {
    const { usersById } = this.props;

    const unformattedMessage = completion.split('**')[1];
    const [userFullName, userIdRaw] = unformattedMessage.split('|');

so it looks like this would happen if completion.split('**') returns an array of less than two elements. In particular this would happen if completion doesn't contain '**'.

Because this is regularly affecting users, and I believe it's causing a crash for them, it would be good to fix.

I think this is ripe for some debugging.

• The first goal would be to try to reproduce it -- go to the compose box and try to enter some text that triggers this bug.
• A useful step for trying to do that would be to read a bit more of the code that leads to here, and try to see when getUserFromMention is called. It seems like that function is assuming that its argument string will contain ** whenever it gets called, but that that isn't always true.
• Once we have a clear reproduction, I suspect the fix may be relatively easy -- the debugging may be the main part of the work.

[im-adithya]

Hey @gnprice! I have tried to reproduce this but failed to do so...
I found that the function getUserFromMention is being called because of handleMessageAutocomplete which is used whenever we type :, # or @... So when the case is @ --> **Username** is added by default (hence it is checking whether the lastWordPrefix is '@' or not in order to ensure the presence of **s) which is then used to split to get unformattedMessage.

It seems like that function is assuming that its argument string will contain ** whenever it gets called, but that that isn't always true.

I tried to replicate this by typing @username but it then doesn't trigger autocomplete so that doesn't even get invoked.
So I think it is correct to assume that the function's argument always contains ** whenever it gets called...

[Gautam-Arora24]

@gnprice I am trying to debug this?

[im-adithya]

@gnprice I would like to work on this?

Hey Gautam, I'm still investigating this issue to reproduce it and will work on this once I find it...

[chrisbobbe]

@Gautam-Arora24 found, in #4474, that unformattedMessage is undefined for user-group mentions (like @*mobile*).

I notice that there's already some code for handling unformattedMessage being undefined, and it quite helpfully mentions user groups. Here:

    // We skip user groups, for which autocompletes are of the form
    // `*<user_group_name>*`, and therefore, message.split('**')[1]
    // is undefined.
    if (unformattedMessage === undefined) {
      return undefined;
    }

That snippet comes from just below the problematic .split call. So perhaps a good solution is to just move that code just above the .split call.