I can think of 2 ways…
1.) While joining a channel via IRC, if the channel exists you will join it and receive these raw notices:
← :matt_thomas!matt_thomas@matt_thomas.tmi.twitch.tv JOIN #savage
→ tmi.twitch.tv MODE #savage
← :matt_thomas.tmi.twitch.tv 353 matt_thomas = #savage :matt_thomas
← :matt_thomas.tmi.twitch.tv 366 matt_thomas #savage :End of /NAMES list
If you try to join a channel that doesn’t exist then you would stop at:
→ tmi.twitch.tv JOIN #derpdddd
Based on that you could have a timer/check for the NAMES or MODE notice within a second or 2, and if it never showed up, notify the use the channel doesn’t exist. This of course requires the user to actually join the channel.
2.) Before executing the JOIN command query the API.
A query to https://api.twitch.tv/kraken/channels/derpddd results in:
{
error: “Not Found”,
status: 404,
message: “Channel ‘derpddd’ does not exist”
}
Whereas a query for an existing channel like https://api.twitch.tv/kraken/channels/savage results in:
{
mature: null,
status: null,
broadcaster_language: null,
display_name: “Savage”,
game: null,
language: “en”,
_id: 11952,
name: “savage”,
created_at: “2007-05-22T10:39:47Z”,
updated_at: “2016-05-11T07:30:24Z”,
delay: null,
logo: null,
banner: null,
video_banner: null,
background: null,
profile_banner: null,
profile_banner_background_color: null,
partner: false,
url: “Twitch”,
views: 874,
followers: 6,
_links: {
self: “https://api.twitch.tv/kraken/channels/savage”,
follows: “https://api.twitch.tv/kraken/channels/savage/follows”,
commercial: “https://api.twitch.tv/kraken/channels/savage/commercial”,
stream_key: “https://api.twitch.tv/kraken/channels/savage/stream_key”,
chat: “https://api.twitch.tv/kraken/chat/savage”,
features: “https://api.twitch.tv/kraken/channels/savage/features”,
subscriptions: “https://api.twitch.tv/kraken/channels/savage/subscriptions”,
editors: “https://api.twitch.tv/kraken/channels/savage/editors”,
teams: “https://api.twitch.tv/kraken/channels/savage/teams”,
videos: “https://api.twitch.tv/kraken/channels/savage/videos”
}
}
Depending on your IRC method, an API call should be fairly easy to implement.