First you have to create the application, go to Twitch developer console and create it. Once done make a copy of the .env.example named .env and fill the CLIENT_ID and CLIENT_SECRET field values provided by Twitch.

You will have something similar to this:

TWITCH_CLIENT_ID=theClientIdProvidedByTwitch
TWITCH_CLIENT_SECRET=aSecretYouHaveToKeepSafe

Now we have to authenticate the account we are going to use using OAuth. In order to get it we have to make a GET request to the following endpoint.

GET https://id.twitch.tv/oauth2/authorize
  ?client_id=<your client ID>
  &redirect_uri=<your registered redirect URI>
  &response_type=code
  &scope=<space-separated list of scopes>

You can get all the available scopes on here. Don't forget to set the exact same redirect_url as you did on the application.

Important: you have to perform this operation with the account you want to use to send the messages. You can have two different accounts, the owner of the application and the authorized account, aka "the chatter". But I recommend you to use the same account for simplicity.

Once authorized, we will get redirected to the specified address. The url will have a GET parameter called code that we will use to obtain the access token.

http://localhost/?code=<code of the authorization>
  &scope=channel:manage:redemptions channel:read:hype_train channel:read:polls channel:read:predictions channel:read:redemptions channel:moderate chat:edit chat:read

Finally make a POST request to the following url.

POST https://id.twitch.tv/oauth2/token
  ?client_id=<your client ID>
  &client_secret=<your client secret>
  &code=<authorization code received above>
  &grant_type=authorization_code
  &redirect_uri=<your registered redirect URI>

This will return a JSON-encoded response. Copy the whole response into a file called tokens.json and place it on the root of this project (same place as is this file). You have to change the key names from kebab_case to camelCase.

The JSON file must have this format (the order is not relevant):

{
  "accessToken": "the access token",
  "expiresIn": 14361,
  "refreshToken": "the refresh token",
  "scope": [
    // list of the scopes
  ],
  "tokenType": "bearer"
}

Starting the service

Install any dependencies executing the following command yarn, then you can start the service using the start script yarn start. Keep the terminal open until you want to close the service.

Development

In order to start the service in development mode use the dev script yarn dev, this will reload the backend whenever you make any change. Keep in mind that some of the features might be disabled in this environment.