youtube-firebase-login.js

// Google API Login
function youtubeLogin() {
    gapi.load('client:auth2', () => {
        gapi.client.init({
            apiKey: config.apiKey,
            clientId: config.clientId,
            discoveryDocs: config.discoveryDocs,
            scope: config.scopes.join(' '),
        }).then(() => {
            gapi.client.load('youtube', 'v3', () => {
                if (gapi.auth2.getAuthInstance().isSignedIn.get()) {
                    // サインイン済みなら
                    getChannelID()
                    var usr = gapi.auth2.getAuthInstance().currentUser.get();
                    var token = usr.getAuthResponse().id_token;
                    firebaseLogin(token);
                    
                } else {
                    gapi.auth2.getAuthInstance().signIn().then(() => {
                        // サインインする
                        getChannelID()
                        var usr = gapi.auth2.getAuthInstance().currentUser.get();
                        var token = usr.getAuthResponse().id_token;
                        firebaseLogin(token);
                    }).catch(error => { console.log(error) })
                }
            })
        }).catch(function(error) {
            console.log(error);
            alert(error.message);
        });
    })
}

// Firebase Login
function firebaseLogin(token) {
    var creds = firebase.auth.GoogleAuthProvider.credential(token);
    firebase.auth().signInAndRetrieveDataWithCredential(creds).then((result) => {
        if (result) {
            let user = result.user;
            console.log(user);
            // ログインできたのでどっかに遷移するとか
        }
     }).catch (error => {
         console.log(error);
     })
}

// Get YouTube ChannelID
function getChannelID() {
    var request = gapi.client.youtube.channels.list({
        mine: true,
        part: 'contentDetails'
      });
    request.execute(response => {
        playlistId = response.result.items[0].id;
        console.log(playlistId); // これがチャンネルIDだああああああああああ
    });
}