YouTube OAuth Access

index.html

<script>
  var GoogleAuth;
  var SCOPE = "https://www.googleapis.com/auth/youtube.force-ssl";
  function handleClientLoad() {
    // Load the API's client and auth2 modules.
    // Call the initClient function after the modules load.
    gapi.load("client:auth2", initClient);
  }

  function initClient() {
    // Retrieve the discovery document for version 3 of YouTube Data API.
    // In practice, your app can retrieve one or more discovery documents.
    var discoveryUrl = "https://www.googleapis.com/discovery/v1/apis/youtube/v3/rest";

    // Initialize the gapi.client object, which app uses to make API requests.
    // Get API key and client ID from API Console.
    // 'scope' field specifies space-delimited list of access scopes.
    gapi.client
      .init({
        apiKey: "API_KEY",
        discoveryDocs: [discoveryUrl],
        clientId: "CLIENT_ID",
        scope: SCOPE
      })
      .then(function() {
        GoogleAuth = gapi.auth2.getAuthInstance();

        // Listen for sign-in state changes.
        GoogleAuth.isSignedIn.listen(updateSigninStatus);

        // Handle initial sign-in state. (Determine if user is already signed in.)
        var user = GoogleAuth.currentUser.get();
        setSigninStatus();

        // Call handleAuthClick function when user clicks on
        //      "Sign In" button.
        $("#sign-in-or-out-button").click(function() {
          handleAuthClick();
        });
      });
  }

  function handleAuthClick() {
    if (GoogleAuth.isSignedIn.get()) {
      // User is authorized and has clicked 'Sign out' button.
      GoogleAuth.signOut();
    } else {
      // User is not signed in. Start Google auth flow.
      GoogleAuth.signIn();
    }
  }

  function setSigninStatus(isSignedIn) {
    var user = GoogleAuth.currentUser.get();
    var isAuthorized = user.hasGrantedScopes(SCOPE);
    if (isAuthorized) {
      $("#sign-in-or-out-button").html("Sign out");
    } else {
      $("#sign-in-or-out-button").html("Sign In");
    }
  }

  function updateSigninStatus(isSignedIn) {
    setSigninStatus();
  }
</script>

<button id="sign-in-or-out-button">Sign In</button>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
<script
  async
  defer
  src="https://apis.google.com/js/api.js"
  onload="this.onload=function(){};handleClientLoad()"
  onreadystatechange="if (this.readyState === 'complete') this.onload()"
></script>