gapi is not defined - Google sign in issue with gapi.auth2.init

I think you'll find with the above example that this won't work either, as gapi.auth2 will not yet be defined (I know this because I made the same mistake myself, today) You first need to call gapi.load('auth2', callback) and pass THAT a callback which then calls gapi.auth2.init. Here is an example of my _onGoogleLoad function, which is the callback for loading the first platform.js script.

var _auth2

var _onGoogleLoad = function () {
  gapi.load('auth2', function () {
    _auth2 = gapi.auth2.init({
      client_id: 'OUR_REAL_ID_GOES_HERE',
      scope: 'email',
      fetch_basic_profile: false
    })
    _enableGoogleButton()
  })
}

After that, you can use the _auth2 variable to actually sign users in.

The problem is not only in gapi. To call init method - auth2 object must be initialized. There is a promise once a google auth object is in fully initialized GoogleAuth.then(onInit, onFailure)

gapi.load('auth2', initSigninV2);

function initSigninV2() {
    gapi.auth2.init({
        client_id: 'CLIENT_ID.apps.googleusercontent.com'
    }).then(function (authInstance) {
        // now auth2 is fully initialized
    });
}

While these answers helped me, I believe there is better answer in official docs.

See Integrating Google Sign-In using listeners

var auth2; // The Sign-In object.
var googleUser; // The current user.

/**
 * Calls startAuth after Sign in V2 finishes setting up.
 */
var appStart = function() {
  gapi.load('auth2', initSigninV2);
};

/**
 * Initializes Signin v2 and sets up listeners.
 */
var initSigninV2 = function() {
  auth2 = gapi.auth2.init({
      client_id: 'CLIENT_ID.apps.googleusercontent.com',
      scope: 'profile'
  });

  // Listen for sign-in state changes.
  auth2.isSignedIn.listen(signinChanged);

  // Listen for changes to current user.
  auth2.currentUser.listen(userChanged);

  // Sign in the user if they are currently signed in.
  if (auth2.isSignedIn.get() == true) {
    auth2.signIn();
  }

  // Start with the current live values.
  refreshValues();
};

/**
 * Listener method for sign-out live value.
 *
 * @param {boolean} val the updated signed out state.
 */
var signinChanged = function (val) {
  console.log('Signin state changed to ', val);
  document.getElementById('signed-in-cell').innerText = val;
};

/**
 * Listener method for when the user changes.
 *
 * @param {GoogleUser} user the updated user.
 */
var userChanged = function (user) {
  console.log('User now: ', user);
  googleUser = user;
  updateGoogleUser();
  document.getElementById('curr-user-cell').innerText =
    JSON.stringify(user, undefined, 2);
};

/**
 * Updates the properties in the Google User table using the current user.
 */
var updateGoogleUser = function () {
  if (googleUser) {
    document.getElementById('user-id').innerText = googleUser.getId();
    document.getElementById('user-scopes').innerText =
      googleUser.getGrantedScopes();
    document.getElementById('auth-response').innerText =
      JSON.stringify(googleUser.getAuthResponse(), undefined, 2);
  } else {
    document.getElementById('user-id').innerText = '--';
    document.getElementById('user-scopes').innerText = '--';
    document.getElementById('auth-response').innerText = '--';
  }
};

/**
 * Retrieves the current user and signed in states from the GoogleAuth
 * object.
 */
var refreshValues = function() {
  if (auth2){
    console.log('Refreshing values...');

    googleUser = auth2.currentUser.get();

    document.getElementById('curr-user-cell').innerText =
      JSON.stringify(googleUser, undefined, 2);
    document.getElementById('signed-in-cell').innerText =
      auth2.isSignedIn.get();

    updateGoogleUser();
  }
}

For a Vue.JS app

• Add an onload hook to the script tag for platform.js
• Set up a function that dispatches an event
• Catch that event in the mounted hook of your component.

Here's a fairly complete example you can just paste into a new vue-cli project.

Don't forget to supply your own client ID!

public/index.html

    <script type="text/javascript">
      function triggerGoogleLoaded() {
        window.dispatchEvent(new Event("google-loaded"));
      }
    </script>
    <script
      src="https://apis.google.com/js/platform.js?onload=triggerGoogleLoaded"
      async
      defer
    ></script>
    <meta
      name="google-signin-client_id"
      content="xxxxxxxxxxxx.apps.googleusercontent.com"
    />

App.vue

<template>
  <div id="app">
    <div id="nav">
      <div id="google-signin-btn"></div>
      <a href="#" class="sign-out" @click="signOut" v-if="profile">Sign out</a>
    </div>
    <div v-if="profile" class="">
      <h2>Signed In User Profile</h2>
      <pre>{{ profile }}</pre>
    </div>
    <div v-if="!profile">
      <h2>Signed out.</h2>
    </div>
    <router-view />
  </div>
</template>

<script>
export default {
  components: {},
  data() {
    return {
      profile: false
    };
  },
  methods: {
    onSignIn(user) {
      const profile = user.getBasicProfile();
      this.profile = profile;
    },
    signOut() {
      var auth2 = gapi.auth2.getAuthInstance();
      auth2.signOut().then(() => {
        location.reload(true);
      });
    },
    renderGoogleLoginButton() {
      gapi.signin2.render("google-signin-btn", {
        onsuccess: this.onSignIn
      });
    }
  },
  mounted() {
    window.addEventListener("google-loaded", this.renderGoogleLoginButton);
  }
};
</script>

This worked for me: https://stackoverflow.com/a/55314602/1034622

Include this script tag

<script src="https://apis.google.com/js/platform.js"></script>