Base64 Encoding of Server Key for Firebase Cloud Messaging
<p>I have been stuck in this problem since 3 days and searched Google, but to no success. I followed the instruction given in <a href="https://developers.google.com/web/fundamentals/getting-started/codelabs/push-notifications/" rel="noreferrer">Push Notifications Example</a>. But I get this nasty error when I try to implement it.</p>

<pre><code>Uncaught (in promise) DOMException: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.
</code></pre>

<p>I figured out the colon(:) is not allowed in Base64 String, but the Server Key given in Firebase in Cloud Messenging tab is</p>

<pre><code>AAAA-8MWvk0:APA91bHmU8XL-rzOn9SPbpG98NSqAJcuhYBBh7oze_XhprBpK7Q9PPWm3PwBo6Llxby4zjhVtgvKPIaSAQGp-8RfMV10_d1GekzICrVX9oYO8pi6dOM4VTp52CCAzug6NYIa10tNddsgE2P5QowGAYcnRHxLkrHXsw
</code></pre>

<p>which contains a colon (don't worry, its just a test app so no problem with privacy). </p>

<p>When I tried to use the legacy server key, it simply throws an error. I tried to use other keys given in Firebase too, but to no success. Please tell me actually which server key to use and how?</p>

<p>I'm attaching my code snippet which actually performs the push subscription.</p>

<pre><code> const API_KEY = "AIzaSyByIOl-mW0pu8SEXFeutB8jq59hhiau0wI";
 var GCM_ENDPOINT = 'https://fcm.googleapis.com/fcm/send';
 const legacy  = 'AIzaSyDGF8t125bJ4wBvYn_UdRewkTxHGr7KpH8';
 const applicationServerPublicKey = 'AAAA-8MWvk0APA91bHmU8XL-rzOn9SPbpG98NSqAJcuhYBBh7oze_XhprBpK7Q9PPWm3PwBo6Llxby4zjhVtgvKPIaSAQGp-8RfMV10_d1GekzICrVX9oYO8pi6dOM4VTp52CCAzug6NYIa10tNddsgE2P5QowGAYcnRHxLkrHXsw';

function urlB64ToUint8Array(base64String) {
 const padding = '='.repeat((4 - base64String.length % 4) % 4);
 const base64 = (base64String + padding)
 .replace(/\-/g, '+')
 .replace(/_/g, '/');
 console.log(base64);
 const rawData = window.atob(base64);
 console.log(rawData);
 const outputArray = new Uint8Array(rawData.length);

 for (let i = 0; i &lt; rawData.length; ++i) {
  outputArray[i] = rawData.charCodeAt(i);
 }
 return outputArray;
}

function endpointWorkaround(pushSubscription) {
 // Make sure we only mess with GCM
 if(pushSubscription.endpoint.indexOf('https://fcm.googleapis.com/fcm/send') !== 0) {
  return pushSubscription.endpoint;
 }

 var mergedEndpoint = pushSubscription.endpoint;
 // Chrome 42 + 43 will not have the subscriptionId attached
 // to the endpoint.
 if (pushSubscription.subscriptionId &amp;&amp;
 pushSubscription.endpoint.indexOf(pushSubscription.subscriptionId) === -1)    {
  // Handle version 42 where you have separate subId and Endpoint
  mergedEndpoint = pushSubscription.endpoint + '/' +
  pushSubscription.subscriptionId;
 }
 return mergedEndpoint;
}

function sendSubscriptionToServer(subscription) {
 // TODO: Send the subscription.endpoint
 // to your server and save it to send a
 // push message at a later date
 //
 // For compatibly of Chrome 43, get the endpoint via
 // endpointWorkaround(subscription)
 console.log('TODO: Implement sendSubscriptionToServer()',    JSON.stringify(subscription));

 var mergedEndpoint = endpointWorkaround(subscription);

 // This is just for demo purposes / an easy to test by
 // generating the appropriate cURL command
 var temp = showCurlCommand(mergedEndpoint);
 return temp;
}

// NOTE: This code is only suitable for GCM endpoints,
// When another browser has a working version, alter
// this to send a PUSH request directly to the endpoint
function showCurlCommand(mergedEndpoint) {
 // The curl command to trigger a push message straight from GCM
 if (mergedEndpoint.indexOf(GCM_ENDPOINT) !== 0) {
  console.warn('This browser isn\'t currently ' + 'supported for this demo');
  return;
 }

 var endpointSections = mergedEndpoint.split('/');
 var subscriptionId = endpointSections[endpointSections.length - 1];

 var curlCommand = 'curl --header "Authorization: key=' + API_KEY + '" --header Content-Type:"application/json" ' + GCM_ENDPOINT + ' -d "{\\"registration_ids\\":[\\"' + subscriptionId + '\\"]}"';

 console.log(curlCommand);
 return subscriptionId;
}

function initialiseState() {
 // Are Notifications supported in the service worker?
 if (!('showNotification' in ServiceWorkerRegistration.prototype)) {
  console.warn('Notifications aren\'t supported.');
  return;
 }

 // Check the current Notification permission.
 // If its denied, it's a permanent block until the
 // user changes the permission
 if (Notification.permission === 'denied') {
  console.warn('The user has blocked notifications.');
  return;
 }

 // Check if push messaging is supported
 if (!('PushManager' in window)) {
  console.warn('Push messaging isn\'t supported.');
  return;
 }
 var prom = new Promise(function(resolve, reject) {
  navigator.serviceWorker.ready.then(function(serviceWorkerRegistration) {
    // Do we already have a push message subscription?
serviceWorkerRegistration.pushManager.getSubscription().then(function(subscription) {
            // Enable any UI which subscribes / unsubscribes from
            // push messages.
            // var pushButton = document.querySelector('.js-push-button');
            // pushButton.disabled = false;

            if (!subscription) {
                // We aren’t subscribed to push, so set UI
                // to allow the user to enable push
        subscribe();
                return;
            }

            // Keep your server in sync with the latest subscription
      var temp = sendSubscriptionToServer(subscription);
      if(temp){
        resolve(temp);
      }else{
        reject("Oops!")
      }

            // Set your UI to show they have subscribed for
            // push messages
            // pushButton.textContent = 'Disable Push Messages';
            // isPushEnabled = true;
        })
        .catch(function(err) {
            console.error('Error during getSubscription()', err);
      reject(err);
        });
});
});
return prom;
}

function unsubscribe() {
 // var pushButton = document.querySelector('.js-push-button');
 // pushButton.disabled = true;

 navigator.serviceWorker.ready.then(function(serviceWorkerRegistration) {
  // To unsubscribe from push messaging, you need get the
  // subcription object, which you can call unsubscribe() on.
  serviceWorkerRegistration.pushManager.getSubscription().then(
  function(pushSubscription) {
   // Check we have a subscription to unsubscribe
   if (!pushSubscription) {
    // No subscription object, so set the state
    // to allow the user to subscribe to push
    //  isPushEnabled = false;
    //  pushButton.disabled = false;
    //  pushButton.textContent = 'Enable Push Messages';
    return;
   }

   // TODO: Make a request to your server to remove
   // the users data from your data store so you
   // don't attempt to send them push messages anymore

   // We have a subcription, so call unsubscribe on it
   pushSubscription.unsubscribe().then(function() {
    //  pushButton.disabled = false;
    //  pushButton.textContent = 'Enable Push Messages';
    //  isPushEnabled = false;
   }).catch(function(e) {
     // We failed to unsubscribe, this can lead to
     // an unusual state, so may be best to remove
     // the subscription id from your data store and
     // inform the user that you disabled push

     console.log('Unsubscription error: ', e);
     //  pushButton.disabled = false;
   });
 }).catch(function(e) {
   console.error('Error thrown while unsubscribing from ' + 'push messaging.', e);
 });
});
}

function subscribe() {
 // Disable the button so it can't be changed while
 // we process the permission request
 // var pushButton = document.querySelector('.js-push-button');
 // pushButton.disabled = true;

 navigator.serviceWorker.ready.then(function(serviceWorkerRegistration) {
  const applicationServerKey = urlB64ToUint8Array(applicationServerPublicKey);
  serviceWorkerRegistration.pushManager.subscribe({userVisibleOnly: true,  applicationServerKey: applicationServerKey})
 .then(function(subscription) {
  console.log(subscription);
  // The subscription was successful
  //  isPushEnabled = true;
  //  pushButton.textContent = 'Disable Push Messages';
  //  pushButton.disabled = false;

  // TODO: Send the subscription subscription.endpoint
  // to your server and save it to send a push message
  // at a later date
  return sendSubscriptionToServer(subscription);
 })
 .catch(function(e) {
  if (Notification.permission === 'denied') {
   // The user denied the notification permission which
   // means we failed to subscribe and the user will need
   // to manually change the notification permission to
   // subscribe to push messages
   console.log('Permission for Notifications was denied');
   //  pushButton.disabled = true;
  } else {
   // A problem occurred with the subscription, this can
   // often be down to an issue or lack of the gcm_sender_id
   // and / or gcm_user_visible_only
   console.log('Unable to subscribe to push.', e);
   //  pushButton.disabled = false;
   //  pushButton.textContent = 'Enable Push Messages';
  }
 });
});
}
</code></pre>