How can I retrieve a service account OAuth2 token from Google Api with Javascript?

There are two major divisions to this task.

1. Configuring
2. Coding

First the Configuration steps.

• If you don't have a google account:
  1. Navigate to google.com
  2. Find and Click "Sign In"
  3. Click "More Options"
  4. Click "Create Account"
  5. Follow the steps to create an account
• Navigate to the api dashboard: console.developers.google.com/apis/dashboard

• Select or create a project by clicking on the current project. The project I have showing is called "My Project"

• Click and enable those API you plan to work with

• navigate to the credentials section: console.developers.google.com/apis/credentials
• Click and select "Service account key"
  • If you create a new service account, for testing set the role to "project" "owner". You'll want to read up on google Api roles eventually. See Managing Roles and Granting Roles to Service Accounts
• Ensure "Key Type" is "Json" and click "Create". You're key/cert will automatically download

Now for the Coding portion.

• First download jsrsasign and add reference to "jsrsasign-all-min.js". If you want you can download just "jsrsasign-all-min.js" from github

• Second update the following script with your cert/key (downloaded earlier):

  function postJWT(jwt, callback) {
      var xhttp = new XMLHttpRequest();
      xhttp.onreadystatechange = function () {
          if (this.readyState == 4) {
              if (this.status == 200 && callback) {
                  callback(this.responseText);
                  return;
              }
              if (console) console.log(this.responseText);
          }
      };
      var parameters = "grant_type=" + encodeURIComponent("urn:ietf:params:oauth:grant-type:jwt-bearer") + "&assertion=" + encodeURIComponent(jwt);
      xhttp.open("POST", "https://www.googleapis.com/oauth2/v4/token", true);
      xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
      xhttp.send(parameters);
  }
  
  function getCert() {
      var cert = //your json key (downloaded earlier) goes here
          {
              "type": "service_account",
              "project_id": "proj..",
              "private_key_id": "e18..",
              "private_key": "-----BEGIN PRIVATE KEY-----\nMII..==\n-----END PRIVATE KEY-----\n",
              "client_email": "service-account@...iam.gserviceaccount.com",
              "client_id": "5761..",
              "auth_uri": "https://accounts.google.com/o/oauth2/auth",
              "token_uri": "https://accounts.google.com/o/oauth2/token",
              "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
              "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/..service-account%40...iam.gserviceaccount.com"
          };      
      return cert;
  }
  function getJWT() {
      var cert = getCert();
      var key = KEYUTIL.getKey(cert.private_key);
      var headers = { "alg": "RS256", "typ": "JWT" };
      var issued = Math.floor(new Date().getTime()/1000);
  
      var claims = {
          "iss": cert.client_email,
          "scope": "https://www.googleapis.com/auth/analytics.readonly",
          "aud": "https://www.googleapis.com/oauth2/v4/token",
          "exp": issued + 3600,
          "iat": issued
      };
  
      var jwt = KJUR.jws.JWS.sign(headers.alg, headers, JSON.stringify(claims), key);
      return jwt;
  }
  

• When you test your code you should receive a json object back with an auth token. You can test your implementation like so:

  postJWT(getJWT(text), function(){
      let token = JSON.parse(response).access_token;
      //Do your api calls here using the token. 
      //Reuse the token for up to 1 hour.
  });
  

Here is an example successful json object with token:

{
    "access_token": "ya29.c.ElkABZznrLNLK6ZAq2ybiH5lsRJpABE8p7MlZZJ0WCKcDNDv75lh-o1iRX__uMNUKSySiawm4YJGsbfqJH2JH61nRK6O2m0GJR7DgkEmo6ZlKtrvzke9C3xpwA",
    "token_type": "Bearer",
    "expires_in": 3600
}

Please note that this approach requires that the key/cert be accessible from your javascript environment. If this environment is public your api is vulnerable.