How to login to Google API with Service Account code example

Example 1: login by google api

<a href="#" onclick="signOut();">Sign out</a><script>  function signOut() {    var auth2 = gapi.auth2.getAuthInstance();    auth2.signOut().then(function () {      console.log('User signed out.');    });  }</script>

Example 2: login by google api

<meta name="google-signin-client_id" content="YOUR_CLIENT_ID.apps.googleusercontent.com">