Deconstruct a JWT token for Firebase

jwtparser.js

// Helper function to extract claims from a JWT. Does *not* verify the
// validity of the token. 
// credits: https://github.com/firebase/angularFire/blob/master/angularFire.js#L370
// polyfill window.atob() for IE8: https://github.com/davidchambers/Base64.js
// or really fast Base64 by Fred Palmer: https://code.google.com/p/javascriptbase64/
function deconstructJWT(token) {
   var segments = token.split(".");
   if (!segments instanceof Array || segments.length !== 3) {
      throw new Error("Invalid JWT");
   }
   var claims = segments[1];
   return JSON.parse(decodeURIComponent(escape(window.atob(claims))));
}

whenexpires.js

// Moment: http://momentjs.com/

function tokHasTimeLeft(tok) {
  if( !tok ) {return false;}
  try {
     var parts = deconstructJWT(tok);
     // default is 24 hrs
     var exp = parts.exp? moment.unix(parts.exp) : moment.unix(parts.iat).add('hours', 24);
     // returns true if token has at least 12 hours left
     return exp.diff(moment(), 'hours') > 12;
  }
  catch(e) {
     console.warn(e);
     return false;
  }
}

Hi, you might want to convert from base64url character set to tandard base64 character by adding this:

claim = claim.replace('-', '+').replace('_', '/');

sources:

• https://stackoverflow.com/a/38552302/2540278
• https://en.wikipedia.org/wiki/Base64#Variants_summary_table