jwt-decode.js

var jwt = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJ0b3B0YWwuY29tIiwiZXhwIjoxNDI2NDIwODAwLCJodHRwOi8vdG9wdGFsLmNvbS9qd3RfY2xhaW1zL2lzX2FkbWluIjp0cnVlLCJjb21wYW55IjoiVG9wdGFsIiwiYXdlc29tZSI6dHJ1ZX0.yRQYnWzskCZUxPwaQupWkiUzKELZ49eM7oWxAQK_ZXw';
console.log(decodeJsonWebToken(jwt));

function decodeJsonWebToken(token){
  var tokenParts = token.split('.');
  return {
    header: decodeTokenPart(tokenParts[0]),
    token: decodeTokenPart(tokenPart[1]),
    signature: tokenPart[2]
  };
}

function decodeTokenPart(tokenPart){
    tokenPart = Base64.decode(tokenPart);
    return JSON.parse(tokenPart);
}