CORS blocks mutation in GraphQL Yoga

The solution to the problem was to write a middleware that sets appropriate response headers so that the fetch doesn't fail.

server.express.use(function(req, res, next) {
  res.header('Access-Control-Allow-Origin', 'http://localhost:7777');
  res.header(
    'Access-Control-Allow-Headers',
    'Origin, X-Requested-With, Content-Type, Accept'
  );
  next();
});

The above is the yoga express server middleware used to solve the problem.