use case for anonymous object to be cast properly to interface

foo.js

// use case for StackOverflow question: 
// https://stackoverflow.com/questions/53328459/prevent-compilation-unless-all-fields-exist

const z = function(req,res,next){
  
  interface ResponseBody {
     foo1: number;
     foo2: string;
     foo3: boolean;
  }
  
  doSomething(v, function(err, d){
    
    if(err){
     return next(err); 
    }
    
    res.json(<ResponseBody>{foo1:d, foo2:"success",foo3:true})
    
  });
  
  
};