Passing server error message to client (using Express and Backbone) - node.js

Question

I've been thrashing for a bit and having difficulty figuring out how to pass server error messages to a client.
On the server I have (simplified):
export function get(req: express.ExpressServerRequest, res: express.ExpressServerResponse) {
res.statusCode = 500;
res.send('CUSTOM ERROR MESSAGE');
}
On the client:
public fetchObject(successF: Function, failF: Function): void {
this.myObj = new MyObj();
this.myObj.fetch({ success: successF, error: failF });
}
private failF(model, xhr, options): void {
// Want to get access to "CUSTOM ERROR MESSAGE"
}
The xhr object responseText is empty and the statusText is always "error".
Any suggestions? Thanks!

Answer 1

Found a solution. Define a class variable and capture the return from the fetch call:
private xhr: XMLHttpRequest = null;
Then:
public fetchObject(successF: Function, failF: Function): void {
this.myObj = new MyObj();
this.xhr = this.myObj.fetch({ success: successF, error: failF });
}
Finally:
private failF(model, xhr, options): void {
doSomething(this.xhr.responseText);
}
this.xhr will contain the reponseText (i.e. 'CUSTOM ERROR MESSAGE'). The local xhr will still be a blank string.
I'm still not sure why this is the case, and if anyone has some insight I'd appreciate it.